AGM Separators for AGM Battery

AGM Battery

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What is an AGM battery used for?

Let us first know what does the acronym, AGM, stand for. It is the abbreviation of the term Absorbent Glass Mat, a fragile, highly porous and paper-like white sheet cut from rolls, made from porous fine fibres of borosilicate glass and used as a battery separator is a type of lead-acid battery called AGM battery valve-regulated lead-acid battery (VRLAB). Simply said, it is a porous battery separator. A battery assembled with AGM separator is called an AGM battery.

AGM Battery separator roll
AGM Battery separator roll

AGM battery application

The VRLA AGM battery is used for all applications where non-spillability and fume-free operations are required. This battery is available in all sizes from 0.8 Ah (12 V) to hundreds of Ah, from 2 V to 12 V configurations. Any voltage value can be offered by a combination of 2 V or 4 V or 6 V or 12 V cells/batteries. They are used in various applications such as solar photovoltaic applications (SPV), uninterrupted power supply (UPS), communication devices, emergency lighting system, Robots, industrial control devices, industrial automation devices, fire-fighting equipment, Community Access Television (CATV), Optical communication devices, Personal Handy-phone Systems (PHS) base stations, microcell base stations, Disaster and crime prevention systems, etc.

Poorly maintained flooded batteries cannot deliver the expected life.
The conventional flooded of lead-acid battery requires some maintenance procedures to be followed. They are:

  1. Keeping the top of the battery clean and dry free from dust and acid droplets.
  2. Maintaining the level of the electrolyte (in the case of a flooded battery) at the appropriate level by topping up with approved water.
    This decrease in electrolyte level is due to the electrolysis (breaking down by using electricity) of water occurring towards the end of a recharge when a part of the water in the dilute acid gets dissociated as hydrogen and oxygen as per the following reaction and vented to the atmosphere stoichiometrically:
    2H2O →2H2 ↑ + O2 ↑

The lead-acid battery contains dilute sulphuric acid as the electrolyte and the terminals of a conventional battery and the external parts such as container, inter-cell connectors, covers, etc. get some sort of acid spray and also get covered with dust. The terminals should be kept clean by wiping with a wet cloth and also by applying white Vaseline periodically so that no corrosion occurs between the terminals and the cable connected to it.

The corrosion product is bluish in colour due to the formation of copper sulphate coming from the brass terminals. If the connectors are made of steel, then the corrosion product will be having a colour of greenish-blue, due to ferrous sulphate. If the product is white in colour, it may be due to lead sulphate (due to sulfation) or due to aluminium connectors being corroded.

Also, acid-fume laden gases emanate from the battery while charging is going on. This fume will affect the surrounding equipment as well as the atmosphere.
The consumer thinks that this is a cumbersome procedure and wants a battery, free from such maintenance work. Scientists and engineers started to think in this line and search for methods to avoid these procedures were taken up in the late 1960s. Only in the late 1960s, the real “maintenance-free” batteries were realized commercially. Sealed nickel-cadmium cells were the forerunner for the VRLAB.

R & D work on small, cylindrical lead-acid cells containing spirally-wound electrodes was started in 1967 in the laboratories of Gates Corporation, USA by John Devitt. In 1968, Donald H. McClelland joined him. Four years later, in 1971, the resulting products were offered for sale: a cell equivalent in size to the conventional manganese dioxide D-cell and another having twice the capacity was offered commercially by Gates Energy Products Denver, CO, USA. [J. Devitt, J Power Sources 64 (1997) 153-156]. Donald. H. McClelland and John L. Devitt of Gates Corporation, USA described for the first time a commercial sealed lead-acid battery based on the oxygen cycle principle [D.H. McClelland and J. L. Devitt US Pat. 3862861 (1975).]

Simultaneously two technologies, one based on gelled electrolyte (GE) and the other on AGM were developed, the former in Germany and the latter in USA, Japan and Europe.
To start with, valve-regulated lead-acid batteries were called ‘maintenance-free’ batteries, electrolyte-starved batteries, sealed batteries and so on. Due to a lot of litigation between the consumers and the manufacturers regarding the use of the term ‘maintenance-free,’ the presently used term “valve-regulated” became widely accepted. Since the VR battery has one-way pressure release valves, the usage of the term “sealed” also is discouraged.

What is the difference between an AGM battery & a standard battery?

An AGM battery and a regular or standard battery use a similar type of plates, mostly, flat plates. This is the only similarity. Some flooded battery also uses tubular plates.

A standard or conventional or flooded battery is entirely different from the AGM battery in the sense that the latter has no free liquid electrolyte, where the electrolyte level has to be maintained by periodically adding approved water to make up for the loss of water due to electrolysis. On the other hand, in the AGM battery, which is a valve-regulated lead acid (VRLA) battery, there is no such requirement, The unique reactions occurring in VR cells take care of the loss by following what is called an “internal oxygen cycle”. This is the main difference.

For the operation of the oxygen cycle, AGM battery has a one-way release valve. Special rubber cap covers a cylindrical exhaust tube. As the internal pressure in the battery reaches the limit, the valve lifts (opens) to release the accumulated gases and before it attains the atmospheric pressure, the valve closes and remains so until the internal pressure again exceeds the vent pressure. The function of this valve is manifold. (i) To prevent accidental ingress of unwanted air from the atmosphere; this results in the discharge of NAM. (ii) For the effective pressure-assisted transport of the oxygen from PAM to NAM, and (iii) to protect the battery from an unexpected explosion; this may be caused by an abusive charge.

In AGM battery, the entire electrolyte is held only in the plates and the AGM separator. Therefore there is no chance of spillage of the corrosive electrolyte, dilute sulphuric acid. For this reason, the AGM battery can be operated on any side, except, upside down. But the flooded battery can be used only in the vertical position. While racking the VRLA batteries, the operation of taking the voltage readings becomes easier in the case of high voltage high capacity batteries.

During the normal operations of VRLAB, there is negligible or no gas emissions. So it is “user-friendly”. Hence AGM battery can be integrated into the electronics equipment. A good example is the personal computer UPS, which normally uses a 12V 7Ah VRLA battery. Because of this reason, the ventilation requirements for VRLA AGM battery are only 25 % of that required for flooded batteries.

Compared to gelled VR or AGM VR batteries, the flooded version suffers from the phenomenon of electrolyte stratification. It is negligible in gelled batteries and in the case of AGM battery it is not as serious as in the flooded batteries. Because of this, the non-uniform utilization of active materials is eliminated or reduced, thus prolonging the life of the batteries.

The manufacturing process in AGM battery involves effective compression of cell elements to suppress the increase in resistance during the life of the battery. A concomitant effect is a decrease in the rate of fall of capacity during cycling/life. This is due to the avoidance of shedding due to the compressive effects.

VRLA Batteries are ready-for-use batteries. It is very easy for installation avoiding the cumbersome and time-consuming initial filling and initial charging, thus minimizing the time required for installation.

Very pure materials are employed in manufacturing VRLA batteries. Because of this aspect and the use of AGM separator, the loss due to self-discharge is very low. For example, the loss is less than 0.1% per day in the case of AGM battery while it is 0.7-1.0% per day for flooded cells. Hence, AGM battery can be stored for longer periods without refreshing charge. Depending on the ambient temperature, AGM battery can be stored without charge up to 6 months (20ºC to 40ºC), 9 months (20ºC to 30ºC) and 1 year if below 20ºC. [panasonic-batteries-vrla-for-professionals_interactive March 2017 p 18]

AGM Battery Capacity retention characteristics
AGM Battery Capacity retention characteristics
Temperature of Storage (ºC) Flooded Flooded Flooded VRLA VRLA VRLA
Period of storage (months) Capacity retention (per cent) Capacity Loss (per cent) Period of storage (months) Capacity retention (per cent) Capacity Loss (per cent)
40 - - - 6 40 60
40 3 35 65 3 70 30
40 2 50 50 2 80 20
40 1 75 25 1 90 10
25 - - - 13 60 40
25 6 55 45 6 82 18
25 5 60 40 5 85 15
25 4 70 30 4 88 12
25 3 75 25 3 90 10
25 1 90 10 1 97 3
10 - - - 12 85 15
10 - - - 9 90 10

The AGM battery can be designed to survive a 30-day short-circuit test and, after recharge, has virtually the same capacity as before the test.

Is an AGM battery the same as gel battery?

