SECTION 7 |
General Information on Lead Acid
Batteries
21
7.8 REDUCTION IN USABLE CAPACITY AT HIGHER DISCHARGE
RATES – TYPICAL IN INVERTER APPLICATION
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 as in cases where
the inverters are driving higher capacity loads, the usable capacity reduces due to
“Peukert Effect”. This relationship is not linear but is more or less according to the
Table 7.3.
TABLE 7.3 BATTERY CAPACITY VERSUS RATE OF DISCHARGE – C-RATE
C-Rate Discharge Current
Usable Capacity (%)
C/20
100%
C/10
87%
C/8
83%
C/6
75%
C/5
70%
C/3
60%
C/2
50%
1C
40%
Table 7.3 shows 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 5 Amperes (50W output
for a 12V inverter and 100W output for a 24V inverter). However, if it is discharged at
a rate of 50 Amperes (500W output for a 12V inverter and 1000W output for a 24V
inverter) then theoretically, it should provide 100 Ah ÷ 50 = 2 hours. However, table
7.3 shows that for 2 hours discharge rate, the capacity is reduced to 50% i.e. 50 Ah.
Therefore, at 50 Ampere discharge rate (500W output for a 12V inverter and 1000W
output for a 24V inverter) the battery will actually last for 50 Ah ÷ 50 Amperes = 1
hour.
7.9 STATE OF CHARGE (SOC) OF A BATTERY – BASED ON “STANDING
VOLTAGE”
The “Standing Voltage” of a battery under open circuit conditions (no load
connected to it) 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 7.4 shows the State of Charge versus Standing
Voltage for a typical 12V/24V battery system at 80°F (26.7ºC).
