gpelectric.com | [page 25]
the 20 h discharge rate 2) and for example a 5h discharge rate 2. See below for an example of how to calculate the
Peukert exponent using these two specications.
5h rating
C
5h
= 75Ah
t
1
= 5h
75Ah
I
1
= = 15A
5h
20h rating
C
20h
= 100Ah (rated capacity)
t
2
= 20h
100Ah
I
1
= = 5A
20h
1 Please note that the rated battery capacity can also be the 1h or even 5h discharge rate.
2 The 5h discharge rate in this example is just arbitrary. Make sure that besides the C20 rating (low discharge current)
a second rating with a substantially higher discharge current is chosen.
Peukert’s exponent n = log 20 - log 5 = 1.26
log 15
- log 5
Please note that Peukert's formula is no more than a rough approximation of reality, and that at very high currents,
batteries will give even less capacity than predicted from a xed exponent.
We recommend not to change the default value in the case of Li-ion batteries: See section 6.
6.2 MIDPOINT VOLTAGE MONITORING
Wiring diagram: see the quick installation sheet. Fig 5-12.
One bad cell or one bad battery can destroy a large, expensive battery bank. A short circuit or high internal leakage
current in one cell for example will result in under charge of that cell and over charge of the other cells.
Similarly, one bad battery in a 24V or 48V bank of several series/parallel connected 12V batteries can destroy the
whole bank.
Moreover, when new cells or batteries are connected in series, they should all have the same initial state-of-charge.
Small dierences will be ironed out during absorption or equalize charging, but large dierences will result in damage
during charging due to excessive gassing of the cells or batteries with the highest initial state of charge.
A timely alarm can be generated by monitoring the midpoint of the battery bank (i. e. by splitting the string voltage
in half and comparing the two string voltage halves).
MORE ABOUT PEUKERT’S
FORMULA & MIDPOINT MONITORING
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