Even though these two types belong to the valve-regulated (VR) type of batteries, the main difference between these two types is the electrolyte. AGM is used as a separator in AGM battery, in which the whole of the electrolyte is contained within the pores of the plates and the pores of the highly porous AGM separator. Typical porosity range for an AGM separator is 90-95%. No extra separator is used. During the filling of electrolyte and subsequent processing, care is taken to see that the AGM is not saturated with the electrolyte and at least 5 % voids are there without being filled with the acid. This is to facilitate the operation of the oxygen cycle.

Oxygen is transported from the positive plate through the separator to the negative plate during charging. This transport can only happen effectively if the separator is not fully saturated. A saturation level of 95% or less is preferred. (POROSITY: It is the ratio in the percentage of the volume of pores in AGM to the total volume of material, including the pores).

But in the gelled electrolyte battery, the electrolyte is mixed with fumed silica powder to immobilize it, so that the gel battery becomes non-spillable. The separator is either polyvinyl chloride (PVC) or cellulosic type. Here the oxygen gas diffuses through the fissures and cracks in the gel matrix. A gel battery may be constructed with pasted type or tubular type plates. Both the types of gel batteries have one-way release valve and operate on the principle of “internal oxygen cycle”.

In both VRLA battery types, sufficient void space is left that allows fast transport of oxygen through the gaseous phase. Only a thin wetting layer at the negative electrode surface has to be permeated by dissolved oxygen, and the efficiency of the internal oxygen-cycle comes close to 100%. When a battery is saturated with the electrolyte initially, it hinders fast oxygen transport, which results in increased water loss. On cycling, such a “wet” cell yields an efficient internal oxygen-cycle.

For most applications, the differences between the two types of VRLA batteries are marginal. When batteries of the same size and design are compared, the internal resistance of the gel battery is slightly higher mainly due to the conventional separator. AGM battery have a lower internal resistance and so AGM battery are preferred for high load application. [D. Berndt, J Power Sources 95 (2001) 2]

In a gel battery, on the other hand, the acid is more strongly bound and therefore the influence of gravity is almost negligible. Thus, gel batteries do not show acid stratification. In general, they are superior in cyclic applications, and tall gel cells can be operated also in an upright position, while with tall AGM battery operation in a horizontal position is usually recommended to limit the height of the separator to about 30 cm.
In gelled electrolyte, most of the oxygen must surround the separator. The polymer separator acts as a barrier for oxygen transport and reduces the transport rate. This is one of the reasons that the maximum rate of the internal oxygen-cycle is lower in gel battery.

Another reason may be that a certain portion of the surface is masked by the gel. Rough figures for this maximum rate are 10 A/100 Ah in AGM battery and 1.5A/100Ah in gel battery. A charging current that exceeds this maximum causes the gas to escape as in a vented battery. But this limitation normally does not influence charging or float behaviour, since VR lead-acid batteries are charged at a constant voltage, and overcharging rates are far below, 1A/100 Ah, even at 2.4V per cell. The more limited maximum rate of the internal oxygen-cycle in gel batteries even offers the advantage that gel batteries are less sensitive to thermal runaway when overcharged at too high a voltage.

The gel batteries are more resistant to thermal runaway tendency than the AGM cells. In an experiment with similar gel and AGM battery (6V/68Ah), the following results are reported by Rusch and his co-workers [https://www.baebatteriesusa.com/wp-content/uploads/2019/03/Understanding-The-Real-Differences-Between-Gel-AGM-Batteries-Rusch-2007.pdf]. After artificially ageing the batteries by overcharge so that they lose 10 % of their water content, the cells were subjected to increased heat evolution by charging at 2.6 volts per cell in a restricted space. The gel battery had a current of 1.5-2.0 A equivalent while the AGM battery had 8-10 A current equivalent (six-fold higher heat evolution).

The temperature of the AGM battery was 100ºC, while that of the gel version remained below 50ºC.  Therefore the float voltage of the gel batteries can be kept at higher level up 50ºC without any danger of thermal runaway.    This will also keep the negative plate in good charge at higher temperatures.

Thermal runaway simulation in agm battery
credits: https://www.baebatteriesusa.com/wp-content/uploads/2019/03/Understanding-The-Real-Differences-Between-Gel-AGM-Batteries-Rusch-2007.pdf]

The AGM battery uses plates generally of a maximum height of 30 to 40 cm in height. If taller plates are employed, then the AGM battery shall be used on its sides. But in a gel battery, no such height restrictions are there. Submarine gel cells with plate height of 1000 mm (1 metre) are already in use.
The AGM battery is preferred for high current, short period applications. The cost of manufacture of AGM battery is higher for high rate capability than the Valve regulated gel battery. But, the gel cells are eminently suited for longer discharge times and give more power per unit currency.

The VRLA flat plate design (OGiV) has the same characteristics as the flooded flat plate design. They are preferable for short bridging times. 

At the 10 min-rate, the power output per manufacturing cost is 30% higher than of the VRLA gel tubular design (OPzV), while at longer discharge times (above 30 minutes ) the tubular VR gel OPzV design gives more power per $. At the 3h-rate, the OPzV gives 15% higher power per $. In the region from 3 h to 10 h, the flooded tubular OPzS gives 10 to 20% more power per $ than the OPzV battery, while in the important region between 30 min and 100 min, flooded tubular (OPzS) gives the same power per $ as VRLA gel tubular (OPzV).

Cell power per $ OPzV set to 100%

What is “internal oxygen cycle” in AGM battery?

In a flooded cell, the gases evolved during an overcharge are vented to the atmosphere. But in a Valve Regulated battery, there is negligible gas evolution because of certain reactions occurring on both the plates. During overcharge of a VR cell, the oxygen evolved from the positive plate passes through the unsaturated pores of the AGM (or the cracks in the gelled electrolyte) and reaches the negative plates and combines with the lead in the negative plate to form lead oxide. Lead oxide has a great affinity for sulphuric acid and so it immediately gets converted to lead

While manufacturing VRLA cells, acid is filled by calculated quantity.
On completion of the formation process, the excess electrolyte (if any) is removed from the cells by a cycling process. At the beginning of cycling (when the cells are filled by more than 96% pores), the oxygen cycle operates with low efficiency, which leads to water loss. When the electrolyte saturation level drops below 96%, the efficiency of the oxygen cycle increases, thus water loss is reduced.

The oxygen gas and H+ ions produced during charging of a VR battery (Reaction A) is made to pass through unsaturated pores available in AGM separator or through cracks and fissures in the gelled electrolyte structure and reach the negative plate where it combines with active lead to form PbO, which gets converted to PbSO4. Water is also formed in this process (Reaction B) along with some heat generation.

(In a flooded lead-acid battery, this diffusion of gases is a slow process, and all the H2 and O2 are vented out. A part of the charging current goes to the useful charging reaction, while a small portion of the current is used in the oxygen cycle reactions. The net result is that water, rather than being released from the cell, is cycled electrochemically to take up the excess overcharge current beyond that used for charging reactions.)

The PbSO4 is converted to Pb and H2SO4   (Reaction C) by an electrochemical route by reacting with the hydrogen ions resulting from the decomposition of water at the positive plates when they are charged.

The reactions are as follows:

At the positive plate:

2H2O → 4H+ + O2 ↑ + 4e                                                                     (A)

At the negative plate:

2Pb + O2 + 2H2SO4 → 2PbSO4 + 2H2O +Heat                                 (B)

2PbSO4 + 4H+ + 4e− → 2Pb + 2 H2SO4                                             (C)

The water produced diffuses through the separator to the positive plates, thus restoring the water decomposed by electrolysis.

The above processes form the oxygen cycle. The latter reduces substantially the water loss during charge and overcharge of the battery, making it maintenance-free.

In the early days of VRLA Battery developments, it was thought essential that the VRLA battery should have 100% efficient oxygen recombination efficiency on the assumption that this would ensure that no gas is vented to the outside atmosphere so that water loss is minimized. In recent years, however, it has become apparent that 100% oxygen recombination may not be desirable, as this may lead to negative-plate degradation. The secondary reactions of hydrogen evolution and grid corrosion are very important in the lead-acid battery and may have a significant impact on VRLA cell behaviour.

