Samlexpower Evolution EVO-2212 Owner's Manual page 18

SECTIOn 1 |
1.4.5 Specifying charging / Discharging currents: c-rate
Electrical energy is stored in a cell / battery in the form of DC power. The value of the stored
energy is related to the amount of the active materials pasted on the battery plates, the surface
area of the plates and the amount of electrolyte covering the plates. As explained above, the
amount of stored electrical energy is also called the Capacity of the battery and is designated by
the symbol "C".
The time in hours over which the battery is discharged to the "End Point Voltage" for purposes
of specifying Ah capacity depends upon the type of application. Let us denote this discharge
time in hours by "T". Let us denote the rate of discharge current of the battery as a multiple of
Ah capacity "C" and call it 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, C-rate is defined as:
"c-rAte" = cAPAcity "c" in Ah ÷ DiSchArge time "t"
Table 1.8 gives some examples of C-rate specifications and applications:
hours of discharge time "t" till
the "end Point voltage"
0.5 hrs.
1 hrs.
5 hrs.
(Inverter application)
8 hrs.
(uPS application)
10 hrs.
(Telecom application)
20 hrs. (
Automotive application)
100 hrs.
note: When a battery is discharged over a shorter time, its specified "C-rate" will be higher. for ex-
ample, the "C-rate" at 5 hour discharge period i.e. C/5 Amps will be 4 times higher than the "C-rate" at
20 hour discharge period i.e. C/20 Amps.
1.4.6 charging / Discharging curves
fig. 1.4 shows the charging and discharging characteristics of a typical 12V / 24V flooded
Lead Acid battery at electrolyte temperature of 80°f / 26.7°C. The curves show the % State
of Charge (X-axis) versus terminal voltage (y-axis) during charging and discharging at different
C-rates. 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 the subsequent explanations.
18 | SAMLEX AMErICA INC.
Safety Instructions & general Information
tAble 1.8 DiSchArge current rAteS - "c-rAteS"
"c-rate" Discharge current
in Amps = capacity "c" in Ah
÷ Discharge time "t" in hrs.
2C
1C
C/5 or 0.2C
C/8 or 0.125C
C/10 or 0.1C
C/20 or 0.05C
C/100 or 0.01C
example of c-rate
Discharge currents
for 100 Ah battery
200A
100A
20A
12.5A
10A
5A
1A