Equalizing Charge Microtex
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Equalizing charge in lead acid battery

The intention of equalizing charge is to bring the on-charge voltage of a lead-acid battery to gassing levels so that all the unconverted lead sulphate is charged to lead and lead dioxide, respectively, in NAM and PAM.

Equalizing charge: Equalizing Batteries

Proper maintenance of lead acid batteries helps in improving the life of the battery. Equalizing charge is one of the most important aspects of this maintenance procedure. 

Equalizing charge definition

For such type of batteries, the intention of equalizing charge is to bring the on-charge voltage of a 12V battery to gassing levels so that all the unconverted lead sulphate is charged to lead and lead dioxide, respectively, in NAM and PAM. When there is free and copious gassing, all the uncharged sulphate ions go into the electrolyte and raise the acid density.

Vinal in his classic book gives the relationship of the voltage of the cells and gassing levels.

Gassing levels and cell voltages on charge of flooded cells

(Credits: Vinal, G.W., Storage Batteries, John Wiley & Sons, New York, 1954, page 262)

Cell Voltage (V) Level of gassing Composition of the gases evolved H2 Percent Composition of the gases evolved O2 Percent
2.2 No gassing - -
2.3 Slight 52 47
2.4 Normal 60 38
2.5 Copious 67 33

Similarly, batteries not properly initial-charged in the factory requires further equalizing charge. This can be evidenced by an increase in the specific gravity of the electrolyte within a few months of commissioning the battery, for example, an inverter battery. Normally, the specific gravity value will be 1.240 before shipping. Once this value is attained, some manufacturers stop charging and assume that the battery has been fully charged.

Actually, had they continued the initial charge further, they could have seen a substantial rise in the specific gravity. This aspect of initial charging indicates the presence of uncharged lead sulfate in the plates. This amount of lead sulphate helped increase the specific gravity of the electrolyte in the process of further charging.

How does equalizing charge help?

The equalizing charge helps in realising the designed life of the battery, avoiding premature failure due to insufficient charging.  A battery receiving regular equalizing charge will live longer than one which does not.  This is particularly true in the case of forklift battery, automotive battery and inverter batteries. We have seen that giving an equalize charge forklift battery ensures better performance of the forklift battery. Extension of battery life via charge equalization control is the established way to better battery performance.

In some countries, the UPS and stationary power supply batteries do not experience power outings even for a few minutes in a year.  In such situations, the manufacturers of batteries advise the consumer to switch off the mains supply for a few minutes.  This will avoid “float passivation”. 

What is an equalizing charge for a battery

All aspects discussed above apply to the VR batteries also. The only difference is that the charging voltage for the equalizing charge is lower. The batteries are to be charged to not more than 14.4 V (for a 12V battery) during an equalizing charge. The gassing rates are:

Gassing levels and cell float voltages on charge of Valve Regulated cells

Equalizing charge voltage

Cell voltage (V) Level of gassing Recombination efficiency (%) Gassing rate * Relative gassing rate
2.25 to 2.3 Negligible gassing ~ 99.87 ~ 0.0185 ~ 1
2.4 Some gassing ~ 99.74 ~ 0.037 ~ 2
2.5 Gassing ~ 97.4 ~ 0.37 ~ 20

*cc/h/Ah/cell from: Credits: C&D Technologies: Technical Bulletin 41-6739, 2012.). 1 cubic foot = 28317 cc (= (12*2.54)3 = 28316.85)

Equalizing charge - How do VRLA batteries differ from flooded lead acid batteries?

The basic chemistry of the two versions of lead acid battery is the same. Discharge reactions are similar, but the charging reactions differ in their intermediate steps.

The gases (hydrogen and oxygen) evolved near the end of charge in a flooded lead acid battery are vented out. Te oxygen gas evolved on the positive plate of VR cells easily move to the negative plate and oxidise the lead, because of higher diffusion coefficients in a gaseous medium. This is a fast reaction in VR cells. Such movement of gases is not possible in flooded cells because of the lower diffusion coefficients. Conditions similar to flooded cells will occur in VR cells also if the AGM is fully saturated and the oxygen recombination reaction will begin only when starved electrolyte condition begins to develop due to water electrolysis and loss of some water.

In a Valve Regulated cell the hydrogen evolution is inhibited by the formation of lead sulphate during charge. This lead sulphate takes the potential of the negative plate to more positive values so that hydrogen evolution is very much reduced. Special alloys are also used in the negative grid which will have higher hydrogen overvoltage.

