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SAMLEX AMERICA INC. | 21
Table 1.11 TERMINAL VOLTAGE AND SOC OF LOADED BATTERY
Discharge
Current:
C-Rate
Terminal Voltage at 80% State
of Discharge (20% SOC)
Terminal Voltage When Completely
Discharged (0% SOC)
12V
24V
12V
24V
C/3 A
10.45V
20.9V
09.50V
19.0V
C/5 A
10.90V
21.8V
10.30V
20.6V
C/10 A
11.95V
23.9V
11.00V
22.0V
C/20 A
11.85V
23.7V
11.50V
23.0V
C/100 A
12.15V
24.3V
11.75V
23.5V
In the example given above, the 10.5V / 21.0V Low Battery / DC Input Alarm would trigger
at around 80% discharged state (20% SOC) when the C-Rate discharge current is C/5 Amps.
However, for lower C-Rate discharge current of C/10 Amps and lower, the battery will be almost
completely discharged when the alarm is sounded.
Hence, if the C-Rate discharge current
is lower than C/5 Amps, the battery may have completely discharged by the time the
Low DC Input Alarm is sounded.
1.4.11 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 10.5V for
a 12V battery (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 batter-
ies 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 (20V for 24V battery). Referring to the Discharge Curves given in Fig 1.4, 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 capac-
ity 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 function are designed to
protect the inverter from excessive current drawn at the lower voltage.
SECTION 1 |
Safety Instructions & General Information
Summary of Contents for Evolution EVO-2212
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