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Measuring the density and specific gravity of battery acid in lead acid batteries

What is battery acid?

The term “battery acid” refers to the electrolyte used in batteries. For lead acid batteries this is sulfuric acid (H2SO4). Sulfuric acid is colorless, odorless, and strongly acidic. 

Why measure the density / specific gravity of battery acid?

Knowing the specific gravity of the electrolyte in batteries gives insight into the level of charge. Due to chemical reactions during discharge, the density of the sulfuric acid electrolyte (or its specific gravity) decreases. Measuring the density of the battery acid therefore gives information about the concentration of H2SO4 and the charging status of the battery. Depending on the result, the operator knows whether the battery needs maintenance or needs to be exchanged. To detect and maintain the weakest cell(s) of the battery, a regular density check is mandatory. 

Battery acid measuring methods

To check the specific gravity of the electrolyte, it is possible to use a hydrometer (also called an “aerometer”) or a digital density meter (also called a “digital hydrometer”).

Using a hydrometer

A lead acid battery hydrometer is a special type of hydrometer which looks like a syringe with a bulb. Inside the bulb there is a float which is calibrated for measuring the Specific Gravity (SG). To use the hydrometer, you suck some of the battery acid (H2SO4) out of the battery up into the bulb and read off the value indicated by the float floating in the sample. 

Using a digital density meter 

A digital density meter (sometimes called a digital hydrometer) can be used to measure the specific gravity of the sulfuric acid electrolyte as long as the measuring cell withstands aggressive acids. The result is typically converted into the right temperature and displayed in the desired unit like SG (Specific Gravity) 80/80 on the digital display.

Density meters (or digital hydrometers) have some advantages over hydrometers: As well as being quicker and giving the result on a digital display already converted to the temperature, cleaning is simpler. Between measurements with a digital density meter cleaning of the cell is not required because the samples are similar, only a rinsing prior to the next measurement is performed. Density measurement on a density meter only requires handling of a small volume of 2 mL.

By following a few simple guidelines it is possible to ensure good density measurement results.

Table 1: Overview of battery acid measuring methods with pros and cons

 

MethodProsCons
Hydrometer
  • Low price
  • Not very accurate
  • Error prone due to manually reading off the value
  • Breaks easily
  • Relatively large amount of samples has to be taken out of the battery
  • Time and skill are required to read off value accurately
Digital density meter
  • Approx. 1 minute for measurement
  • Easy and safe filling with built in manual pump
  • Only 2 mL sample required
  • Automatic calculation of specific gravity (SG) from measured density due to integrated conversion tables
  • High accuracy
  • Relatively high price compared to hydrometers

Common units

The density of sulfuric acid (H2SO4) is measured and converted into specific gravity at either 70 °F to obtain SG 70/70 or at 80 °F to obtain SG 80/80.