Equalizing charge: Construction-wise VRLA batteries have the following differences:

  • The electrolyte volume is less in VRLA battery. This is intentionally kept so because there should be a passage for the oxygen evolved from the PAM to contact NAM via unsaturated pores in the absorptive glass mat (AGM) separator. To compensate for the reduced volume of the electrolyte, higher density acid is used in VR batteries. This will also compensate for the reduced low rate capacities.
  • The elements are highly compressed in VRLA battery. This aspect plays the most important role in enhancing the life of the batteries. The plate-separator-container wall compression is an integral part of the design. This ensures good electrolyte diffusion between plates and separator. The life is also increased because of the reduction in positive active material expansion and the resultant capacity loss.

  • VRLA batteries have a one-way resealing valve in each cell or there may be a common valve for a few cells (particularly in small capacity cells). This multi-functional valve works in the following manner:
    i. Prevents accidental ingress of atmospheric air (oxygen).
    ii. Helps in pressure-assisted oxygen transport from PAM to NAM
    iii. Prevents explosion in the event of undue pressure development inside the battery due to abusive charging or malfunctioning of the charger.
  • Proper functioning of VRLA batteries depends on the internal oxygen cycle, which in turn depends on a leak-proof construction: the lid to cover seal and the pot to cover seal. The internal oxygen cycle helps in reducing the hydrogen evolution and thus reduces water loss.

Internal Oxygen cycle

During the charging of VRLA battery:
At the positive plate, O2 gas is evolved and protons and electrons are produced.
2H2O → 4H+ + O2 ↑ + 4e- ……… Eq. 1

The oxygen gas, hydrogen ions and electrons evolved as a result of electrolysis of water on the positive plate pass through empty pores, gas-filled pores and electrolyte channels in the AGM separator (or the fine cracks in the gelled electrolyte matrix in the case of gelled VR batteries) and reach the negative plates. This gas combines with the lead in the NAM to become PbO and the reduced oxygen combines with the hydrogen ions to form water.  This oxide combines chemically with the sulphate ions to form lead sulphate

2Pb + O2 → 2PbO
2PbO + 2H2SO4 → 2PbSO4 + 2H2O
2Pb + O2 + 2H2SO4 → 2PbSO4 + 2H2O + Heat ……… Eq. 2
But, this being a charging process, the lead sulphate thus produced again has to be converted to lead; sulphuric acid is generated by an electrochemical route by reacting with the protons (hydrogen ions) and electrons resulting from the decomposition of water at the positive plates when they are charged.

2PbSO4 + 4H+ + 4e → 2Pb + 2H2SO4         ……… Eq. 3

When the NAM gets converted to PbSO4 during a charge, the potential of the negative plate becomes more positive (as in the case of a discharge).  This helps in hindering the hydrogen evolution reaction.  Very small amounts of hydrogen gas are produced, but the one-way valve ensures that the pressure inside the jar does not reach dangerous levels by venting the hydrogen to the atmosphere, thus protecting the battery from bulging and other defects.   


The last reaction restores the chemical balance of the cell. The net sum of the reactions (Eq 1) to (Eq 3) being zero, the electrical energy spent during charge is converted into heat rather than to chemical energy [Ref R.F. Nelson, Proc. 4th Int Lead Acid Battery Seminar, 25-27 Apr 1990, San Francisco, USA, ILZRO, Inc.,1990, pp.31-60].

Lead acid cell - discharge reactions explained
Equalizing Charge
Recombination reaction in a VRLA cell

The most important advantage of a VRLA cell is no addition of water is required as a maintenance procedure. The next advantage is that it evolves negligible amount of gases in the course of its operation, because of near 100 % recombination at the recommended float voltages of 2.25 to 2.3 V per cell. Moreover, there are no transport restrictions in moving these batteries from place to place.

Primary and Rechargeable batteries

A battery is defined as an electrochemical device that can convert chemical energy into electrical energy through redox reactions and thus act as an electrochemical power source. But, it is not a perennial source of power. The battery will supply power only until there are sufficient active materials to sustain the energy-producing reactions. Once the voltage level of the battery attains a certain lower level defined by the chemistry of the system, the reactions have to be reversed, i.e., the battery has to receive direct current. This act of supplying a direct current in the reverse direction of the discharge to a discharged battery to reverse the discharge reactions is called “charging”.

This will regenerate the original active materials from the discharge products and will also increase the battery voltage to higher values, again defined by the chemistry of the system. This statement is applicable to batteries which are called secondary or storage batteries. It is not relevant to primary cells such as those used in electric torches and wristwatches. The lowering of the battery voltage during a discharge occurs due to depletion of the active materials and several other reasons.

An independent unit of battery is called “cell”. A battery is a combination of two or more cells connected in several different fashions to attain the designed voltage and capacity ratings or total kWh rating. Most commonly, a monobloc battery is employed in automobiles and small capacity valve-regulated lead acid battery (VRLA) and tubular batteries (up to 12V/200 Ah); beyond this capacity single cells are used for getting the required kWh ratings by combining them in series or series-parallel arrangements.

