acid used in battery

Acid used in Battery

Ultimate guide to Acid used in battery

The term battery acid usually refers to sulphuric acid for lead-acid batteries. Sulphuric acid is the aqueous acid electrolyte used in lead acid batteries. Sulphuric acid is diluted with chemically clean & pure water (demineralized water) to obtain about 37% concentration by weight of acid. The concentration of acid differs from battery manufacturer to manufacturer. The lead acid battery use a combination of positive & negative electrodes placed inside a plastic compartment using a medium of the electrolyte as a transport mechanism for the electronic movement of ions generated in the electrochemical reactions that take place inside the cell.

How many different battery acids are there? Which of the following acids are used in a battery?

Battery acids are aqueous electrolytes generally and are those salts, acids or alkalis that can dissolve in water to form Acid electrolytes Alkaline electrolytes & neutral electrolytes. Acid electrolytes include sulphuric acid, perchloric acid, hydrofluosilicic acid etc. Sodium chloride is a neutral electrolyte.

Purchasing battery acid

Battery acid is not an item you can purchase in a normal store. You would need to purchase battery acid from an authorized chemical dealer or from a battery acid supplier. Buying from a battery acid supplier will ensure you get the correct specific gravity as required for small quantities.

What is DM water?

Demineralized water or DM water is almost equivalent to distilled water and has no dissolved ions. All dissolved minerals (salts) like calcium and magnesium carbonates, bicarbonates, salts of iron and other dissolved impurities are removed by Ion Exchanger. Both Cations (Positive metallic ions) and Anions (negative ions) are removed by  the resins used, both double – bed and single bed resins are available.  The conductivity of the water is continuously monitored. Time of regeneration is indicated by a higher conductivity. This is a signal for regeneration after the designed capacity of say, 10,000 litres are treated. The resins have a designed life and the resins need replacement after 3-5 yrs

Guide to make battery acid to correct specific gravity:

Electrolyte is a mixture of concentrated sulphuric acid (Specific Gravity about 1.840) and distilled/demineralized water (Specific Gravity about 1.000). Acid and water are combined, by adding the acid to the water, never the reverse, until the required density is secured.

Do not add water to acid – Only add Acid to water.

Different specific gravity sulphuric acid is used in lead acid batteries. The common working specific gravities of sulphuric acid corrected at 27 deg C for different types of batteries are given below:

Battery Application Specific Gravity Typical Range
Automotive Batteries 1.270 - 1.290
Traction Batteries 1.275 - 1.285
Stationary Batteries 1.195 - 1.205
AGM VRLA Batteries 1.300 - 1.310
Tubular Gel VRLA Batteries 1.280 - 1.290
SMF Monobloc Batteries 1.280 - 1.300

Battery acid preparation

Caution: When working with acid or electrolyte always use protective goggles, rubber gloves and rubber apron.

  1. Cleaned vessels of Hard Rubber/Plastic, Porcelain or Lead lined boxes are to be used.
  2. The acid to be used in the battery for initial filling, is of battery grade specific gravity as mentioned in the Manufacturer data sheet.
  3. If the acid is obtained in concentrated form it is necessary to dilute it to required Specific Gravity. The acid and the distilled water to be used for diluting should conform to IS: 266-1977 and IS: 1069-1964 respectively.
  4. Remember, NEVER POUR WATER TO ACID, ALWAYS ADD ACID TO WATER. For diluting, use only glass rod/lead lined paddle for mixing.
  5. Mixing of Electrolyte

Specification of water and acid for use in lead-acid battery

Elements - permissible limits Water Acid
Suspended matter Nil Nil
Iron 0.10 ppm 10 ppm
Chlorine 1 ppm 3 ppm
Manganese 0.10 ppm Nil
Total dissolved solids 2 ppm Nil
Electrical Conductivity micro ohms / cm 5 max not applicable

Measuring specific gravity of sulphuric acid and the correction for temperature: The acid gravity is read by the hydrometer and the temperature is read by a mercury-in-glass type thermometer. Avoid parallax error by keeping the acid level in the hydrometer in the same level of the eye. The correction is done by adding 0.0007 in case of acid being at a temperature higher than the reference temperature and subtracting 0.0007 in case the acid is at lower temperature than the reference temperature for each deg C. Suppose we measure a batch of acid as 1.250 at 40 deg C, the corrected specific gravity at 30 deg C for that batch of acid will be – 1.250 + (40-30) X 0.0007 = 1.257.

