# How to determine the capacity of an automotive battery when not given?

#### Lumenosity

Joined Mar 1, 2017
487
Suppose you have a car battery that only stated cranking amps and group (29DC)
How would you determine the Amp Hour Capacity? Can it be calculated from the Group Number or CCA?

Now suppose you had a battery that stated the capacity as 114Ah @ 1amp .....
How would you determine the capacity at 2amps discharge rate? Is it simply half the 114Ah 1 amp value?

Thanks

#### Alec_t

Joined Sep 17, 2013
12,001
Is it simply half the 114Ah 1 amp value?
Not exactly. The higher the current draw the less the capacity. Ask the manufacturer.

#### crutschow

Joined Mar 14, 2008
26,998
The only accurate way to determine its capacity is to do a discharge test at a low current (a few amperes) and note when the voltage drops below its complete discharge value.

#### MrAl

Joined Jun 17, 2014
8,149
Suppose you have a car battery that only stated cranking amps and group (29DC)
How would you determine the Amp Hour Capacity? Can it be calculated from the Group Number or CCA?

Now suppose you had a battery that stated the capacity as 114Ah @ 1amp .....
How would you determine the capacity at 2amps discharge rate? Is it simply half the 114Ah 1 amp value?

Thanks
Hi,

The capacity of a battery is almost always given as a C/20 rate discharge.
Thus if the battery is rated at 100 ampere hours, then the discharge rate for that test is 100/20=5 amps.
If the battery is rated at 200 ampere hours, then the discharge rate is 200/20=10 amps.

Now if you want to discharge at a higher current than the standardized test current, you need to either find this out from the manufacturer which may provide a chart, or you have to calculate the "P" factor. The P factor is used in an equation that calculates the capacity from the test data which would usually be from the 20 hour rate (above) and from another discharge rate test such as maybe the 10 hour rate and maybe a third test at maybe a 5 hour test. You can then calculate the P factor and that wll classify the battery so you can calculate other rates. There is also an aging factor though which means the battery capacity goes down over time. This is harder to calculate so you probably have to rely on the manufacturer for this data.

Obviously the best way is to get it from the manufacturer, but unfortunately this may be hard or impossible to get sometimes. This means we have to implement tests that tell us the data we need to know such as the P factor tests, and also we can run aging tests but that really has to be done right in the application itself so that we can get real time data that can be used to predict when we are going to have to replace the battery. For a simple example, if we log the discharge over time and the time to discharge to a certain level starts to decrease too much, then we might trigger an alarm that warns us that we have to replace the battery soon.

• nsaspook

#### nsaspook

Joined Aug 27, 2009
8,268
Suppose you have a car battery that only stated cranking amps and group (29DC)
How would you determine the Amp Hour Capacity? Can it be calculated from the Group Number or CCA?
No, you can't calculate a Engine starting battery CCA to a Deep cycle battery C/20 discharge rate directly (its like converting miles per hour into miles per gallon.). As stated above it can be determined experimentally and usually you will be disappointed on how low the actual discharge rate is because starting batteries are designed for very shallow discharges. If you have a deep cycle application, use a deep cycle battery.