 A lead acid battery of 48V/1500 Ah (or 72 kWh) rating may have 24 numbers of 2V/1500 Ah capacity cells connected in a simple series manner or 48 cells of 2V/750 Ah capacity cells connected in a series-parallel way. That is 24 cells connected in series to make 48V/750Ah (or 36 kWh) battery. Another such 48V/750 battery will be connected in parallel to the first one to make it 48V/1500 Ah (72 kWh) battery.

Another example from a lithium-ion (Li-ion) electric vehicle (EV) battery:
Depending on the battery-pack size, EV maker Tesla uses about 6,000-8,000 cells per pack, each cell being of 3.6V/3.1 to 3.4 Ah capacity to build a 70 or 90 kWh battery pack.

70 kWh Tesla EV battery uses about 6000 cells of the type 18650 NCA cells of 3.7 V/3.4 Ah, connected in a complicated series-parallel arrangement. It has a range of 325 km per charge. (Here the figure 18650 refers to a particular type of Li-ion cell having approximate dimensions of length (or height) 65 mm and diameter of 18 mm. The term “NCA” stands for the cathode material used in this cell, e.g., N =Nickel, C= Cobalt and A = aluminium, that is nickel-cobalt-aluminium oxide cathode material)
90kWh pack has 7,616 cells in 16modules. The weight is 540 kg. It has a range of 426 km per charge.

Components of a battery cell:

The most essential components of a battery are:
a. Anode (Negative plate)
b. Cathode (Positive plate)
c. Electrolyte (In lead acid battery, the electrolyte is also an active material, but not so in most other systems)
The above three are called active components
Of course, there are inactive components like
a. Jar
b. Current collecting grids
c. Bus bar or connector straps
d. Separators
e. Inter-cell connectors
f. Terminal posts, etc

In a lead-acid battery, the electrolyte (dilute sulphuric acid) does take part in the energy-producing reaction as can be seen from the cell reaction given below. The sulphuric acid is consumed to convert the lead dioxide and lead to lead sulphate and so the density of the electrolyte decreases as the discharge reaction proceeds. On the contrary, when the cell is charged, the density of the electrolyte rises as the charge reaction proceeds. The reason is that the sulphate ions absorbed by both the active materials during discharge are released in the electrolyte and so the density of the electrolyte increases.

Discharge and charge reactions

The reactions of a galvanic cell or battery are specific to the system or the chemistry:

For example, the lead acid cell:

Pb + PbO2 + 2H2SO4   Discharge ↔ Charge  2PbSO4 + 2H2O    E° = 2.04 V

In a Ni-Cd cell

Cd  +  2NiOOH  + 2H2O Discharge ↔ Charge  Cd(OH)2 + 2Ni(OH)2    E° = 1.32 V

In a Zn-Cl2 cell:

Zn + Cl2 Discharge ↔ Charge   ZnCl2                                           E° = 2.12 V

In a Daniel cell (This is a primary cell; here note the absence of reversible arrows)

Zn + Cu2+ Discharge ↔ Charge Zn2+ + Cu(s)                                              E° = 1.1 V

Equalizing charge voltage: more on charging of a battery

As described above, a storage battery is not a perennial source of power. Once it is exhausted, it has to be recharged to get power from it again. The batteries are expected to give a certain lifetime, called life expectancy. To obtain the designed life and reliability, the storage batteries are to be properly charged and maintained as per the instructions provided by the manufacturers. Proper charging methods should be used to obtain maximum possible life out of the battery.

Reactions in lead acid cell:

During discharge: PbO2 + Pb + 2H2SO4     2PbSO4 + 2H2O

The discharge will proceed only till certain amounts of conducting materials are there in the cell; thereafter the rate of drop in voltage will be so fast that the end voltage will be reached soon.  So there is what is called a cut-off voltage or end voltage, beyond which discharge should not be continued.  Further discharge will make the recharge difficult and may lead to unexpected catastrophic results. 

The batteries are to be charged immediately after discharge at the rates recommended by the manufacturer or as per the instruction supplied by them. 

Recombination reaction in an AGM Battery
Recombination reaction in a VRLA cell

What happens during a discharge and charge reactions inside a cell?

Electrolyte:           2H2SO4  = 2H+ + 2HSO4‾  

Negative plate:      Pb° = Pb2+ HSO4 + 2e

                                 Pb2+ + HSO4‾ = PbSO4 ↓  + H+

            ⇑                                                ⇓

Positive plate:       PbO2  = Pb4+ + 2O2-

                                Pb4+ + 2e = Pb2+

                                Pb2++ 3H+ + HSO4‾ +2O2- =PbSO4 ¯ ↓+ 2H2O

Sulphuric acid being a strong electrolyte, it is dissociated as hydrogen ions and bisulphate ions (also called hydrogen sulphate ion). 


