28 | SAMLEX AMERICA INC.
TABLE 7.5 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
11.50V
23.0V
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 trig-
ger 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.
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, corro-
sion, 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.
7.12 LOW DC INPUT VOLTAGE SHUT-DOWN IN INVERTERS
As explained above at Section 7.10, 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 sound-
ed 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 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
SECTION 7 |
General Information - Battery Related
