If you know your Peukert # for your battery you can use this formula to see how long your battery will last for a particular load. This is kinda technical so stop reading here if ya don't wanna go nuts!
Peukerts Equation: I ^{n} x T = C
The AMPS use (I) to the Peukert # exponent (N) multiplied by time (T) = the battery amp hour capacity (at the 100 hour discharge rate...or 1.1 times the 20 hour capacity)
Example...let imagine a 200 a/h battery (at 20 hr. rating) with a typical flooded battery Peukert rating of 1.2 and you want to know how long you can run your 5amp refrigerator.
Well 5 amps raised to the 1.2 power is almost 7.
The 200 amp at 20 hr battery needs to be raised by 10% to 220
So we have 7(T) =220 or a bit over 31 hours to total discharge of the battery...so figure on 15 hours before you need to recharge.
If everything remained the same BUT you changed to a 10 amp fridge...
the 10 to the 1.2 is almost 16 and you thus get
16T=220...or 13.5 hours till total discharge and just 67 hours before you need to recharge.
So...you can see that as current draw increases...the capacity of the battery in hours drops even more quickly...and a higher Peukert number accelerates this.
Now conversely...a low Puekert # means the capacity is relatively unnaffected by the rate of amp draw. This is one of the big advantages of AGM batteries as they can have P numbers in the 1.08 type range.
So....if you are looking at building a large battery bank with big amp draws...checking into your PARTICULAR brand and model battery choices Peukert # may lead you to one brand and one type over another. Peukert #'s are generally hard to find and you may need to write to the mfr. you are considering.
Hope a few find the above interesting.