The rates of the two reactions need to be balanced, otherwise, one of the electrodes — usually the negative — may not become fully charged. The negative electrode may actually self-discharge at the reversible potential and therefore its potential will have to rise above this value (i.e., become more negative) to compensate for self-discharge and to prevent capacity decline [M.J. Weighall in Rand, D.A.J; Moseley, P.T; Garche. J; Parker, C.D.(Eds.) Valve-Regulated Lead- Acid Batteries, Elsevier, New York, 2004, Chapter 6, page 177].

Charging of valve regulated & flooded lead acid cells
Credits: Sketch by Dr PG Balakrishnan

The actual structure of the Absorbent Glass Mat separator exercises an important influence on the efficiency of oxygen recombination. An AGM separator with a high surface area and a small average pore size may wick acid to a greater height and provide higher resistance to the diffusion of oxygen. This may imply the use of an AGM separator with a high percentage of fine fibres, or a hybrid AGM separator containing, for example, organic fibres.

What is the difference between an AGM battery & a tubular battery?

AGM battery invariably employs flat plates, having a thickness between 1.2 mm to 3.0 mm depending on the applications, whether it is for starting, lighting and ignition (SLI) purpose or stationary purpose. Thicker plates are used for stationary applications. But a tubular battery uses tubular plates, the thickness of which may vary from 4 mm to 8 mm. Mostly, the tubular plate batteries are used in stationary applications.

In AGM battery, the entire electrolyte is held inside of the plates and the AGM separator. Therefore there is no chance of spillage of the corrosive electrolyte, dilute sulphuric acid. For this reason, the AGM battery can be operated on any side, except, upside down. But the tubular batteries have an excess of liquid electrolyte and can be used only in an upright position. We can measure the density of the electrolyte in tubular cells, but not in AGM battery.

The AGM battery operates in a semi-sealed atmosphere with a one-way release valve on the principle of oxygen cycle and therefore there is negligible water loss. Hence, there is no necessity for adding water to this battery. But the tubular battery is a vented type and all the gases evolved during overcharge are vented to the atmosphere; this results in water loss and hence the electrolyte level goes down necessitating periodic water addition to maintain the level of the electrolyte.

Because of the flooded nature, the tubular cells can tolerate overcharge and a higher temperature. This type has got a better heat dissipation. But the AGM battery is not tolerant to high-temperature operation, since these batteries are inherently prone to exothermic reactions due to internal oxygen cycle. AGM battery can be operated up to 40ºC, while the other type can tolerate up to 50ºC.

The polarization of positive and negative plates during a float charge at 2.30 V per cell (OCV = 2.15 V)

Flooded -New Flooded -End of life Gelled - New Gelled - End of life AGM - New AGM - End of life
Positive plate polarisation (mV) 80 80 90 120 125 (to 175) 210
Negative plate polarisation(mV) 70 70 60 30 25 0 (to -25) sulphated)
Polarization of three types of batteries

Polarization of three types of batteries
The IEC 60 896-22 has as the highest requirement 350 days at 60°C or 290 days at 62.8°C.
Life test at 62.8ºC as per IEEE 535 – 1986

Battery Type Days at 62.8ºC Equivalent years at 20ºC
OGi (Flooded flat plate) 425 33.0
OPzV (VR tubular) 450 34.8
OPzS (Flooded tubular) 550 42.6

How long does an AGM battery last?

A definite statement cannot be made on the usable life of any type of battery. Before one answers “how many years an AGM battery may last”, the conditions under which the battery operates should be clearly defined;

for example, whether it is simply floated across a particular voltage or is it cyclically operated. In the float operated manner, the battery is continuously float-charged at a particular voltage and it is called upon to supply current only when the main power is not available (Example: Telephone exchange batteries, UPS batteries, etc., where the life is expressed in years). But in the case of a traction battery, which is employed in factories for material handling purposes, and electric vehicles, the batteries experience deep discharges up to 80 % at 2 to 6-hour rate, the life will be shorter.

The life of the AGM battery depends on a number of operating parameters like:

Effect of temperature on life
The effect of temperature on the operational life of the lead-acid battery is very significant. At higher temperatures (and at charging voltages beyond the recommended values) dry-out happens faster, leading to the premature end of life. The corrosion of the grid is an electrochemical phenomenon. At higher temperatures, the corrosion is more and so the growth (both horizontal and vertical) is also more. This results in the loss of grid-active material contact and hence impaired capacity. Increasing temperature accelerates the rate at which the chemical reactions occur.

These reactions adhere to the Arrhenius relationship which, in its simplest form, states that the rate of electrochemical process doubles for each 10oC rise in temperature (keeping other factors such as float voltage
constant). This can be quantified using the relationship [Piyali Som and Joe Szymborski, Proc. 13th Annual Battery Conf. Applications& Advances, Jan 1998, California State Univ., Long Beach, CA pp. 285-290]
Life Acceleration Factor = 2((T−25))/10)
Life Acceleration Factor = 2((45-25)/10) = 2(20)/10) = 22 = 4
Life Acceleration Factor = 2((45-20)/10) = 2(25)/10) = 22.5 = 5.66
Life Acceleration Factor = 2((68.2-25)/10) = 2(43.2)/10) = 24.32 = 19.97
Life Acceleration Factor = 2((68.2-20)/10) = 2(48.2)/10) = 24.82 = 28.25

A battery operated at a temperature of 45ºC can be expected to age four times faster or have 25% of the life expected at 25ºC.
A battery operated at a temperature of 68.2ºC can be expected to age 19.97 times faster or have 20 times of the life expected at 25ºC. A battery operated at a temperature of 68.2ºC can be expected to age 28.2 times faster and have that much more of the life expected at 20ºC.

Accelerated life test and equivalent lives of batteries

Life at 20ºC Life at 25ºC
Life at 68.2ºC 28.2 times more 20 times more
Life at 45ºC 5.66 times more 4 times more

The expected float life of the VRLA battery is greater than 8 years at room temperature, arrived at by using accelerated testing methods, specifically, at high temperatures.
Cycle life of 12V VRLA (Delphi) has been studied by R. D. Brost. The study was carried out to 80% DOD at 30, 40 and 50ºC. The batteries were subjected to 100% discharge at 2 hours after every 25 cycles at 25ºC to determine the capacity. The results show that the cycle life at 30ºC is about 475 while, the number of cycles is 360 and 135, approximately, at 40ºC and 50ºC, respectively. [Ron D. Brost, Proc. Thirteenth Annual Battery Conf. Applications and Advances, California Univ., Long Beach,1998, pp. 25-29]

Temperature dependence of life of VRLA Batteries
Credits: [Ron D. Brost, Pro. Thirteenth Annual Battery Conf. Applications and Advances, California Univ., Long Beach, 1998, pp. 25-29]

Depth of discharge and life
The cycle life of sealed lead-acid is directly related to the depth of discharge (DOD). The depth of discharge is a measure of how deeply a battery is discharged. When a battery is fully charged, the DOD is 0%. Conversely, when a battery is 100% discharged, the DOD is 100%. When the DOD is 60 %, SOC is 40 %. 100 – SOC in % = DOD in %

The typical number of discharge/charge cycles for VR batteries at 25°C with respect to the depth of discharge is:
150 – 200 cycles with 100% depth of discharge (full discharge)
400 – 500 cycles with 50% depth of discharge (partial discharge)
1000 + cycles with 30% depth of discharge (shallow discharge)
Under normal float operating conditions, four or five years of dependable service life can be expected in stand-by applications (up to ten for the Hawker Cyclon line), or between 200 and 1000 charge/discharge cycles depending on the average depth of discharge. [Sandia Report SAND2004-3149, June 2004]

Flat plate technology AGM battery can deliver
400 cycles at 80% discharge
600 cycles at 50% discharge
1500 cycles at 30% discharge

Effect of position on cyclic life of VRLA Batteries

Credits: [R.V. Biagetti, I.C. Baeringer, F.J. Chiacchio, A.G. Cannone, J.J. Kelley, J.B. Ockerman and A.J. Salkind, , Intelec 1994, 16th International Telecommunications Energy Conference, October, 1994, Vancouver, BC., Canada, as cited by A.G. Cannone, A.J. Salkind and F.A. Trumbore , Proc. 13th Annual Battery Conf. Applications and Advances, California Univ., Long Beach, 1998, pp. 271-278.]

