Battery “State Of Charge” Chart

June 18, 2013, by Ken Jorgustin

12-volt-battery-voltage

If you want to know the approximate State Of Charge (SOC) of a 12-volt battery (or 6-volt, or other battery bank combination), I put together a chart which indicates the percentage charge as it relates to battery voltage or specific gravity.

Here’s the chart, and the reason I did it…

 
Actually, I made a first version of the chart several years ago when I first put together an off-grid solar power system and needed to know this information to maintain my batteries in an optimum condition. This latest version includes several battery bank voltage scenarios for quick reference. My solar power system charges a 48-volt battery bank, while my RV battery bank is made up of 12-volt batteries in parallel.

I had recently installed a new charger/inverter system for my 5th-wheel trailer, and while I am currently on an excursion up in the White Mountain region of New Hampshire I have been giving it a real-world test for the first time, and have compared my chart to actual conditions. So far, so good…

 

STATE OF CHARGE

Open circuit voltage, or specific gravity per cell

battery-state-of-charge

Download full size PDF

Notation regarding measuring the battery voltage…

The battery must be in ‘open circuit’ condition, or in other words… the battery must NOT be under load and it must NOT be charging. To be somewhat accurate, the battery should be in that condition for an hour or two before taking a measurement, while for a much more accurate measurement you should wait 6 hours up to 24 hours.

If testing specific gravity (deep-cycle flooded/wet batteries), fill the hydrometer 2-3 times before drawing a sample from the battery.

For longest life, batteries should stay in the green zone (40% or more SOC). Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably. It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity, but in many batteries this is difficult or impossible.

There are more sophisticated methods and systems to measure the state of charge, such as one which uses a battery ‘shunt’ which actually keeps track of the current flow out of the battery as well as the charging current that flows back into the battery. The system I’m currently using in the RV is built by Magnum Energy (Made in USA), and tracks the current flow in and out of the battery bank as well as voltage, temperature compensation, efficiencies, and more…

Although you can get pretty technical with all this, the voltage chart versus percentage state of charge will give you a pretty good indication assuming that your battery or battery bank is in good condition.

A side note… if using batteries for an RV or trailer, you should NOT use ‘flooded/wet’ batteries which will out-gas. Instead, consider AGM batteries (Absorbed Glass Mat) which are sealed, spill proof, maintenance free and have the electrolyte absorbed in a fiber-glass mat between the plates.