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10 | SAMLEX AMERICA INC.
SECTION 3 | Description & Principle of Operation
Reduction in Usable Capacity at Higher Discharge Rates
As stated above, the rated capacity of the battery in Ah is normally applicable at a
discharge rate of 20 Hours. As the discharge rate is increased, the usable capacity
reduces due to “Peukert Effect”. This relationship is not linear but is more or less
according to the Table below:
HOURS OF
DISCHARGE
DISCHARGE
RATE
DISCHARGE RATE
FOR 100Ah BATTERY
USABLE
CAPACITY
20 HRS. C/20 A 5A 100 %
10 HRS. C/10 A 10A 87 %
8 HRS. C/8 A 12.5A 83 %
6 HRS. C/6 A 16.7A 75 %
5 HRS. C/5 A 20A 70 %
3 HRS. C/3 A 33.3A 60 %
2 HRS. C/2 A 50A 50 %
1 HRS. C A 100A 40 %
Table 3.2. Battery Capacity versus Rate of Discharge
Using the above Table will show that a 100 Ah capacity battery will deliver 100% (i.e.
full 100 Ah) capacity if it is slowly discharged over 20 hours at the rate of C/20 A or 5A.
However, if it is discharged at a rate of 2 Hrs. (C/2A or 50A) then theoretically, it should
provide 100 Ah ÷ 50A = 2 Hours. However, the Table above shows that for 2 Hours
discharge rate (C/2A or 50A), the capacity is reduced to 50% (i.e. 50 Ah). Therefore, at 50
Ampere discharge rate the battery will actually last for 50 Ah ÷ 50A = 1 Hour.
State of Charge (SOC) of a Battery
The “Standing Voltage” of a battery can approximately indicate the State of Charge
(SOC) of the battery. The “Standing Voltage” is measured after disconnecting any
charging device(s) and the battery load(s) and letting the battery “stand” idle for 3 to
8 hours before the voltage measurement is taken. Table 3.3 shows the State of Charge
versus Standing Voltage for a 12V lead acid battery system (6 cells in series) at around
80ºF (26.7ºC). For 24-volt systems, multiply by 2 (12 cells in series); for 48-volt systems,
multiply by 4 (24 cells in series).