Effect of position on cyclic life of VRLA Batteries

The figure shows the average capacities for two batteries positioned in the normal upright position, on their sides with their plate’s vertical and with plates in the horizontal position. In the vertical position, the electrolyte develops stratification due to gravity effects and this aggravates as the cycling proceeds and the capacity decline in this position is very fast. However, when cycled in a side vertical position the decline in capacity is not so fast and the cycling in the horizontal position gives the best life. The figure is a plot of capacity vs. cycle number for the 11-plate Cell 52 cycled successively in the horizontal, vertical and horizontal positions.

This cell was cycled alone with the trickle/charge and charge voltage limits set at 2.4 V and the trickle/charge time and current set at 3 hours and 0.3 A. Prior to the vertical cycle 78, the cell was float charged for 4 days. For the horizontal cycling, the coulombic efficiency is relatively high and constant, as is the charge acceptance. However, during the vertical cycling, the charge acceptance declines significantly with cycling while the efficiency remains relatively constant. When horizontal cycling was resumed, with no extended float charge, the discharge capacity (also charge time) is seen to rise quickly back to the level prior to the vertical cycling.

Effects of both temperature and charge/float voltage on battery life 

The effects of both temperature and float voltage on life are interrelated and interactive. Figure shows the expected life of a VR GNB Absolyte IIP battery for various float voltages and temperatures. It is assumed that the float voltage and temperature are held constant throughout the life of the battery.

Credits: [Piyali Som and Joe Szymborski, Proc. 13th Annual Battery Conf. Applications & Advances, Jan 1998, California State Univ., Long Beach, CA pp. 285-290, as given by P.G. Balakrishnan, Lead Storage Batteries, Scitech Publications (India) Pvt. Ltd., Chennai, 2011, page 14.37 ]

Combined effect of temperature & float voltage on GNB Absolyte IIP product
Charge voltage and life of Drysafe Multicraft batteries (12 V, 25 Ah5)
Credits: [R. Wagner, J. Power Sources 53 (1995) 153-162]

Wagner has reported the test results carried out with three different charging regimes for cyclic batteries and shows that the use of a higher charging voltage (14.4 V CV mode) gives longer life and there is negligible water loss in this case. Charge voltage and life of Drysafe Multicraft batteries (12 V, 25 Ah5)
25ºC; C/5 test every 50 cycles; discharge: 5 A to 10.2 V; charging as labelled in the figure

Effect of tin addition to positive grid alloy in VRLA batteries 

Tin additions to pure lead have greatly diminished the problems experienced on cycling batteries with grids made from this metal. Small amounts of tin (0.3–0.6 wt.%) increase significantly the charge-acceptance of pure lead.  An alloy with calcium content of 0.07 % and tin 0.7% gives the least growth when tested as bare grids as well as in float life tested cells. [H.K. Giess, J Power Sources 53 (1995) 31-43]

Effect of Maintenance of the Life of the Battery
Maintaining the batteries in good condition by following certain procedures will help in realising the expected life from batteries. Some of them are
a. Periodical cleaning of the outside
b. Periodical bench charge (Equalization charge)
c. Periodical check-up of the electrolyte level etc.

The manufacturing of batteries is done with several quality control procedures and SOPs so that a high-quality product is an outcome. Any genuine defect is bound to show up immediately after the batteries are put into service or within a few days from that. The more strenuous the service, the earlier will a defect manifest itself. The premature failures are rather an indication of the poor performance than of inherent defects in the system. The better the maintenance, the higher will be the life of batteries.

AGM vs flooded battery - what you need to know?

AGM battery are very clean in the external appearance during the operative life. But the flooded battery is smeared with dust and acid spray during operation. Moreover, the terminals are encrusted with corrosion product, if not maintained properly.
AGM battery and flooded (flat plate) batteries use flat plates or grid plates, having a thickness between 1.2 mm to 3.0 mm depending on the applications, whether it is for starting, lighting and ignition (SLI) purpose or stationary purpose. Thicker plates are used for the latter purpose.

In AGM battery, the whole of the electrolyte is contained in the plates and the separator. Therefore there is no chance of spillage of the corrosive electrolyte, dilute sulphuric acid. For this reason, the AGM battery can be operated on any side, except, upside down. But the flooded batteries have an excess of liquid electrolyte and can be used only in an upright position. We can measure the density of the electrolyte in tubular cells, but not in AGM cells. But by measuring the stabilised open circuit (OCV) of the battery, one can know the specific gravity value at that condition.

There is empirical rule
OCV = Specific gravity + 0.84 for single cells
Specific gravity = OCV – 0.84
For 12 Volt batteries, we have to divide the OCV of the battery by 6 to arrive at the cell OCV.
OCV of the battery = 13.2 V
Therefore cell OCV = 13.3/6 = 2.2 V
Specific gravity = 2.2 V – 0.84 = 1.36
Therefore the Specific gravity is 1.360

The AGM battery operates in a semi-sealed atmosphere with a one-way release valve on the principle of oxygen cycle and therefore there is negligible water loss. Hence, there is no necessity for adding water to this battery. But the flooded battery is a vented type and all the gases evolved during overcharge are vented to the atmosphere; this results in water loss and hence the electrolyte level goes down necessitating periodic water addition to maintain the level of the electrolyte.

Because of the flooded nature, these cells can tolerate overcharge and a higher temperature. This type has got a better heat dissipation. But the AGM battery are not tolerant to high-temperature operation, since these batteries are inherently prone to exothermic reactions due to internal oxygen cycle. AGM battery can be operated up to 40ºC, while the other type can tolerate up to 50ºC.

Absorbent glass mat AGM battery - what is absorbed? How? Why absorbent? More details of the AGM separator

Absorbent glass mat (AGM) is the name given to the type of glass fibre separator used in valve-regulated (VR) batteries. AGM has to absorb a lot of electrolyte (up to six times its apparent volume) and retain it for facilitating cell reactions. This is made possible by its high porosity. By absorbing and retaining the electrolyte the battery is made unspillable.

The essential manufacturing process of micro-glass fibres which are used to manufacture AGM separator is shown in the figure. The glass raw materials are melted in a furnace at around 1000ºC. Molten glass is then drawn from bushings to form primary coarse glass fibres with a diameter of a few hundred microns. These are then converted by a combustion gas to fine fibres (0.1 to 10 μm) which are collected on to a moving conveyor net by vacuum from below. The traditional method of manufacturing absorptive glass mats AGM for valve-regulated lead-acid batteries is to blend two or more types of fibres together in an aqueous acidic solution.

This process reduces the length of fibres to about 1 to 2 mm and causes some fibrillation. This blend is deposited on to either a moving endless wire or a roto-former (another version of an endless wire). The sheet acquires consistency as the water is withdrawn; it is then pressed and dried against heated drums.

Wet laying process results in AGM sheet fibre orientation which gives anisotropic network. The pores and channels measured in the z-direction (i.e., in a direction vertical to the plane of the sheet) are larger (10 to 25 μm, 90 % of the total pores) than those in x and y planes (2 to 4 μm). There is about 5 % of very large pores between 30 and 100 μm (probably due to edge effects during sample preparation and are not truly representing the typical structure). This manufacturing method is known as a flame attenuation process.

The first step in the production of AGM is the dispersion and agitation of the glass fibres in a large amount of acidified water. The mixture of fibres and water is then deposited on a surface where vacuum is applied and most of the water is removed. The formed mat is then slightly pressed and dried by means of heated rolls. At the end of the drying section, the water content of the mat is below 1 wt.%. A roto-former device for forming and de-watering AGM sheets is shown below.