On starting a discharge, the porous lead in the negative plate gets oxidized to lead ions (Pb2+) and because it is always in contact with the electrolyte sulphuric acid, it gets converted to lead sulphate (PbSO4); the latter gets deposited as a white material on the pores, surface and cracks of the negative plates. The former reaction (the lead becoming lead ions) is electrochemical in nature while the latter (lead ions becoming lead sulphate) is a chemical reaction.

We say that lead dissolves as lead ions in the vicinity of the reaction site and immediately deposits as lead sulphate after combining with the bisulphate ions from the electrolyte on the negative active material (NAM). Such type of reaction is called a dissolution-deposition or dissolution-precipitation mechanism in electrochemistry.
Similarly, the positive active material (PAM) combines with the electrons coming from the NAM and becomes lead ions, which combines with the bisulphate ions from the electrolyte and deposits as lead sulphate on the positive active material, following the same dissolution-deposition mechanism.

During a recharge: 2PbSO4 + 2H2O Charge→ PbO2 + Pb + 2H2SO4

The reaction products obtained during discharge on the positive and negative plates are converted back to the original materials during a charge. Here, the reactions have reverse designations to those of a discharge. The positive plate undergoes oxidation, while the opposite polarity plate undergoes reduction.

Equalizing charge: When is the Full charge complete

The batteries are assumed to have completed the normal recharging if the following conditions are met:

Parameter Flooded Lead acid Battery Valve Regulated Lead Acid Battery (VRLA)
Charging voltage & current A constant current charge is assumed here: the voltage of a battery at the end of a charge should be constant for a particular current. The value may be 16.2 to 16.5v for a a 12v battery For a constant impressed voltage (say 13.8v to 14.4v for a 12v battery), the current should be constant for at least two hours
Specific gravity of electrolyte Specific gravity of electrolyte should also reach constant value. This value will depend on the fully charged battery when it was supplied by the manufacturer. Specific gravity of electrolyte cannot be measured.
Nature of gassing Uniform and copious gassing on both the plates. The volume of gases evolved will be 1:2 as in water, i.e., 2 volumes of hydrogen for 1 volume of oxygen. At the levels of charging voltage recommended for VRLABs, negligible gassing is observed. At 2.25 to 2.3 volts per cell (Vpc) float charge, no gas evolution is observed. At 2.3 Vpc, a 12V 100Ah VRLAB may emit 8 to 11 ml/h/12V battery. But at 2.4 Vpc it is almost double, 18 to 21 ml/h/12V battery. (i. pbq VRLA Batteries, January, 2010. ii. C&D Technologies: Technical Bulletin 41-6739, 2012.)

Equalizing Charge: what is an equalizing charge for a battery

  • A newly assembled lead acid battery requires initial filling and initial charging.
  • A discharged battery requires a normal recharge.
  • Batteries connected to appliances and equipment are normally not fully charged, in the sense they do not reach the full charge voltage of > 16 V for a 12V battery. For example, in the (starting, lighting and ignition) SLI application in automobiles, the maximum voltage that the battery can attain is about 14.4 V for a 12V battery. Similarly, the charging voltages of inverter/UPS battery do not go beyond 13.8 to 14.4 V. In such applications, the process of accumulation of unconverted lead sulphate in both positive and negative plates goes on increasing as the life of the battery increases.

The reason is that the values of voltages referred above are not sufficient to restore all the discharged products to the original active materials. Such batteries require periodic recharge to bring all the cells to full charge and to the same level. This will also help in removing the effects of stratification of the electrolyte. Such extra-appliance charging is called a bench charge or equalizing charge.

Conclusions on Equalizing charge:

The equalizing charge is a part of the maintenance procedure. The maximum voltage at which the equalizing charge can be carried out depends on the type of the lead acid battery, whether it is of flooded type or VRLA type. The former type of cells can be charged at constant current to a voltage of 16.5 V for a 12V battery to bring all the cells in a battery to the same level.

However, the VRLA cells must be charged only by constant voltage method and this impressed voltage should not exceed the recommended maximum voltage of 14.4 V for a 12V battery. Where the facility of constant voltage charging is not available, the VRLA batteries can be charged at constant current with constant monitoring of the terminal voltage (TV) of the battery. Whenever the TV nears or exceeds the 14.4 V level, the charging current should be continuously reduced so that the TV is not allowed to go beyond 14.4 V

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