Samlex America PST-200S-24E - Manual (Page 21)

20 | SAMLEX AMERICA INC. SAMLEX AMERICA INC. | 21
seCtIOn 7 | General Information on Batteries for
Powering Inverters
the DC input cables are thick enough to allow a negligible voltage drop between the
battery and the inverter) .
Inverters are normally 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 10.7V for a 12V battery or 21.4V for 24V battery at C-Rate dis-
charge current of C/5 Amps and electrolyte temp. of 78°C (by convention, battery data
is normally presented at 78°C). 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 normally estimated based on the measured termi-
nal 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 78°f. (By convention, battery data is normally presented at 78°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
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 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