SECTION 7 |
General Information on Lead Acid
Batteries
24
7.12 LOW DC INPUT VOLTAGE SHUT-DOWN IN INVERTERS
As explained above, at around 80% State of Discharge of the battery at C-Rate
discharge current of around C/5 Amps, the Low DC Input Voltage Alarm is sounded
at around 11V for a 12V battery (at around 22V for 24V battery) to warn the user to
disconnect the battery to prevent further draining of the battery. If the load is not
disconnected at this stage, the batteries will be drained further to a lower voltage
and to a completely discharged condition that is harmful for the battery and for the
inverter.
Inverters are normally provided with a protection to shut down the output of the
inverter if the DC voltage at the input terminals of the inverter drops below a
threshold of around 10.5V for a 12V battery (21V for 24V battery). Referring to the
Discharge Curves given in fig 7.1, the State of Discharge for various C-Rate discharge
currents for battery voltage of 10V / 20V is as follows: (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):
- 85% State of Discharge (15% State of Charge) at very high C-Rate discharge current
of C/3 Amps.
- 100% State of Discharge (0 % State of Charge) at high C-Rate discharge current of
C/5 Amps.
- 100% discharged (0% State of Charge) at lower C-Rate discharge current of C/10
Amps.
It is seen that at DC input voltage of 10V / 20V, the battery is completely discharged
for C-Rate discharge current of C/5 and lower.
In view of the above, it may be seen that a fixed Low DC Input Voltage Alarm is not
useful. Temperature of the battery further complicates the situation. All the above
analysis is based on battery electrolyte temperature of 80°F. The battery capacity
varies with temperature. Battery capacity is also a function of age and charging
history. Older batteries have lower capacity because of shedding of active materials,
sulfation, corrosion, increasing number of charge / discharge cycles etc. Hence, the
State of Discharge of a battery under load cannot be estimated accurately. However,
the low DC input voltage alarm and shut-down functions are designed to protect the
inverter from excessive current drawn at the lower voltage.
