Peukert's Equation

Peukert's Equation describes the effect of different discharge rates on battery capacity. As the discharge rate increases, the available battery capacity decreases. The table and examples on the following page illustrate this effect and how to use the table to estimate the exponent "n". The tables on pages 40 and 41 have typical values of "n" for common batteries.

Making two discharge tests, one at a high discharge rate and one at a low rate, that bracket your normal range of operation, allows you to calculate an "n" that will describe this varying effect. The Link 10 uses an "n" equal to 1.25, which is typical for many batteries.

At some low to moderate discharge rate, typically a battery's 20-hour rate, the logrithmic effect of Peukert's Equation is greatly reduced. The effect of discharge rates smaller than this is nearly linear. Battery manufacturer specifications of battery capacity in amp hours is typically given at the 20- hour rate. From this description, if a battery is discharged at this rate for the period of time called out, you will be able to remove the rated capacity.

The equation for Peukert's Capacity (Cp ) is:

C p = I n t where

n =

log t2 - log t1

 

 

log I1 - log I2

By doing two discharge tests and knowing I1 & I2 (discharge current in amps of the two tests), and t1 & t2 (time in hours for the two tests) you can calculate n (the Peukert coefficient). You will need a calculator that has a Log function to solve the equation above. See example on page 42. After you solve for your Peukert's coefficient you may enter it using Advanced Function F8.

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