PSE-12275A Manual
20 | SAMLEX AMERICA INC.
Inverters are provided with a buzzer alarm to warn that the loaded battery has been
deeply discharged to around 80% of the rated capacity. Normally, the buzzer alarm is
triggered when the voltage at the DC input terminals of the inverter has dropped to
around 10.5V for a 12V battery or 21V for 24V battery at C-Rate discharge current of C/5
Amps and electrolyte temp. of 80°F. The inverter is shut down if the terminal voltage at
C/5 discharge current falls further to 10V for 12V battery (20V for 24V battery).
The State of Discharge of a battery is estimated based on the measured terminal voltage
of the battery. The terminal voltage of the battery is dependent upon the following:
- Temperature of the battery electrolyte: Temperature of the electrolyte affects the
electrochemical reactions inside the battery and produces a Negative Voltage
Coefcient – 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. 6.1 (page 18) show the % State of Charge versus the termi-
nal voltage of typical battery under different charge /discharge currents, i.e. “C-Rates”
and xed 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).
Low DC Input Voltage Alarm in Inverters
As stated earlier, the buzzer alarm is triggered when the voltage at the DC input termi-
nals of the inverter has dropped to around 10.5V for a 12V battery (21V 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 follows (Refer to Fig. 6.1, page 18):
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 10.45V 20.9V 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
SECTION 6 | General Information on
Lead Acid Batteries