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In view of the above, it may be seen that a xed 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 function is designed to protect the inverter from excessive current drawn at
the lower voltage.
1.4.14.2 Low DC Input Voltage Shutdown in Inverters
As explained above at Section 1.4.14, 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 10.5V for a 12V battery or, at around 21V
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 10V for a 12V battery or, 20V for 24V battery.
Referring to the Discharge Curves given in Fig 1.4.1, 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 xed Low DC Input Voltage Shutdown 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 shut-down function is designed to protect the inverter from excessive current
drawn at the lower voltage.
SECTION 1.4 | General Information – Battery Related
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