Manufacture of AGM Separator
Credits: S. Vijayarajan in 2-day workshop on VRLA batteries ILZDA, New Delhi, 28-29 Aug 1997 pp. 16-19
A roto-former device for forming & de watering AGM sheets
Credits: [A.L. Ferreira, J Power Sources 78 (1999) 42]

Unlike the conventional separators (such as PVC or PE separators), the AGM has to perform several additional functions in addition to those performed by PVC or PE separators. Some authors call it the fourth active material in lead-acid batteries.

a. It acts as a reservoir of electrolyte. It’s highly porous nature enables it to absorb and retains up to six times its volume.
b. It should be sufficiently resilient and compressible in wet and dry conditions so that it can be handled in the various unit operations, without being damaged or torn.
c. The structure should be suitable for operation of oxygen cycle prevalent in VR batteries, allowing gaseous oxygen to flow through its unfilled pores, although it is wetted by the electrolyte almost to 95 % of its pores.

d. The conventional separators have small and tortuous pore structure, with little or no directional variations. But the AGM made by the wet laying of micro fibreglass material has high porosity and relatively large pores with considerable directional differences. These characteristics affect the distribution and movement of gases and liquids in the elements. [Ken Peters, J. Power Sources 42 (1993) 155-164]

The important characteristics of AGM separators are:
i. True (BET) surface area (m2/g)
ii. Porosity (%)
iii. Average pore size (μm)
iv. Thickness under compression (mm)
v. Basis weight or Grammage (g/m2) (weight of AGM sheet per square metre)
vi. Wicking height (mm) (The height the acid column reach when a piece of AGM is kept immersed in acid)
vii. Tensile strength

Typical properties of AGM separators are given in the following table:

Ref. W. BӦhnstedt, J Power Sources 78 (1999) 35–40

Property Unit of measurement Value
Basic weight (Grammage) g/m2 200
Porosity % 93-95
Mean pore size μm 5-10
Thickness at 10kPa mm 1.3
Thickness at 30kPa mm 1.0
Puncture strength(N) N 7.5

Ref:  Ken Peters, J. Power Sources 42 (1993) 155-164

Property Unit of Meaurement Value
Surface area
Coarse fibres m2/g 0.6
Fine fibres m2/g 2.0 to 2.6
Maximum pore size
Coarse fibres μm 45
Fine fibres μm 14
Wicking height, 1.300 specific gravity acid Unit of measurement Coarse fibres (0.5 m2/g) Fine fibres (2.6 m2/g)
1 minute mm 42 33
5 minute mm 94 75
1 hour mm 195 220
2 hours mm 240 370
10 hours mm 360 550

Notes:
1. As the fibre diameter increase, the pore size also increases.
2. As the fibre diameter increase, tensile strength decreases.
3. As the fibre diameter increase, cost decreases.
4. The coarse fibre layer will wick to a limited height, but at a very fast rate

5. The finer fibre will carry the acid to greater heights, although slowly
By including a denser layer (with small pores, which is created by finer glass fibres) within a multi-layered AGM separator, a finer overall pore structure is created. Thus, maximum pores are reduced by a half and the average pores are also nearly halved. The impact on the minimum pores is a reduction by one quarter. The synergy which exists between fine and coarse glass fibres is detected in all the wicking characteristics of the multi-layered AGM [A.L. Ferreira, J Power Sources 78 (1999) 41–45].

The coarse fibre layer will wick to a limited height, but at a very fast rate, whereas the finer side will carry the acid to greater heights, although slowly. Thus, the individual advantages of the two types of fibre are combined. By virtue of the better wicking properties, the critical process of initial filling of VRLA batteries is improved and the particular problem of filling tall plates with tight plate spacing is lessened. The maximum height after an extended period of wicking test is found to be inversely proportional to the pore size. That is, the smaller the pores, the greater is the wicking height.

The capillary forces dictate the electrolyte flow. The pore size distribution in, active materials of positive and negative plates has only minimal difference between dimensional planes. In freshly formed plates, about 80 % of porosity consists of pores smaller than 1 μm as against the 10 to 24 μm diameter pores in the z plane and 2 μm pores in the other two planes. Therefore the acid fills the plates (small pores) first (i.e., preferential filling of plates). Then the AGM is filled to the calculated void volume bringing the AGM to a partially saturated level so that “pushing out” of electrolyte during charge can provide gas channels for oxygen transport.

AGM Battery, comparison between AGM, flooded & Gel battery

Sl No. Property Flooded AGM VR Gelled VR
1 Active materials Pb/PbO2/H2SO4 Pb/PbO2/H2SO4 Pb/PbO2/H2SO4
2 Electrolyte (Dilute sulphuric acid) Flooded, excess, free Absorbed and retained by plates and absorbent Glass Mat (AGM) separator Immobilised by gelling with fine silica powder
3 Plate thickness Thin - medium Medium Thick
4 Number of plates (for same capacity battery, same dimensions) Most More Least
5 Maintenance Yes Nil Nil
6 Acid leakage spillability Yes No No
7 Electrolyte stratification in tall cells Very high Medium Negligible
8 outside of battery Becomes dusty and sprayed with acid droplets No No
9 Electrolyte level To be adjusted Not necessary Not necessary
10 Separator PE or PVC or any other polymeric material Absorbent glass mat (AGM) PE or PVC or any other polymeric material
11 Gases evolved during charge Stoichimetrically vented to atmosphere Recombined (internal oxygen cycle) Recombined (internal oxygen cycle)
12 one-way release valve Not provided. Open vents Yes. Valve-regulated Yes. Valve-regulated
13 Internal resistance Medium Low High
14 Safe DOD 50% 80% 80%
15 Cold-cranking OK Very good Not suitable
16 High discharge (High Power) Good Best Medium
17 Deep cycling Good better very good
18 Cost Lowest Medium High
19 Charging Normal Careful Careful
20 Maximum charging voltage (12v battery 16.5 V 14.4 V 14.4 V
21 Charging mode Any method Constant-voltage (CV) or CC-CV Constant-voltage
22 Overcharging Can withstand Cannot Cannot
23 Heat dissipation Very good Not bad Good
24 Fast charging Medium Very good Not advisable

Misconceptions about AGM battery

Charging and chargers
Misconception -1
Any regular charger can be used for AGM battery – False

All batteries require bench charging (or full charge) once in a while to equalize the cells’ imbalance.
This is done by removing the battery from the appliance and charging separately what is generally called bench charging.

The meaning of a full charge:
For a flooded battery:
i. All the cells in a battery should reach the uniform end of charge voltage, 16.5 V for a 12 V battery.
ii. All the cells should gas uniformly and copiously at the end of charge.
iii. The variation in specific gravity in the cells and between the cells should be removed.
iv. If facilities are available, the cadmium potential readings on positive and negative plates can be recorded. For a fully charged positive plate, the cadmium potential reading is in the range of 2.40 to 2.45 V and for negative plates, the values are in the range of 0.2v to – 0.22v

The meaning of a full charge:
For a VRLA AGM battery:
i. The terminal voltage would reach 14.4 V (for a 12 V battery)
ii. The current at the end of charge would be about 2 to 4 mA per Ah (i.e., 0.20 A to 0.4 A for a 100 Ah battery
The value of end of charge voltage for a12 V battery varies between a flooded and a VR battery.
The maximum charging voltage is about 16.5 V for a 12 V flooded battery, while it is only 14.4 V for VR batteries (both AGM and gelled batteries).

If a normal constant current charger is used for charging a VR battery, the voltage may exceed the limit of 14.4 V. If it goes undetected, the battery will get warmed up. Still, later the battery gets heated up and ultimately the container will bulge and may also burst if the one-way release valve does not function properly. This is because the battery’s recombination reactions cannot cope up with the excess oxygen gas produced by the higher charging current. Inherently, the recombination reaction is exothermic (heat-producing) in nature. The higher current will add to the heat of this reaction and may lead to thermal runaway.

In contrast, the flooded battery can go up to 16.5 V for a full charge with copious gassing without any damage up to 50ºC.
Chargers meant for VRLA batteries are controlled chargers. They are
a. Constant current- Constant voltage (CC-CV)
or
b. Constant voltage (CV) chargers.

While charging, one has to select the suitable voltage. For a 12V battery, a voltage range of 13.8 to 14.4 V can be selected for a full charge. Since the VR AGM battery can absorb any strength of initial current without any damage, the initial current can be set at any level (usually 0.4C amperes; but in fact or rapid charge, up to 5C A). The higher the selected voltage and current, the lower will be the time taken for a full charge.

For a fully discharged battery, it will take about 12 to 24 hours for a full charge. In the CC-CV mode, the initial current will be constant for about 3 to 6 hours, depending on the previous discharge. If the battery was only 50 % discharged previously, the CC mode will operate for about 2 to 3 hours and then switch over to CV mode. If it is 100 % discharged previously, the CC mode will operate for about 5 to 6 hours and then switch over to CV mode

Misconception -2

AGM battery or gel battery replacement is the same as flooded-battery replacement

Equivalent capacity batteries can be replaced if the space is ok.
But recent vehicles (e.g., GM) have a battery-sensor module on the negative battery cable. Ford has a battery-monitoring system (BMS). Other manufacturers have similar systems. These systems require recalibration with a scan tool. This is necessary because of the improvements in the manufacturing systems. These batteries have a lower internal-resistance due to improved separators and thinner plates with improved paste formulations. If the system is not recalibrated, the alternator might overcharge the new battery and cause the battery to fail soon after replacement.
So, one can install an AGM battery in place of an OEM flooded-battery. An AGM automotive battery will give the vehicle higher cold cranking amperes (CCA).

