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 on pages 25, 26 and 27 have typical values of "n" for common batteries. Page 25 is a look-up table, pages 26 & 27 have "n" values for common batteries, and page 27 has the formula for calculating "n" for other batteries.

The LINK 2000 uses Peukert's equation only for calculating the Time Remaining of operation function. The amp hours display is always the actual number of amp hours consumed. This means that if you rapidly discharge a battery, your time remaining number may show zero hours remaining before you see the total number of amp hours of battery capacity consumed.

If battery Type 0 is selected, the initial Peukert exponent is set at 1.25. This is an appropriate mid-range value for many common liquid cells. If any other battery type is selected, the initial Peukert exponent is set at 1.11. This is an appropriate mid-range value for many common gel and AGM cells. Please note that you may only declare one battery type using function F02. This is because we strongly recommend against mixing AGM, gel, and liquid electrolyte batteries in the same system. The selected Peukert exponent is applied to both battery banks. Set the exponent to the correct value for the bank which you use most often.

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 2000 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. 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:

 

log t2 - log t1

C p = I n t where

n =

 

 

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 that has a Log function to solve the equation above. You may also use the 20-hour discharge rate and the number of reserve minutes as the two discharges to solve Peukert's equation. See example on page 27. After you solve for your Peukert's exponent you may enter it using Function F08.

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