Lesson 2/Learning Event 1
As a battery is charged, some of the water in the electrolyte is broken down and passes
off in the form of gas. If charging is continued after the battery is fully charged, the
amount of gas given off increases and the battery overheats. This is called overcharging
and can damage the battery. The gas given off by a battery during the charging process is
explosive and can be easily ignited by a spark.
Repeated charging and discharging slowly wears out the battery. It causes the lead
peroxide to fall off the positive plates into the sediment space in the bottom of the
container. The sediment may build up high enough to cause a short circuit between the
negative and positive plates, but normally the cell will be worn out before the sediment
reaches the bottom of the plates.
The strength of the battery electrolyte is determined by comparing its weight to that of an equal
volume of pure water. Pure water has a specific gravity (weight) of 1.000. Let's suppose that we
compare 1 gallon of water to 1 gallon of a second substance and find that the second substance
weighs 2 times more than the water. The second substance is said to have a specific gravity of
2.500 or 2 times that of water. Pure sulfuric acid has a specific gravity of 1.835.
Since electrolyte is a mixture of sulfuric acid and water, the specific gravity of electrolyte will be
more than the 1.000 of the water but less than the 1.835 of the acid. The mixture that is generally
placed in batteries has a specific gravity of 1.280, and by volume it contains 73-percent water and
27-percent sulfuric acid.
Often the specific gravity of the electrolyte is simply referred to as the gravity of the battery. Also,
it is customary to omit the decimal point and refer to a specific gravity of 1.280 as "twelve eighty,"
1.200 as "twelve hundred," and so forth. Variations of gravity in the third decimal place are
referred to as points. For example, 1.284 is four points higher than 1.280.