The meaning of a full charge:
For a flooded battery:
i. All the cells in a battery should reach the uniform end of charge voltage, 16.5 V for a 12 V battery.
ii. All the cells should gas uniformly and copiously at the end of charge.
iii. The variation in specific gravity in the cells and between the cells should be removed.
iv. If facilities are available, the cadmium potential readings on positive and negative plates can be recorded. For a fully charged positive plate, the cadmium potential reading is in the range of 2.40 to 2.45 V and for negative plates, the values are in the range of 0.2v to – 0.22v

Can you charge an AGM battery with a regular charger?

If a normal constant current charger is used for charging AGM VR battery, the voltage should be monitored closely. It may exceed the limit of 14.4 V. If it goes undetected, the battery will get warmed up. Still, later the battery gets heated up and ultimately the container will bulge and may also burst if the one-way release valve does not function properly. This is because the battery’s recombination reactions cannot cope up with the excess oxygen gas produced by the higher charging current. Inherently, the recombination reaction is exothermic (heat-producing) in nature. The higher current will aggravate the situation and add to the heat of this reaction and may lead to thermal runaway.

Hence, it is not advisable to use the regular charger for AGM battery charging.

But, if you follow the procedure given below or have the advice of a VRLA battery expert, you can use the regular charger very carefully.

The procedure is to follow the terminal voltage (TV) readings and record them at 30-minutes intervals. Once the TV reaches 14.4 V, the current should constantly be reduced so that the TV never goes beyond 14.4 V. When the current readings show very low values (2 to 4 mA per Ah of battery capacity), the charging can be terminated. Also, the leads of a thermocouple or thermometer bulb can be attached to the negative terminal of the battery and similar to TV readings, temperature readings should also be recorded. The temperature should not be allowed to exceed 45ºC.

Can you jump start an AGM battery?

Yes, if the voltage ratings are the same.
The chemistry of both the flooded and AGM battery is the same. Only, most of the electrolyte is absorbed in the AGM. Hence, using any battery of the same voltage rating to jump-start an AGM battery for a few seconds will do no harm to either of the batteries.

How can I tell if I have an AGM battery?

  • Examine the top of the container and also sides to see any screen printing indicating that it is a VRLA battery. If you do not find any user-accessible device written on the top and a piece of advice not to add water, then it is an AGM battery.
  • If any free electrolyte is visible after removing vent plugs, then also it is not an AGM battery
  • The nameplate or the screen printing on the battery container or the Owner’s Manual can give a good idea about the type of the battery in question. If you do not have any of these three, examine the top of the battery for any venting system or something like a magic eye. You can also look for electrolyte level markings on the sides of the battery container. If you see any of the three (vents, magic eye and electrolyte level markings), it indicates that it is not an AGM battery.

There is another method, but a time-consuming one. The battery has to be charged fully and after an idle period of 2 days, the open-circuit voltage (OCV) is measured.

If the OCV value is from 12.50 to 12.75 V it may a flooded battery
If the OCV value is from 13.00 to 13.20 V it may a VRLA battery (capacity < 24 Ah)
If the OCV value is from 12.80 to 12.90 V it may a VRLA battery (capacity ≥ 24 Ah)

These statements are made on the assumptions that for flooded batteries, the final specific gravity is about 1.250. For VRLA batteries of capacities 24Ah and smaller values, the final specific gravity is about 1.360 and for VRLA batteries of higher capacities, the final specific gravity is about 1.300

How do I know if my AGM battery is bad?

  • Check for any external damage, cracks and leakage or corrosion products. If you find anyone of these, the battery is BAD
  • Measure the OCV of the battery. If it shows a value lower than 11.5 V, most probably, it is BAD. But before that, see if you can find out the date of despatch or supply. If the battery is older than 3 to 4 years, it can be assumed to be BAD.
  • Now, the battery should be checked for charge acceptance by using a charger whose DC voltage output is 20 to 24 V or more (for a 12 V battery). Charge the battery for one hour, give a rest period of 15 minutes and now measure the OCV. If it has increased, then continue charging for 24 hours by a constant voltage method, taking all the necessary precautions for a VR battery charging. After giving a rest period of 2 hours, test the battery for capacity using any appliance (e.g., a suitable DC bulb, inverter, emergency lamp, UPS for a PC, etc). If the battery is able to deliver 80 % or more capacity, the battery is GOOD.
  • If the OCV does not increase after 1-hour charge, it means that the battery cannot hold a charge. The battery can be labelled as BAD.

Is an AGM battery worth the extra money?

Yes.
Even though the cost of the battery is a little higher, the maintenance required for AGM is almost zero. There is no necessity for topping up, no cleaning of the corroded terminals is required, lesser number of equalizing charges, etc.; the operational cost over the entire life of an AGM battery is very low, bringing the cost of the AGM VR battery to a level equal to that of flooded batteries.
This is particularly advantageous when the place is inaccessible in a remote unattended area.

Does an AGM battery need to be vented

In the event of an abusive overcharge, the low-pressure one-way release valves fitted in the covers of VRLA batteries open up and re-seat after releasing the excess pressure. Hence, there is no necessity to vent the VRLA battery.
In the case of valve malfunctioning, the excess pressure may not be released by lifting up. If the valve does not re-seal, then also the cells will be open to the atmosphere and the negative active material (NAM) will get discharged, thus resulting in sulfation and insufficient charge and battery capacity run down.

Can I trickle charge an AGM battery?

Yes.
Actually AGM battery are under float charge in most of the UPS/emergency power supply. When the batteries are floated at 2.25 to 2.3 V per cell, a small trickle current is always flowing through the battery to keep it in a fully charged condition.
In case, huge numbers of batteries are in stock, then also each individual battery can be kept under trickle charge.
At a typical float-charge voltage of 2.25 V per cell, the float current is at 100 to 400 mA per 100 Ah for VR AGM batteries. Compared with a flooded battery’s equilibrium float current of 14 mA per 100 Ah, the VR battery’s higher float current is due to the effect of the oxygen cycle.

[R.F. Nelson in Rand, D.A.J; Moseley, P.T; Garche. J ; Parker, C.D.(Eds.) Valve-Regulated Lead- Acid Batteries, Elsevier, New York, 2004, pp. 258].

Can a dead AGM battery be charged?

Yes. We can say definitely only after charging the battery for some time. It also depends on the age of the battery.
The dead AGM battery has a very high internal resistance. To overcome this high internal resistance, a battery charger which can supply 4 V per cell DC output is required, with a digital ammeter and digital voltmeter.

While charging a dead AGM battery, to start with, the terminal voltage (TV) will be very high (as high as 18-20 V for a12 V battery) and the current almost zero. If the battery is capable of revival, the TV will slowly come down (almost to 12 V) and the ammeter simultaneous will begin to show some current. This indicates that the battery comes alive. The TV will slowly begin to increase now and the charging shall be continued and finished in the usual manner.

An unconventional way is to carefully remove the vent valves and add a little water at a time until we see a few drops excess water. Now, without replacing the valves, charge the battery by a constant current mode (C/10 amperes) until the terminal voltage goes to higher values than 15 V (Remember. we have not closed the valves). Give a little rest period and discharge the battery through suitable resistance or bulb. Measure the time of discharge to reach 10.5 V in the case of a 12 V battery). If it is delivering more than 80 % of the capacity, it is revived. Please take personal safety precautions at all times.

What voltage is a fully charged AGM battery?

A fully charged battery under cyclic operation will have a Terminal Voltage (TV) of 14.4 V (for 12V batteries). After about 48 hours rest period, the TV will stabilise at 13.2V (if the specific gravity for initial filling was 1.360) (1.360 + 0.84 = 2.20 per cell. For a 12V battery, OCV = 2.2 *6= 13.2V). If the capacity of the battery is higher than 24Ah, the specific gravity will be 1.300. Hence the stabilised OCV will be 12.84V

What is the maximum charging voltage for a 12 volt AGM battery?

