How to measure battery State-of-Charge (SOC)

Essentially, battery state-of-charge (SOC) can be easily measured using an amp-hour meter, by voltage, or by specific gravity. Measuring SOC by voltage or specific gravity requires specialized knowledge and a great deal of care, so it is recommended that owners utilize amp-hour meters for all battery systems.

An amp-hour meter is somewhat like a fuel gauge for batteries, providing all the information necessary to keep the batteries charged. While using an amp-hour meter, users can see the system’s voltage, current, and battery condition.

Midnite Solar battery capacity meter
Midnite Solar battery capacity meter

The voltage of a particular state-of-charge will vary depending on which state the battery is in; charging or discharging. Also affecting the variation is the current in relation to the size of the battery. It is important to remember that voltage will vary with temperature.

While the battery is charging, a lower outdoor temperature will increase battery voltage. For example. the full-charge voltage on a 12 VDC battery is 0.9 VDC higher at the lower temperature of 32℉ than at 70℉. The opposite is true for discharging. While discharging, a higher outdoor temperature results in higher voltage.

While a battery is idle, the temperature has little effect, although very high temperatures may increase the self-discharge rate.

A hydrometer may also be used to measure battery state-of-charge in flooded lead-acid batteries by measuring the amount of electrolytes near the plates. However, the electrolytes can only be measured at the very top of the battery, which may or may not be near the plates.

To get an accurate reading from the hydrometer, take the readings after the battery has been idle for a long while, or after several hours of the battery slowly discharging. Near the end of the charging cycle,  gas bubbles rise through the sulfuric acid. It takes a few hours to mix enough to get an accurate reading with the hydrometer.

Whether a hydrometer or an amp-hour meter is used to measure battery state-of-charge, using them will allow you to determine where your batteries are in the stages of charging or discharging.