Acid filling

Acid filling lead acid battery

Filling lead acid battery

For the user or dealer, there are 2 types of batteries which need to be Acid-filled and first charged.
1. Dry and uncharged
2. Dry-charged
1. Dry and uncharged:

Filling lead acid battery - Dry & uncharged

The positive plates are already charged and negative plates are in a partially charged condition. On initial filling, strictly follow the procedure given by the battery manufacturer.                             

Every type of battery will have a stipulated final specific gravity after charge. Let us say this is 1.250. The battery manufacturer will be advising you to fill about 30 points less than this value for initial filling, say 1.210 or 1.200. The objective is to get a specific gravity of 1.250 after initial charging. Fill to the maximum level marked on the container. You will find that the temperature increases by 5-10 degrees C. Allow to soak and cool.

CHARGING: Connect to the charger. Set the lowest current setting on the charger. After one hour, increase the current to 5-10 % of the rated Ah of the battery. Charge for 20 hrs. Check specific gravity using a hydrometer. Measure the sp gr and voltage every one hour. Gassing starts. Allow the gases to escape by opening the vents. When there is no further change in specific gravity disconnect the charger.
If the final specific gravity is below 1.250 add acid of higher gravity (1. 4) and adjust to 1.250. Add water if the specific gravity is higher to get 1.250. Adjustment is to be done in all the cells

Put back the vent plugs and wash the top with water and wipe it clean. No acid should be present on top of the battery cover. Allow to dry. The battery is now ready for use.

Filling lead acid battery - Dry charged battery

As the name implies, a dry-charged battery is already charged. And needs only a short booster charge to be ready for fitment.

Fill the battery with acid of specific gravity 1.240 -1,245. Measure the temperature before and after filling and note the difference. If the temperature difference is only 3-4 degrees C, charge at 10 % current (of rated Ah) for 2 hrs. If the temperature difference is more than 5-8 degree C, charge for 5 hrs or more. Measure the voltage and specific gravity for 3 consecutive hours. Stop when the voltage and specific gravity are equal for 3 consecutive hours.

  • Green Plate battery charging
  • Dry and uncharged
  • Dry-charged

Assembly of Pasted, cured and dry positive and negative battery plates is called green plate battery. The battery plates will have the composition as given below

  • Unconverted Lead oxide (major constituent- over 90-94 %)
  • Tribasic and Tetrabasic lead sulphate – 4-5  %
  • Other lead compounds like hydroxides, carbonate etc – 1-2 %
  • Free Lead < 1-2%

Thus the major constituent is lead oxide. It reacts with sulphuric acid to form lead sulphate (neutralisation), generating a considerable amount of heat. Since the volume of sulphuric acid in each cell is limited, the increase in temperature is very pronounced. There is a corresponding drop in specific gravity of acid as well.

During initial charging, the lead sulphates formed from PbO (and 3BS and 4BS) have to be converted to the respective active materials during charging. The energy input and time for formation are the highest. The low specific gravity at start of charging helps in good charge acceptance. This is a good feature since solubility of lead sulphate increases at low specific gravity and reactions are faster.

Filling lead acid battery - Steps to be taken for Green Plate battery charging:

  • The acid should be chilled to 5-8 ⁰C to partially offset the high temperature caused by the reactions to form lead Sulphate.
  • Choose a specific gravity for initial filling 40-45 points less than the final specific gravity required.
  • After filling, the temperature will gradually increase
  • Cool by circulating water up to ¾th level of container height. Use an industrial fan for faster cooling
  • Connect to the charger after about 3 hours of soaking/cooling.

Use stepped current method of Pavlov– 2% of rated capacity for 1st hour, 4 % for the next 1 hr and 8% for the next 1 hour
After 3 hrs, a higher current not more than 14% of capacity can be used
Discontinue charging when the temperature of electrolyte rises to 48 ⁰ C. Temperature up to 55 ⁰ C is acceptable for 10 minutes
Continue charging at constant current till 400 % of Ah capacity is put in.
At this point, a discharge at C10 or C20 is recommended for 2-3 hrs for inverter batteries. For automotive batteries, this step is not essential. Inverter battery customers’ expect full capacity at the start. Hence the discharge step for inverter batteries.

In the dry and uncharged battery, the positive plates are already converted with PbO2 content of over 85-90 %. Only a short charge is adequate. On the other hand, the negative plates which were fully formed in open formation are oxidised on exposure to air.  About 50 % of the spongy lead is converted to lead oxide. This oxide reacts with acid used in initial filling.  The heat generated is only a quarter of the green plate. So also the energy required for conversion to active materials is less.

For the initial filling, use a specific gravity less by 30-35 points from the final designed specific gravity.


In dry-charged batteries, as the name implies both positive and negative plates are already charged. Only a short boost charge is required before using as a flooded battery.

This system is nowadays used for VRLA batteries since VRLA uses a high Specific Gravity acid and volume of acid filled is low. At high specific gravity, the solubility of lead-sulphate is low and conversion is correspondingly slow. Therefore it is always advisable to use fully formed positive plates and dry-charged negative plates to get good conversion of active materials and get full Ah capacity in the battery.

Filling lead acid battery

For filling VRLA batteries, use a specific gravity only 5 points less the target of 1.300. Since there is practically no PbO in both plates, there is less chemical reaction and no heat generation.

  The filling of VRLA batteries will take a longer time as it uses vacuum filling procedure. A finite time period of 10 – 30 minutes is required for the battery plates to get fully wetted. During this period, the negative plates will get oxidised partially. This calls for a longer period of charging.

Use the consecutive hourly voltage readings as a guide to decide completion of charge.

what is c rate in battery

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