Filter
Top Products
Battery Maintenance
975-0012-01-02 Rev A B–17
State of Charge The battery’s state-of-charge should be checked often and only when the battery at
a state of rest (when the battery is not powering loads or actively being charged).
First thing in the morning is usually the best time to check the state of charge. If
the batteries are readily accessible, measure the voltage across the individual
battery terminals. There should be less than a 0.2 volt difference between each
battery.
To determine the individual cell voltage, divide the voltage by the number of cells
in the battery (25.2 volts divided by 12 cells = 2.1 volts per cell). If a greater
difference is measured, the batteries may need to be equalized (liquid lead-acid
types only) or replaced.
All batteries in the bank should measure the same voltage (this is not an accurate
measurement for cross-tied batteries’ as each battery is in parallel with another
battery making individual battery measurements impossible).
The voltage should match the following table for the entire battery bank output.
These values indicate the overall battery’s state of charge for the entire bank.
Individual cell voltages (if available) are also shown as a percentage of charge.
The values given are for a temperature of 77 °F (25 °C). Cooler temperatures
produce lower voltage measurements.
Table B-6
Battery State-of-Charge
System Voltage
Percent of Full
Charge 12 Volt 24 Volt 48 Volt
Individual
Cell
Voltage
100% 12.7 25.4 50.8 2.12
90% 12.6 25.2 50.4 2.10
80% 12.5 25.0 50.0 2.08
70% 12.3 24.6 49.2 2.05
60% 12.2 24.4 48.8 2.03
50% 12.1 24.2 48.4 2.02
40% 12.0 24.0 48.0 2.00
30% 11.8 23.6 47.2 1.97
20% 11.7 23.4 46.8 1.95
10% 11.6 23.2 46.4 1.93
0% <
11.6 < 23.2 < 46.4 < 1.93
Important:
Table B-6 only applies for batteries that have been at rest for a minimum of
3 hours (i.e., no inverting, no charging).