Peukert in brief

The usual form of Peukert's equation is T=C/In

Where:

T = time in hours
C = the Peukert capacity of the battery (ie at the 1 amp discharge rate)
I = the discharge current
n = Peukert's exponent.

This equation will only work on batteries that are specified at the "Peukert Capacity" ie the 1 amp discharge rate. They very rarely are.

Batteries are usually specified at an "hour" rate, for instance 100Ahrs at 20 hours. Or 90Ahrs at 10 hours etc.

If your batteries are specified in such a way (and they nearly always are) then the equation must be modified to take this into account.

The modifed equation is T=C(C/R)n-1/In or T=R(C/R)n/In

Where:

T = time in hours
C = the specified capacity of the battery (at the specified hour rating)
I = the discharge current
n = Peukert's exponent
R = the hour rating (ie 20 hours, or 10 hours etc)

Alternatively, do this:

R(C/R)n = the "Peukert Capacity".

So in the case of a battery specified as being 100Ahrs@20 hours with a Peukert's exponent of 1.25 we get:

20(100/20)1.25 = 149.5Ahrs. This is the "peukert capacity". ie the capacity of the battery when discharged at 1 amp.

If you use this figure as the capacity of the battery then the usual Peukert's equation of T=C/In can be used.

There is a slightly different Peukert calculator here that operates by adjusting the specified battery capacity to the "Peukert Capacity" then showing run times calculated using the usual T=C/In

This spreadsheet is slightly different from the one here as the first one calculates the "Peukert capacity" then

runs the usual equation (T=C/In) on the discharge rates whereas the second one calculates the run times directly on the specified battery capacity using our modified Peukert's equation (T=C(C/R)n-1/In). The only difference in results is the adjusted Peukert Amps. You will notice that the final run times and available capacity are identical in both spreadsheets thus showing that both versions of the equation are indeed valid.