AGM battery meant for cyclic operation are to be charged under constant potential or constant voltage mode (CV mode), at 14.4 to 14.5 V with an initial current being normally limited to 0.25 C amperes (i.e., 25 amperes for a 100 Ah battery) Some manufacturers allow up to 14.9 V with the initial current being limited to 0.4 C for cyclic use (i.e., 40 amperes for a 100 Ah battery). [panasonic-batteries-vrla-for-professionals_interactive March 2017, p.22]

What causes AGM batteries to fail?

Valve-regulated lead-acid (VRLA) batteries have been proposed as energy sources for several applications because of their good power performance and low price. They are also eminently suitable for float applications. Unfortunately, however, intensive utilization of the positive active-mass (particularly at high rates of discharge) causes softening of this material and, thereby, reduces battery cycle-life. Also, grid growth and grid corrosion, water loss and sulphation due to stratification and insufficient charging are some of the failure mechanisms. Most of the failures are associated with positive plates.

Corrosion, grid growth and positive active material expansion and softening
In the operation of batteries, the tendency of the growth of the positive grids is evident during repetitive charge and discharge, which causes both horizontal and vertical growth of grids. The grids get corroded during the entire life of the battery. As a result of this grid growth, the contact between the PAM and grid is lost, resulting in capacity decay.

The grid growth may cause an internal short between the positive plate and the negative strap of the cell. Continuing the charge of a bank of cells/batteries with one or two short-circuited cell will aggravate the temperature rise and lead to thermal runaway.

Drying out (water loss) and Thermal runaway

Dry out is also a problem with AGM battery. This is due to charging with inappropriately higher voltage, combined with higher temperature. Due to dry out, the recombination reaction rate is increased and the consequent temperature rise aggravates the situation, leading to thermal runaway.

Another cause is the malfunctioning of the valve. If it does not close properly after opening, the atmospheric oxygen (air) enters the cell and oxidises the NAM resulting in sulfation. The gases will be vented and dry out will occur. Dry-out allows oxygen recombination to proceed at a high
rate resulting in enhanced temperature.

Acid stratification in AGM battery

The tendency of the sulphuric acid electrolyte to increase in density as we go down the depth of a tall cell is known as stratification. Concentration gradients (‘acid stratification’) occur readily in the electrolyte of flooded cells. As the cells are charged, sulphuric acid is produced at a high
concentration adjacent to the plate surface and sinks to the base of the cell because it has a higher relative density than the rest of the electrolyte. If left uncorrected, this situation will lead to a non-uniform utilization of active material (with reduced capacity), aggravated local corrosion and, consequently, shortened cell-life.

Flooded cells are periodically set to produce gas during charging, which stirs the electrolyte and overcomes these problems. The immobilization of the electrolyte in a VRLA cell with an AGM separator reduces the tendency for acid stratification but also removes the possible remedy for the problem since gassing is not an option. A gelled electrolyte practically eliminates stratification effects because the molecules of acid immobilized in the gel are not free to move under the influence of gravity.

Leaks due to manufacturing defects

Improper design or workmanship may result in cover to pillar seal leaks. Cover to container seals may also leak. (Manufacturing defects). Missing or improper selection or malfunctioning of valves may also result in leaks of gases to the atmosphere. Non-closure after the opening of valves may result in accelerated dry out and capacity loss.
Mechanical damage may cause cells to leak leading to failure similar to pillar to cover leakage. Grid growth may produce cracks in the container. A slight acid film may form around the crack due to capillary action. If the acid film is in contact with uninsulated metal components, the ground-fault current could lead to thermal runaway or even fire [panasonic-batteries-vrla-for-professionals_interactive March 2017, p. 25].

Negative group bar corrosion

The group bar connection to the plate lugs may become corroded and possibly disconnected. The group bar alloy needs to be correctly specified and the connection between the group bar and the plate lugs needs to be carefully made, especially if this is a manual operation.

What should a 12 volt AGM battery read when fully charged?

While on charge and at or near the end of the charge, the terminal voltage (TV) may read 14.4 for a full charge.
The open-circuit voltage (OCV) will slowly decrease and will stabilise after about 48 hours at the rated OCV. Rated, in the sense that the OCV depends on the electrolyte specific gravity originally used.
OCV of the battery = 13.2V if the specific gravity used is 1.360. If the specific gravity is 1.300 the OCV will be 12.84V

Can you put an AGM battery in any car?

Yes. Provided, the capacities are the same and the battery box accommodates the new battery.
It is better to monitor the Terminal Voltage (TV) while being charged by the alternator for a few hours in a fully charged condition. The TV should not exceed14.4 V. Then it is ok to use that battery in that particular vehicle.
If it is a recent model new car the battery requires recalibration with a scan tool.

Why are AGM battery so expensive?

The AGM battery is more costly than flooded batteries but less costly than gel batteries.
The following reasons contribute to the higher cost:
i. Material purity.
(a) All materials that go into the AGM battery are costlier. The lead-calcium alloy is costlier than the conventional low antimony alloys. This alloy is preferably made from primary lead. The tin component in the positive grid alloy is the costliest item. Tin is added from 0.7 to 1.5 % in the positive grid alloy. The Indian market rate for Tin in May 2020 was Rs.1650 (LME 17545 USD per ton on 10-7-2020).
(b) The oxide is preferably made from 4Nines (99.99 %) primary lead, which adds to the cost.
(c) AGM is costlier.

(d) The acid for preparing the electrolyte and for other processes is purer than that used in conventional batteries.
(e) ABS plastic is more costly.
(f) The valves are to be checked for performance individually.
(g) The COS alloy is also costly
ii. Processing cost
(a) Special compression tools are employed for assembly of cells.
(b) An accurate and chilled acid filling is required
(c) The AGM battery are cycled a few times before shipping
(d) The assembly area must be kept free from dust to keep the self-discharge rate to a low level.
These are the causes for the higher cost of the AGM battery.

Is AGM battery better than lead acid flooded cells?

Yes.
i. The AGM battery is non-spillable. There is no requirement of topping up with water every now and then.
ii. They are more resistant to vibration. This is particularly useful applications like trailer-boats and where the roads are bumpy with several potholes.
iii. Because AGM batteries use pure alloys and pure materials, they perform batter with respect to self-discharge. These batteries can be left unattended for a longer time than flooded batteries.
iv. The AGM batteries can be located in a cooler part of the car (instead of fitting it in the hot engine compartment), thus reducing the operating temperature of the battery.

v. The maintenance cost of the AGM battery is lower and calculated over the entire life of the battery, the higher initial cost is off-set by this saving.
vi. The AGM battery can accept higher charging current because of their lower internal resistance)

Is a deep cycle battery an AGM battery?

All deep cycle batteries need not be AGM battery.
A deep cycle battery can be any type of battery like lead-acid or Li-ion or any other chemistry.

What is a deep cycle battery? A deep cycle battery can deliver every time about 80% of its rated capacity over its useful life. The battery requires that it be recharged every time after it is discharged.
Most of the people searching to buy batteries end up with an automotive lead-acid battery, because it is the cheapest available one. If a customer wants a battery for repetitive cycling, he has to search for a suitable battery meant for cyclic application.
An AGM battery with a label of “deep-cycle battery” is definitely a deep cycle battery. Such batteries invariably have thicker plates than the automotive batteries.

How many volts should a 12 volt battery read?

A 12-volt battery should read more than12V if it is in good condition.
The following table gives some values:

Sl No Battery type Open circuit voltage (V) Remarks
1 Automotive 12.40 to 12.60 Fully charged condition
2 Automotive 12 Fully discharged condition
3 AGM Batteries 13.0 to 13.2 Batteries with capacities ≤ 24Ah. Fully charged condition
4 AGM Batteries 12.7 to 12.8 Batteries with capacities ≥ 24Ah Fully charged condition
5 Gelled VR Batteries 12.7 to 12.8 Fully charged condition
6 AGM Batteries/Gelled batteries 12.0 Fully discharged conditions
7 Inverter batteries 12.4 to 12.6 Fully charged condition
8 Inverter batteries 12 Fully discharged condition
How far can you discharge an AGM battery?

As in the case of any other battery, a 12V AGM battery can be discharged down to 10.5V (1.75 V per cell) at low currents (up to 3-hour rate) and for higher rates of discharge down to 9.6V (1.6 V per cell). Further discharge will make the terminal voltage go down very fast. No meaningful energy can be obtained beyond these end voltage values.

