SBM-01 high precision battery monitor
6
2.2 Peukert’s exponent
As mentioned earlier in chapter 1.2 the Peukert efficiency describes that
if you discharge a battery faster than the 20hr rating, it’s Amphour size
decreases. The amount of battery size decrease is called ‘Peukert
exponent’ and can be adjusted from
1.00
up to
1.50
in Function F10. The
higher the Peukert exponent the faster the battery size shrinks with
increasing discharge rate. An ideal (theoretical) battery has a Peukert
Exponent of 1.00 and doesn’t care how big the discharge current is. Of
course these batteries do not exist, and an F10 setting of
1.00
is only
implemented to bypass Peukert compensation in the SBM-01.
The default setting for the Peukert exponent is 1.25, and is an
acceptable average value for most lead acid type of batteries. However
for precise battery monitoring, entering the right Peukert exponent is
essential. If the Peukert exponent is not provided with your battery, you
can calculate it by using other specifications which must be provided with
your battery. The Peukert equation is stated below :
Cp = I
n
⋅
t where Peukert exponent ‘n’ =
2
log
1
log
1
log
2
log
I
I
t
t
−
−
The battery specifications needed for calculation of the Peukert
exponent, are the rated battery capacity (usually the 20hr discharge
rate
(1)
) and for example a 5hr discharge rate
(2)
. See the calculation
example below to define the Peukert exponent using these two
specifications :
5hr rating, C5 =
75Ah
→
t1 = 5hr
→
I1 = 75Ah/5hr = 15A
20hr rating, C20 = 100Ah (rated capacity)
→
t2 = 20hr
→
I2 = 100Ah/20hr = 5A
Peukert exponent n =
5
log
15
log
5
log
20
log
−
−
=
1,26
