SECTION 7 |
General Information on Lead
Acid Batteries
23
Coefficient – during charging / discharging, the terminal voltage drops with rise in
temperature and rises with drop in temperature.
- The amount of discharging current or “C-Rate”: A battery has non linear internal
resistance and hence, as the discharge current increases, the battery terminal
voltage decreases non-linearly.
The discharge curves in fig. 7.1 show the % State of Charge versus the terminal
voltage of typical battery under different charge /discharge currents, i.e. “C-Rates”
and fixed temperature of 80°F. (Please note that the X-Axis of the curves shows the
% of State of Charge. The % of State of Discharge will be 100% - % State of Charge).
7.11 LOW DC INPUT VOLTAGE ALARM IN INVERTERS
As stated earlier, the buzzer alarm is triggered when the voltage at the DC input
terminals of the inverter has dropped to around 10.7V for a 12V battery (21.4V for
24V battery) at C-Rate discharge current of C/5 Amps. Please note that the terminal
voltage relative to a particular of State Discharge decreases with the rise in the value
of the discharge current. For example, terminal voltages for a State of Discharge of
80% (State of Charge of 20%) for various discharge currents will be as given at table
7.5 (Refer to fig 7.1 for parameters and values shown in table 7.5):
TABLE 7.5 TERMINAL VOLTAGE AND SOC OF LOADED BATTERY
Discharge Current:
C-Rate
Terminal Voltage at 80% State
of Discharge (20% SOC)
Terminal Voltage When Completely
Discharged (0% SOC)
12V
24V
12V
24V
C/3 A
10.70V
21.4V
09.50V
19.0V
C/5 A
10.90V
21.8V
10.30V
20.6V
C/10 A
11.95V
23.9V
11.00V
22.0V
C/20 A
11.85V
23.7V
11.50V
23.0V
C/100 A
12.15V
24.3V
11.75V
23.5V
In the example given above, the 10.9V / 21.8V Low Battery / DC Input Alarm would
trigger at around 80% discharged state (20% SOC) when the C-Rate discharge
current is C/5 Amps. However, for lower C-Rate discharge current of C/10 Amps and
lower, the battery will be almost completely discharged when the alarm is sounded.
Hence, if the C-Rate discharge current is lower than C/5 Amps, the battery may have
completely discharged by the time the Low DC Input Alarm is sounded.
