Samlexpower Pure Sine Wave PST-600-12 Guide Du Propriétaire page 21

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cables are thick enough to allow a negligible voltage drop between the battery and
the inverter) .
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. 7.1 (page 19) show the % State of Charge versus the termi-
nal voltage of a 12V 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. 7.1, page 19):
Discharge Current:
C-Rate
C/3 A
C/5 A
C/10 A
C/20 A
C/100 A
General Information on Batteries for
Powering Inverters
Terminal Voltage at 80% State
of Discharge (20% SoC)
10.45V
10.90V
11.95V
11.85V
12.15V
Terminal Voltage When Completely
Discharged (0% SoC)
09.50V
10.30V
11.00V
11.50V
11.75V
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