26 | SAMLEX AMERICA INC.
TABLE 7.3 BATTERY CAPACITY VERSUS RATE OF DISCHARGE – C-RATE
C-rate Discharge Current Usable Capacity (%)
C/20 or, 0.05C 100%
C/10 or, 0.10C 87%
C/8 or, 0.125C 83%
C/6 or, 0.17C 75%
C/5 or, 0.20C 70%
C/3 or, 0.34C 60%
C/2 or, 0.50C 50%
1C 40%
Table 7.3 shows that a 100 Ah capacity battery will deliver 100% (i.e. full 100 Ah) capac-
ity 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 charger or 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).
TABLE 7.4: SOC VERSUS STANDING VOLTAGE (TYPICAL FLOODED BATTERY)
Percentage of
Full Charge
Standing Voltage
of Individual Cells
Standing Voltage of
12V Battery
Standing Voltage of
24AVV Battery
100% 2.105V 12.63V 25.26V
90% 2.10V 12.6V 25.20V
80% 2.08V 12.5V 25.00V
70% 2.05V 12.3V 24.60V
60% 2.03V 12.2V 24.40V
50% 2.02V 12.1V 24.20V
40% 2.00V 12.0V 24.00V
30% 1.97V 11.8V 23.60V
20% 1.95V 11.7V 23.40V
10% 1.93V 11.6V 23.20V
0% = / < 1.93V = / < 11.6V = / < 23.20V
SECTION 7 | General Information - Battery Related
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