The AMP Hour (AH) specification provides a measurement of battery capacity. In other words, it is an indication of how much energy can be stored by the battery.
A typical Amp Hour specification might read, “100 AH @ 20HR”.
The specification is saying that the battery will provide 5 amps of current at a usable voltage continuously for 20 hours. The “5 amps” was calculated by dividing 100 by 20.
It should be noted that a usable voltage is considered to about 10.5 volts and above on a battery that is under load (or has devices connected).
IMPORTANT! Having defined the AH specification, it is important to understand what the Battery Specification does not say.
A Common Misunderstanding Associated with Amp Hours
Consider the 100 Amp Hour battery. As indicated above, it will provide 5 amps of current for twenty hours while maintaining a voltage above 10.5 volts.
A common mistake is made when it is assumed that the 100 AH battery will also provide 100 amps for 1 Hour. It won’t. In fact, a battery of this type may only provide about 40 minutes of continuous 100 amp service at best.
This is due to a well known characteristic associated with lead acid batteries. Specifically the capacity will decrease as the rate of discharge increases.
In other words the relationship between battery capacity (how much energy is available) and the rate of discharge is not a linear one.
The phenomenon being described here is known as Peukert’s law.
Other Amp Hour Rates
As indicated above, the Amp Hour specification on 12 volt batteries is normally based a twenty hour rate. In fact, the specification is so standardized that battery labels often do not include this information.
That said, it is important to be aware that deviations from this norm are not uncommon.
Some battery manufacturers will establish different rates for their amp hour specification. For example five and ten amp hour rates are not uncommon.
In the case of a battery manufacturer who specifies a 100 AH based on five hour rate, the claim is that the battery will provide 20 amps for five hours before dropping below 10. 5 volts.
Determining How Long a Battery Will last Using Amp Hours
While you may be able gauge the relative capacity of one battery compared to another, you may have some difficulties when you try to determine exactly how long your battery will last. There are a couple basic reasons for this.
First, the AH rating is one that is based on a specific period of time and a specific current draw. Because of Peukert’s Law, the relationship is not linear, therefore simple calculations are not always practical.
Second, current draw is rarely constant in most vehicle applications. Some times all the lights are on, sometimes they are not. Sometimes the stereo volume in turned up and sometimes it is turned down. This very dynamic environment makes it difficult to determine exactly how long a battery will last.
As previously mentioned, the AH hours is gauge that is useful for relative comparisons. When comparing one battery to another for example, the Amp Hour specification will tell us that a 120 amp hour battery will have more capacity than a 80 amp hour battery.
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