26 | SAMLEX AMERICA INC.
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 (i) 10.5V for a 12V battery pack/ battery bank or, (ii) 21V for 24V battery bank or, (iii)
42V for 48V battery bank 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 (i) 10V for 12V battery pack/ battery bank or, (ii)
20V for 24V battery bank or, (iii) 40V for 48V battery bank.
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. 1.4.1 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).
1.4.14.1 Low DC Input Voltage Alarm in Inverters
As stated earlier at Section 1.4.14, the buzzer alarm is triggered when the voltage at the DC input terminals of the
inverter has dropped to around (i) 10.5V for a 12V battery pack/ battery bank or, (ii) 21V for 24V battery bank or, (iii)
42V for 48V battery bank at C-rate discharge current of C/5 Amps. Please note that the terminal voltage relative to
a particular State of 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 1.4.5 (
Refer to Fig. 1.4.1 for parameters and values shown in Table 1.4.5
):
TABLE 1.4.5 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
48V
12V
24V
48V
C/3 A
10.45V
20.9V
41.8V
09.50V
19.0V
38.0V
C/5 A
10.90V
21.8V
43.6V
10.30V
20.6V
41.2V
C/10 A
11.50V
23.0V
46.0V
11.00V
22.0V
44.0V
C/20 A
11.85V
23.7V
47.4V
11.50V
23.0V
46.0V
C/100 A
12.15V
24.3V
48.6V
11.75V
23.5V
47.0V
In the example given, the 10.5V / 21.0V / 42.0V 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.
SECTION 1.4 |
General Information
–
Battery Related
Содержание Evolution EVO-4248SP
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