Samlexpower PSE-12125A, Owner'S Manual

Page: 20 / 52

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
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. 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 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).
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