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24
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 (C
p
) is:
By doing two discharge tests and knowing I
1
& I
2
(discharge current in amps of
the two tests), and t
1
& t
2
(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.
C
p
= I
n
t where
log
t
2
- log t
1
log I
1
- log I
2
n =