26 | SAMLEX AMERICA INC.
SAMLEX AMERICA INC. | 27
Check the individual cell voltages / specific gravity. If the inter cell voltage difference is
more than a 0.2V, or the specific gravity difference is 0.015 or more, the cells will require
equalization. Please note that only non-sealed / vented / flooded / wet cell batteries are
equalized. Do not equalize sealed / VRLA type of AGM or Gel Cell Batteries.
7.10 STATE OF DISCHARGE OF A LOADED BATTERY – LOW BATTERY /
DC INPUT VOLTAGE ALARM AND SHUTDOWN IN INVERTERS
Most inverter hardware estimate the State of Discharge of the loaded battery by measur-
ing the voltage at the inverter’s DC input terminals (considering that the DC input 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.
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 show the % State of Charge versus the terminal voltage
of typical Flooded Lead Acid 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).
7.11 LOW DC INPUT VOLTAGE ALARM IN INVERTERS
As stated earlier at Section 7.10, 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 a 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 given at Table 7.5
(Refer to Fig 7.1 for parameters and values shown in Table 7.5):
SECTION 7 |
General Information - Battery Related
