03/03/97
26
HIGH DISCHARGE RATES
& PEUKERT'S EQUATION
Peukert's Equation describes the effect of different discharge rates on battery ca-
pacity. 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 & 28 have typical values of "n" for
common batteries.
The
Link 20 uses Peukert's equation in calculations to forecast the Time Remain-
ing and run the light bars. The Amp hours display is always the actual number of A hrs
consumed. This means that if you heavily discharge a battery, your time remaining
display may show zero hours remaining before the expected number of A hrs of battery
capacity is consumed.
Making two discharge tests, one at a high discharge rate (to get I
1
[current] and t
1
[time])
and one at a low rate (to get I
2
[current] and t
2
[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 (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 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.
C
p
= I
n
t where
log
t
2
- log t
1
log I
1
- log I
2
n =
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