HIGH DISCHARGE RATES & 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 tables and examples on the following pages illustrate this effect and how to use the table to estimate the exponent "n". The tables on pages 27 and 28 have typical values of "n" for common batteries.

The Link 20 uses Peukert's equation in calculations to forecast the Time Remaining and run the light bars. The amp hours display is always the actual number of amp hours consumed. This means that if you heavily discharge a battery, your time- remaining display may show zero hours remaining before the expected number of amp hours of battery capacity is consumed.

Making two discharge tests, one at a high discharge rate (to get I1[current] and t1[time]) and one at a low rate (to get I2[current] and t2[time]), that bracket your normal range of operation, allows you to calculate an "n" which will describe this varying effect. The Link 20 uses a default value of "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 logarithmic 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. If a battery is discharged at, or slower than, the 20-hour rate, you should be able to remove the rated capacity if the battery is healthy.

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 exponent). You will need a calculator with a log function to solve the equation above.

Instead of doing two discharge tests yourself, you may use the 20-hour discharge rate and the number of reserve minutes as the two discharges to solve Peukert's equation. See the example given on page 29. After you solve for your Peukert's exponent you may enter it using Advanced Function F08.

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