In my March 2 post<\/a> 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.<\/p>\n Peukert’s Law gives the equation<\/p>\n t=T(C\/(I*T))k<\/sup>, where<\/p>\n t is the actual time the battery will deliver arbitrary current I,<\/p>\n T is the discharge time corresponding to the given amp*hour rating,<\/p>\n C is the given amp*hour rating,<\/p>\n k is a physical constant that depends on the type of battery (around 1.4 for lead-acid).<\/p>\n 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.<\/p>\n Example: Imagine a battery rated 120Ah over 20 hours. How long can it deliver 120A?<\/strong><\/p>\n Solution: Theoretically, a 120Ah battery can deliver 120A for 1 hour, although we already know not to expect it. Peukert’s Law gives<\/p>\n t=20(120\/(120*20))1.4<\/sup><\/p>\n t=20(0.05)1.4<\/sup><\/p>\n t=0.30 hours, or about 18 minutes.<\/p>\n I’ll be talking more about lead acid batteries in future posts:)<\/p>\n Source:<\/p>\n all-about-lead-acid-batteries.capnfatz.com<\/a><\/p>\n batteryuniversity.com<\/a><\/p>\n batteryuniversity.com<\/a><\/p>\n