Reserve capacity for a lead acid battery answers the question, “How will long will my battery last at the specified current draw?”.
The measurement is normally based on a 25 Amp current draw at eighty degrees Fahrenheit.
For example, a specification may read, “RC @ 25A = 160 minutes”.
Such a specification would indicate that at 80 degrees F, the battery can supply 25 amps of current at a useable voltage for 160 minutes.
The definition of useable voltage is any voltage that is at or above 10.5 volts.\
A Common Misunderstanding Associated with Reserve Capacity
Like Amp Hours, there is a tendency to want to use the Reserve Capacity (RC) rating to determine how long a battery will last under a different current draw.
For example, given a battery with a 160 minute reserve capacity at 25 amps, one might wrongly conclude that the battery should last 80 minutes with a 50 amp continuous draw.
Such a conclusion would be incorrect.
As battery current draw increases, the battery capacity decreases. In other words, the relationship is non linear. This phenomenon is known as Peukert’s Law.
In reality, the battery with 160 amp reserve capacity at 25A at will last significantly less than 80 minutes at 50A. Indeed, it may be more realistic to expect it to last 65 to 70 minutes.
Differences between Amp Hours and Reserve Capacity
Another battery specification that is closely related to Reserve Capacity is called Amp Hours. Both relate to battery capacity, however they each answer a different question about the battery.
Reserve Capacity answers the question, “Given a particular current draw (or load), how long will the battery last?”
Amp Hours, on the other hand, answers the question, “What is the maximum current that can be drawn from the battery continuously and provide 20 hours of service?”
Determining How Long a Battery Will last Using Reserve Capacity
Reserve Capacity can be a much more useful specification when attempting to determine how long a battery will last. This is because the current draw (or load) included in this specification is usually higher than the current draw in an Amp Hour specification.
That said, there are limits to using Reserve Capacity for determining how long a battery will last. These limits exist for two very fundamental reasons.
- Peukert’s Law applies here. As the law indicates, battery capacity decreases as the current draw increases. Similarly, battery capacity increases as current draw decreases.
- Battery loads tend to be dynamic. In other words, they may be changing all the time depending on what devices are on and how they are being used. Coupled with Peukert’s Law, figuring out how long a battery will last can represent a pretty monumental task.