Battery State-of-Charge
Battery state-of-charge (SOC) can be measured by an amp-hour meter, voltage or by specific gravity. Some care and knowledge is required to interpret state-of-charge from voltage or specific gravity readings. So we recommend amp-hour meters for all systems with batteries.
Amp-Hour Meters
An amp-hour meter is like having a “gas gauge” for batteries. It gives the user all the information they need to keep their batteries charged. At a glance the user can see system voltage, current and battery condition. (See the meter section for more information on amp-hour meters.)
Measuring Battery State-of-Charge
Battery voltage will vary for the same state-of-charge depending on whether the battery is being charged or discharged, and what the current flow is in relation to the size of the battery. The chart below will give you an idea of state-of-charge for various battery conditions in flooded cell lead-acid batteries. Voltage varies with temperature. While charging, a lower temperature will increase battery voltage. Voltage on a 12 volt battery is 0.9 volts higher at 32°F than at 70°F. While discharging, a higher temperature will increase battery voltage. There is little temperature effect while a battery is standing. (This information courtesy of Ralph Heisy, Bogart Engineering.)
Battery Condition |
Battery Voltage |
@ 77°F |
12 Volt |
24 Volt |
Battery during equalization charge |
Over 15 |
Over 30 |
Battery near full charge while charging |
14.4 to 15.0 |
28.8 to 30.0 |
Battery near full discharge while charging |
12.3 to 13.2 |
24.6 to 26.4 |
Battery fully charged with light load |
12.4 to 12.7 |
24.8 to 25.4 |
Battery fully charged with heavy load |
11.5 to 12.5 |
23.0 to 25.0 |
No charge or discharge for 6 hours - 100% charged |
12.7 |
25.4 |
No charge or discharge for 6 hours - 80% charged |
12.5 |
25 |
No charge or discharge for 6 hours - 60% charged |
12.2 |
24.4 |
No charge or discharge for 6 hours - 40%charged |
11.9 |
23.8 |
No charge or discharge for 6 hours - 20% charged |
11.6 |
23.2 |
No charge or discharge for 6 hours - Fully discharged |
11.4 |
22.8 |
Battery near full discharge while discharging |
10.2 to 11.2 |
20.4 to 22.4 |
Hydrometers
A hydrometer is very accurate at measuring battery state-of-charge if you measure the electrolyte near the plates. Unfortunately, you can only measure the electrolyte at the top of the battery. When a battery is being charged or discharged, a chemical reaction takes place at the border between the lead plates and the electrolyte. During charging, the electrolyte changes from water to sulfuric acid. The acid becomes stronger and the specific gravity rises as the battery charges. It takes several hours for the electrolyte to mix so that you get an accurate reading at the top of the battery. Always try to take readings after a long period of no charge or discharge.
State of Charge |
Specific Gravity |
100% Charged |
1.265 |
75% Charged |
1.239 |
50% Charged |
1.2 |
25% Charged |
1.17 |
Fully Discharged |
1.11 |
This chart gives state of charge verses specific gravity of the electrolyte. These readings are correct at 75°F.
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