How many volts should a fully charged AGM battery have?

A fully charged battery (under cyclic operation) will have a TV of 14.4 V (for 12 V batteries). After about 48 hours rest period, the TV will stabilise at 13.2 ± 0.5 V (if the specific gravity for initial filling was 1.360, usually for AGM battery having capacities £ 24 Ah) (1.360 + 0.84 = 2.20 per cell.  For a 12 V battery, OCV = 2.2 *6= 13.2 V). 

If the capacity of the battery is higher than 24 Ah, the specific gravity will be 1.300.  Hence the stabilised OCV will be 12.84 ± 0.5 V.

Float operated batteries will have float charging voltage of 2.25 to 2.3 V per cell (13.5 to 13.8 V for a 12 V battery). The stabilised voltage values will be as given above. Invariably it would be 12.84 ± 0.5 V.

Can an AGM battery explode?

Yes, some times.
There are no explosion hazards as the gassing is very limited. Even so, most of the VRLA Batteries have been provided with explosion-proof vents for protection against explosion in the event of user abuse
If the battery is abusively charged or if the charging component of an inverter/UPS is not properly functioning, the charging current will be driving the battery to thermal runaway conditions and the battery may explode.
If the terminals are shorted also (abusive use of a battery), the battery may explode. If there is a crack or improper joining of parts while lead burning (“cold welds”), this crack will be a cause of the fire and the battery may explode as a result.

The main cause for an explosion inside or near a battery is the creation of a “Spark”. A spark can cause an explosion if the hydrogen gas concentration in the battery or vicinity is about 2.5 to 4.0% by volume. The lower limit for the explosive mixture of hydrogen in air is 4.1%, but, for safety reason hydrogen should not exceed 2%. The upper limit is 74%. A heavy explosion occurs with violence when the mixture contains 2 parts of hydrogen to 1 of oxygen. This condition will prevail when a flooded battery is overcharged with vent plugs tightly screwed to the cover.

How do you charge AGM battery?

All VRLA batteries are to be charged by one of the two following methods:
a. Constant current-constant voltage method (CC-CV)
b. Constant voltage method (CV)
If the charging voltage by CV is 2.45 V per cell, the current (0.4C A) will remain constant for about one hour and then begins to decrease and stabilise at about 4 mA/ Ah after about for 5 hours. If the charge voltage is 2.3 V per cell the current (0.3C A) will remain constant for about two hours and then begins to decrease and stabilise at a few mA after about for 6 hours.

Likewise, the duration for which the current will remain constant depends on the initial current, such as 0.1C A, 0.2C A, 0.3C A and 0.4C A and also the charge voltage, such as 2.25 V, 2.30 V, 2.35, 2.40 Vans 2.45 V. The higher the initial current or voltage, the lesser will be the time of residence in that current level.
Also, the time for a full charge will be less if the current or voltage selected is higher.
The VRLA battery does not restrict the initial current; hence the higher initial current will shorten the time required for a full charge.

In CC charge the voltages are not usually controlled. Therefore the danger of cells remaining for an appreciable amount of time at high voltages is possible. Then gassing and grid corrosion can occur. On the other hand, the CC mode of charging ensures that all cells will be able to achieve full recharge on each cycle or during float charging. Overcharge is possible during CC charging. On the other hand, undercharging is the primary danger with CV modes

Pros & Cons of AGM Battery

Advantages & Disadvantages 

ADVANTAGES:

1 AGM battery are eminently suited for high power drains because of their low internal resistance and in places where the obnoxious fume and acid spray is prohibited.
2 AGM battery are non-spillable and require no addition of water periodically. They are therefore maintenance-free in this sense.
3 AGM battery can be used on their sides, except upside-down. This is an advantage in fitting it inside the appliance
4 AGM battery can be fitted anywhere in a car, not necessarily in the engine compartment.

5 AGM battery are highly resistant to vibration because of their method of manufacture using AGM and compression. Therefore it is excellently suited for sea-faring boats and in places where the road are notorious for potholes, ups and downs.
6 AGM battery have longer lives compared to flooded batteries. The plates are comparatively thicker. Thicker plates mean longer life. The user cannot tamper with the battery or its electrolyte and add impurities and thus cause premature failure.

7 Because the AGM battery are made with very pure materials in a clean atmosphere, the self-discharge rate is very low. The rate for AGM battery is 0.1 % per day while it is almost 10 times for a flooded battery. So, batteries meant for long time storage need refreshing charges less frequently. The loss is only 30 % after 12 months if stored at 25ºC and at 10ºC, it is only10 %.
8 Because of negligible stratification, lesser equalisation charges are needed.

9 The hydrogen gas evolution during float is reduced by a factor of 10 in the case of AGM battery. The ventilation of the battery room may be reduced by a factor of 5 according to the safety standard EN 50 272-2.
10 No acid protection of the floor and other surfaces in the battery room is required.

DISADVANTAGES:

1. The disadvantages are a minimum. The cost of the battery is comparatively higher.
2. If it is abusively charged or if the charger is not functioning properly, the battery may bulge, burst or sometimes explode.
3. In the case of SPV applications, AGM battery are not 100 % efficient. A part of the energy is lost in the charge-discharge process. They are 80-85 % efficient. We can explain this in the following lines: Consider that am SPV panel produces 1000 Wh of energy, the AGM battery would be able to store 850Wh only due to the inefficiency mentioned above.

4. Oxygen ingress through leakages in the container, lid or pole bushing discharges the negative plate.
5. The polarization of the negative plate is reduced due to oxygen recombination on the negative plate. In improper cell designs, the negative polarisation is lost and the negative plate discharges, although the float voltage is above open-circuit.
6. To avoid drying out, the maximum operating temperature is reduced from 55°C to 45°C.
7. VRLA cells do not allow the same inspection possibilities such as acid density measurements and visual inspection, so the awareness of a full functioning battery is reduced

Do AGM battery require maintenance?

No. But, they require a refreshing charge if kept unused. The batteries can be kept idle for a maximum of 10 to 12 months at normal temperatures. At lower temperatures, the loss will be far less.

How do you maintain an AGM battery?

Normally, there is no need for maintenance of AGM battery. Although the VRLAB manufacturers state that there is no need for equalizing charge during float charge operation, to get a higher life from the battery, it is better to bench charge the batteries once in 6 months (batteries older than 2 years) or 12 months (new batteries). This is to equalise all the cells and bring them to the same State-of-charge (SOC).

Do you need to charge a new AGM battery?

Generally, all the batteries lose capacity due to self-discharge during storage and transportation. Therefore it is advisable to give refreshing charge for a few hours depending on the time elapsed between the date of manufacture and installation/commissioning. The 2 V cells can be charged at 2.3 to 2.4 V per cell until the terminal voltage reads the set values and maintain it at this level for 2 hours.

Are AGM batteries safer?

AGM battery (and gel batteries) are far safer than flooded batteries. They are unspillable and do not emit hydrogen gas (if properly charged following the manufacturer’s instructions). If any regular or normal charger is used for charging AGM battery, care should be exercised not to allow the temperature to go to more than 50ºC and the terminal voltage beyond 14.4 V (for a 12V battery).

What is float voltage for AGM battery?

Most of the manufacturers specify 2.25 to 2.30 V per cell with temperature compensation of – 3 mV/cell (reference point is 25ºC).
For cyclic batteries, the charging voltage in CV mode is 2.40 to 2.45 per cell (14.4 to 14.7 V for 12V batteries).
At a typical float-charge voltage of 2.25 V per cell, VRLA battery has a float current of 45 mA per 100 Ah due to the effect of the oxygen cycle, with an equivalent energy input of 101.3 mW (2.25*45). In the equivalent flooded battery, the float current is 14 mA per 100 Ah, which corresponds to an energy input of 31.5 mW (2.25V*14 mA).

Thus the VRLA float current is more than three times Credits: [R.F. Nelson in Rand, D.A.J; Moseley, P.T; Garche. J ; Parker, C.D.(Eds.) Valve-Regulated Lead- Acid Batteries, Elsevier, New York, 2004, pp. 258].

Can I use a trickle charger on an AGM battery?

Yes. What is a trickle charge? It is the method of giving a continuous charge using a small current. This is to compensate for the self-discharge in the AGM battery when it is not connected to any load.

This was an unexpected long article!!

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