Samlex America PST-200S-24E - Manual (Page 18)

18 | SAMLEX AMERICA INC.
seCtIOn 7 | General Information on Batteries for
Powering Inverters
as the “C-Rate”. If the battery delivers a very high discharge current, the battery will be
discharged to the “End Point Voltage” in a shorter period of time. on the other hand,
if the battery delivers a lower discharge current, the battery will be discharged to the
“End Point Voltage” after a longer period of time. Mathematically:
eQUAtion 1:
Discharge current “C-Rate” = Capacity “C” in Ah ÷ Discharge Time “T”
Table 7.2 below gives some examples of C-Rate specifications and applications:
tABLe 7.2: DiScHARGe cURRent RAteS - “c-RAteS”
Hours of discharge time
“T” till the “End Point
Voltage”
C-Rate Discharge Current in Amps Example of C-Rate
Discharge Currents for
100 Ah battery
fraction Decimal Subscript
0.5 Hrs. 2C 2C 2C 200A
1 Hrs. 1C 1C 1C 100A
5 Hrs. C/5 0.2C C5 20A
8 Hrs.
(UPS application)
C/8 0.125C C8 12.5A
10 Hrs.
(Telecom application)
C/10 0.1C C10 10A
20 Hrs.
(Automotive application)
C/20 0.05C C20 5A
100 Hrs. C/100 0.01C C100 1A
note: When a battery is discharged over a shorter time, its specified “C-Rate” discharge current
will be higher. for example, the “C-Rate” discharge current at 5 Hour discharge period i.e. 0.2C
/ C5 / C/5 Amps will be 4 times higher than the “C-Rate” discharge current at 20 Hour discharge
period i.e. 0.05C / C20 / C/20 Amps.
charging / Discharging curves
fig. 7.1 (page 19) shows the charging and discharging characteristics of a typical, 6 cell,
12V, Lead Acid battery at electrolyte temperature of 80°f (by convention, battery data
is normally presented at 80°f). The curves show the % State of Charge (X-axis) versus
terminal voltage (Y-axis) during charging and discharging at different C-Rates. for 24V
battery, multiply voltage on Y-axis by 2 for 48V battery, multiply voltage on Y-axis by 4
(Please note that X-axis shows % State of charge. State of Discharge will be = 100% -
% State of Charge). These curves will be referred to in subsequent explanations.
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 (page 19).