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4.3.4
Peukert's formula: about battery capacity and the rate of discharge
Please see section 2.3 for a general explanation.
The value which can be adjusted in Peukert's formula is the exponent
n: see the formula below.
In the BMV Peukert's exponent can be adjusted from 1.00 to 1.50. The
higher the Peukert exponent the faster the effective capacity "shrinks"
with increasing discharge rate. An ideal (theoretical) battery has a
Peukert Exponent of 1.00 and has a fixed capacity; regardless of the
size of the discharge current. The default setting for the Peukert
exponent is 1.25. This is an acceptable average value for most lead
acid batteries.
Peukert's equation is stated below:
n
where Peukert's exponent n =
Cp
I
t
=
⋅
The battery specifications needed for calculation of the Peukert
exponent are the rated battery capacity (usually the 20 h discharge
1
rate
) and for example a 5 h discharge rate
example of how to calculate the Peukert exponent using these two
specifications.
5 h rating
C
t
I
1
Please note that the rated battery capacity can also be defined as the 10 h or even 5 h
discharge rate.
2
The 5 h discharge rate in this example is just arbitrary. Make sure that besides the C
rating (low discharge current) a second rating with a substantially higher discharge current
is chosen.
75
Ah
=
5
h
5
h
=
1
75
Ah
15
A
=
=
1
5
h
log
t
log
t
−
2
log
I
log
I
−
1
2
. See below for an
1
2
20
17
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