Samlex America PST-30S-24E - Manual (Page 21)

20 | SAMLEX AMERICA INC. SAMLEX AMERICA INC. | 21
seCtIOn 7 | General Information on Batteries for
Powering Inverters
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 (by convention, battery data is normally presented
at 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 at fig. 7.1 show the % State of Charge versus the terminal 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:
Discharge Current:
C-Rate
Terminal Voltage at 80% State
of Discharge (20% SoC)
Terminal Voltage When Completely
Discharged (0% SoC)
C/3 A 10.45V 09.50V
C/5 A 10.90V 10.30V
C/10 A 11.95V 11.00V
C/20 A 11.85V 11.50V
C/100 A 12.15V 11.75V