Tutoring physics or chemistry, you might encounter Peukert’s Law, although it’s probably used more by industry.
In my March 2 post I mentioned reserve capacity and Amp*hours as two ways to measure a battery’s potential output. Numerically they are convertible backwards and forwards, but the reality is not necessarily so simple, because the speed of discharge affects the total output a battery can manage. Specifically, a higher discharge rate lessens the battery’s efficiency, so that the total output will decrease the faster the battery is discharged.
Peukert’s Law gives the equation
t is the actual time the battery will deliver arbitrary current I,
T is the discharge time corresponding to the given amp*hour rating,
C is the given amp*hour rating,
k is a physical constant that depends on the type of battery (around 1.4 for lead-acid).
Because of Peukert’s Law, an amp*hour rating must be given with a specific time for which it’s valid. (My reading suggests that 20 hours might be a typical time.) Therefore, an amp*hour rating might read “120Ah over 20 hours”. Such a rating implies a discharge rate of 6A for 20 hours. How long the battery can deliver a different amperage can be calculated by Peukert’s Law.
Example: Imagine a battery rated 120Ah over 20 hours. How long can it deliver 120A?
Solution: Theoretically, a 120Ah battery can deliver 120A for 1 hour, although we already know not to expect it. Peukert’s Law gives
t=0.30 hours, or about 18 minutes.
I’ll be talking more about lead acid batteries in future posts:)
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.