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Table 1.4.3 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 1.4.3 above 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.
1.4.13 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 1.4.4 shows the State of Charge versus Standing Voltage for a typical
12V/24V battery system at 80°F (26.7ºC).
TABLE 1.4.4 SOC VERSUS STANDING VOLTAGE (TYPICAL FLOODED BATTERY)
Percentage of
Full Charge
Standing Voltage
of Individual Cells
Standing Voltage of
12V Battery
Standing Voltage
of 24V 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
Check the individual cell voltages / specic gravity. If the inter-cell voltage difference is more than a 0.2V, or the
specic gravity difference is 0.015 or more, the cells will require equalization. Refer to Section 1.4.3.4 and 1.4.4
regarding details on equalization). Please note that only non-sealed / vented / ooded / wet cell batteries are
equalized. Do not equalize sealed / VRLA type of AGM or Gel Cell Batteries.
1.4.14 State of Discharge of a Loaded Battery – Low Battery / DC Input Voltage Alarm and
Shutdown in Inverters
Most inverter hardware estimate the State of Discharge of the loaded battery by measuring the voltage at the
inverter’s DC input terminals [considering that the DC input cables are thick enough to allow a negligible voltage drop
between the battery and the inverter].
SECTION 1.4 | General Information – Battery Related
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