Lesson 2/Learning Event 1
As the battery wears out, active material drops off the plates, which reduces the battery's
capacity and causes a larger voltage drop under the load.
As outdoor temperature drops, the electrolyte gets thicker, which slows down its action
and causes a larger voltage drop during cold weather.
The current required by the electrical systems of two vehicles are often different, even though they
require the same voltage. For instance, both the - and 5-ton trucks have 24-volt electrical
systems, but the batteries used in the - ton truck will not supply enough current to satisfactorily
crank the large engine of the 5-ton truck.
We can tell which battery has a larger capacity or current-producing capability by physical size, just
like judging a bigger can will hold more water, but that is not an exact measurement.
The amount of water the cans will hold is measured in pints, quarts, or gallons. Battery capacity is
measured in ampere-hours, which is the number of amperes the battery will deliver multiplied by
the number of hours the battery will deliver it. For example, suppose a battery will deliver 5
amperes for 20 hours, then 5 amperes multiplied by 20 hours equals 100 ampere-hours.
If the ampere-hour rating is based on nothing more than the method described above, the rating
would change if the rate of discharge changed. Suppose that we increase the load on the battery
from 5 to 10 amperes. The battery would produce the 10 amperes for a period less than 20 hours,
so its rating would be less than 100 ampere-hours. As you can see, some standard procedure must
be used to rate the ampere-hours so that the ratings will mean the same in all cases.
The capacity of automotive batteries is rated by a standard procedure known as the 20-hour rating.
This rating gives the number of ampere-hours a battery will deliver if it is discharged at a uniform
rate for 20 hours, at a temperature of 80, and with a battery voltage of 1.75 volts per cell at the
end of the 20-hour period.
Two sizes of batteries, type 2HN and 6TN, are commonly used in tactical wheeled vehicles. Both
types are 12-volt batteries. The smaller 2HN battery is rated 45 ampere-hours; the larger 6TN
battery is rated 100 ampere-hours.