Technical Manual

Document No. 6-0101 Rev. F Page 32 of 40
APPENDIX C – CHARTS AND GRAPHS
Battery Load Voltage vs. DOD
Below are listed the 1 hour, 8 hour, 20 hour and 120 hour load voltages during a discharge
cycle to 100% discharge (10.5V endpoint) for a 12V battery at 77°F (25°C).
DOD (%)
1 hr. Rate
8 hr. Rate
20 hr. Rate
120 hr. Rate
10
12.23
12.60
12.65
12.69
20
12.16
12.51
12.55
12.58
30
12.07
12.39
12.42
12.45
40
11.96
12.25
12.28
12.32
50
11.83
12.11
12.15
12.18
60
11.70
11.98
12.02
12.05
70
11.55
11.79
11.83
11.88
80
11.38
11.59
11.61
11.65
90
11.15
11.32
11.34
11.40
100
10.50
10.50
10.50
10.50
NOTE: Multiply by 2X for a 24V battery bank and 4X for a 48V battery bank.
Note that these voltages are approximate and will vary as the battery ages. They are, however,
a fair indicator of state of charge and can be used when setting low voltage alarms or
disconnects. For example, if the average load is 50A and the battery has a rated capacity of 400
AH at the 8-hour rate, the minimum discharge voltage would be 12.11 volts for a 50% DOD.
SOC (%) vs. OCV
An easy method to estimate the State of Charge (SOC) of the battery is by measuring its Open
Circuit Voltage (OCV). This measurement should be made after the battery has been at rest for
a minimum of four hours with the battery shut off from its charging source and load. The
reference temperature is 77°F (25°C), but the OCV does not change appreciably at other
temperatures (temperature coefficient is 0.10 millivolts/cell per °F).
State of
Charge (%)
OCV of
2V battery
OCV of
6V battery
OCV of
12V battery
OCV of
24V battery
OCV of
48V battery
100
>2.13
>6.39
>12.78
>25.56
>51.12
90
2.11
6.33
12.66
25.32
50.64
80
2.09
6.27
12.54
25.08
50.16
75
2.08
6.24
12.48
24.96
49.92
70
2.07
6.21
12.42
24.84
49.68
60
2.05
6.15
12.30
24.60
49.20
50
2.03
6.09
12.18
24.36
48.72
40
2.01
6.03
12.06
24.12
48.24
30
1.99
5.97
11.94
23.88
47.76
25
1.98
5.94
11.9
23.76
47.52
20
1.97
5.91
11.82
23.64
47.28
10
1.95
5.85
11.70
23.40
46.80
0
<1.93
<5.79
<11.58
<23.16
<46.32
These voltage levels are applicable for aged batteries. The voltage levels for a new battery will
be somewhat higher at a given state of charge. For example, a new battery typically has an
OCV of 2.17 volts per cell (13.0 for a 12V battery) or higher at 100% SOC.