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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 / 48V battery system at 80°F (26.7ºC).
TABLE 7.4: STATE OF CHARGE VERSUS STANDING VOLTAGE
Percentage of
Full Charge
Standing Voltage of
Individual Cells
Standing Voltage of
24V Battery
Standing Voltage of
48V Battery
100%
2.105V
25.26V
50.52V
90%
2.10V
25.20V
50.40V
80%
2.08V
25.00V
50.00V
70%
2.05V
24.60V
49.20V
60%
2.03V
24.40V
48.80V
50%
2.02V
24.20V
48.40V
30%
1.97V
23.60V
47.20V
20%
1.95V
23.40V
46.80V
10%
1.93V
23.20V
46.40V
0%
= / < 1.93V
= / < 23.20V
= / < 46.40V
Check the individual cell voltages / specific gravity. If the inter cell voltage difference is more than a 0.2V, or
the specific gravity difference is 0.015 or more, the cells will require equalization. Please note that only the
non-sealed / vented / flooded / wet cell batteries are equalized. Do not equalize sealed / VRLA type of AGM
or Gel Cell Batteries.
SECTION 7 |
General Information on Lead Acid Batteries
