SamplexPower PST-1500-12 Mode D'emploi page 20

SECTION 7 |
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 dis-
charge 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
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 "Stand-
ing 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 measure-
ment 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).
20 | SAMLEX AMERICA INC.
General Information on Lead Acid
Batteries
C/20
C/10
C/8
C/6
C/5
C/3
C/2
1C
Usable Capacity (%)
100%
87%
83%
75%
70%
60%
50%
40%