SAMLEX AMERICA INC. | 33
SECTION 7 |
General Information on Lead Acid Batteries
7.12 LOW DC INPUT VOLTAGE SHUTDOWN IN INVERTERS
7.12.1 General
As explained earlier at Section 7.11, 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 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 corresponding to 100% discharged condition that
is harmful for the battery and for the inverter. The inverter is, therefore, shut down when the DC input
voltage reaches the threshold of 100% discharged condition.
This is done to protect the inverter against higher DC input currents at lower than normal DC input
voltages. Higher than normal DC input currents produce over heating in the input section of the inverter
and reduce reliability of components.
As explained above, 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 corresponding
to 100% discharged condition. In this inverter, this threshold is programmable through parameter "UVP
Setting" (See Section 7.12.2). Referring to the Discharge Curves given in Fig 7.1, the State of 100% Discharge
for various C-Rate discharge is as follows:
- 19.0V at very high C-rate discharge current of C/3 Amps.
- 20.5V at high C-Rate discharge current of C/5 Amps.
- 22.0V at lower C-rate Discharge current of C/10 Amps.
It is seen from the above that the terminal voltage at 100% discharged condition varies with the C-rate
discharge current.
17.12.2 Programming of Parameter "UVP Setting"
In this inverter, the threshold of Low DC Input Voltage Shutdown is programmable and is called "UVP
Setting" (UVP stands for Under Voltage Protection). The default value is 20V for PSR-1200-24 and 40V for PSR-
1200-48. This default value is close to the terminal voltage at 100% discharge condition at C-rate discharge
current of C/5 Amperes (Refer to Fig 7.1). For programming details, refer to Fig 10.1 / Table 10.1 under Main
Menu for Parameter Group 2 – "Input Setup" / Sub Menu for Parameter Group 2, Item (c) "UVP Setting"
(Section 10.2.2.3).
7.13 DEPTH OF DISCHARGE OF BATTERY AND BATTERY LIFE
7.13.1 General
The more deeply a battery is discharged on each cycle, the shorter the battery life. Using more batteries than the
minimum required will result in longer life for the battery bank. A typical cycle life chart is given in the Table 7.6:
TABLE 7.6: TYPICAL CYCLE LIFE CHART
Depth of Discharge
% of Ah Capacity
Cycle Life of Group 27 /31
Cycle Life of Group 8D
Cycle Life of Group GC2
10
1000
1500
3800
50
320
480
1100
80
200
300
675
100
150
225
550