So, the Generalised formula is

  • S.G.(30 deg C) = S.G.(t deg C) +0.0007 ( t – 30 )
  • Where, t is the temperature of the electrolyte; S.G. (30 deg C) = Specific gravity at 30 deg C;  S.G. (t deg C) = specific gravity measured at t deg C.    

To make 10 litres of dilute battery acid from concentrated Sulphuric acid 1.840 Sp Gr

To Achieve Specific Gravity After Mixing Quantity of water in Litres Quantity of 1.840 Specific Gravity Acid in Litres
1.200 8.67 1.87
1.240 8.16 2.36
1.260 8.33 2.50
1.190 8.7 1.80

Diluting Concentrated Sulphuric Acid of Density 1.835 Sp Gr.

To Achieve Specific Gravity when cooled Quantity of water in Litres Quantity of 1.835 Sp Gr Sulphuric Acid in Litres
1.400 1690 1000
1.375 1780 1000
1.350 1975 1000
1.300 2520 1000
1.250 2260 1000
1.230 3670 1000
1.225 3800 1000
1.220 3910 1000
1.210 4150 1000
1.200 4430 1000
1.180 5050 1000
1.150 6230 1000

Diluting Sulphuric Acid of Density 1.400 Sp. Gr. to get lower specific gravity

To Achieve Specific Gravity when cooled Quantity of water in Litres Quantity of 1.400 Sp Gr Sulphuric Acid in Litres
1.400 nil 1000
1.375 75 1000
1.350 160 1000
1.300 380 1000
1.250 700 1000
1.230 850 1000
1.225 905 1000
1.220 960 1000
1.210 1050 1000
1.200 1160 1000
1.180 1380 1000
1.150 1920 1000

Specific gravity of acid in different types of batteries

the Specific gravity of a fully charged cell in a lead-acid battery varies from 1.200-1.320. When a lower specific  gr of 1.200 is used, a larger volume is used per Ah per cell. For example:

  • Stationary cells Sp gr 1.200 has about 18-20 ml acid per Ah per cell
  • UPS batteries have sp gr of 1. 240-1.250 and use 14 to 16 ml acid per acid per cell
  • Traction batteries sp gr 1.250-1.260 use 13-15 ml acid per Ah per cell
  • Automotive  batteries sp gr.   1.260-1.270 use 12-13 ml acid per Ah per cell
  • VRLA batteries sp gr 1.3-1.32 use 9 ml of acid per Ah per cell
  • VRLA gel use same sp gr. of 1.300 use 10-11 ml acid per Ah per cell

This shows that the mass of sulphuric acid used per Ah per cell is almost the same for all batteries. It also shows that Volume of acid used multiplied by concentration of acid in wt % is the same for all batteries. This can be verified by calculations using the following table:

Specific Gravity @ 20 oC
Temperature coefficient per oC H2SO4 Weight % H2SO4 Vol % Freezing Point oC
Water 0.0 0.0 0
1.020 0.022 2.9 1.6 -
1.050 0.033 7.3 4.2 -3.3
1.100 0.048 14.3 8.5 -7.8
1.150 0.060 20.9 13 -15
1.200 0.068 27.2 17.1 -17
1.250 0.072 33.4 22.6 -52
1.300 0.075 39.1 27.6 -71

Freezing of battery acid

The table gives the freezing point of electrolyte at different sp.gr. when battery is used in colder climates. If the acid freezes, the ice formed expands and the container may crack. The table helps us to identify the safe temperatures the battery can withstand.

Caution: It is necessary to ensure that the battery is kept in a charged condition in winter in the cold regions. If kept in a discharged condition, the acid may freeze and break the container

It needs to be emphasized that Lead-acid has the widest temperature range in which it can work unlike other competitive technologies which have narrow ranges. Although the performance at low temperature is not up to the desired level, stipulating performance criterion like CCA (Cold  Cranking Amperes) mitigates this issue.

Wrong gravity of acid while charging

I used the wrong gravity of acid for initial filling & the battery charging was done for a short period. Now the battery is not having capacity – what should I do to recover this battery?

There is no standard procedure to revive the battery in such situations, however you can attempt to recover the battery using the following procedure:

If the specific gravity used was lower than the usual standard gravity, dump the acid following all safety & environmental norms. Fill with correct grade battery acid and charge the usual way. It will accept charge and can be fully charged. An adjustment of final specific gravity will be necessary for all cells.

If the specific gravity used was higher, the same procedure can be used. Adjusting specific gravity at end of charge could be tedious. One or two batteries can be handled in this manner. Obviously handling larger quantity is going to be a serious challenge. Always take care that you are filling the correct specific gravity at the time of initial charge.

Do get in touch with us, if you have any questions on battery acid.

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