The ‘State Of Charge’ (SOC) of a battery is a measurement of how much energy is remaining (percent). It’s like a fuel gauge. Measuring and knowing the SOC of a battery or battery bank is useful when applying towards alternative energy, or any other situation where you need to know its condition.
There are several ways to determine a battery’s SOC, including measuring a battery’s chemistry (specific gravity), its voltage (volt meter), or tracking the current flow in and out of the battery with a ‘shunt’ and associated metering.
I put together the following chart which indicates the state-of-charge (percent) as it relates to battery voltage or specific gravity. Voltages and Specific Gravity are listed for a 6-volt or 12-volt battery, and battery banks of 24 and 48 volts.
Here’s the chart, and a few notes…
(UPDATED) I made a first version of the chart several years ago for my own interest when I first put together a small off-grid solar power system. This latest version includes several battery bank voltage scenarios for quick reference.
How I determined the voltage values: I researched as many battery manufacturers that I could find regarding their own published SOC data. Some were slightly different from each-other with regards to their SOC values, however I averaged all of them together to come up with a chart which represents what I believe to be a good general indication.
Note: Voltage measurements are only approximate to determine SOC, and measuring battery voltage is NOT the most accurate way to do this (there are variables under varying circumstances). A more accurate method is to measure the specific gravity of each cell within the battery, however for many batteries this is difficult or impossible (AGM batteries, for example). Many (most) alt-energy systems incorporate a DC-shunt which keeps track of SOC by monitoring the current flow in and out of the battery or battery bank, which is a very accurate way to track state-of-charge.
Note: For best accuracy when measuring battery voltage, the battery must be in ‘open circuit’ condition (at rest). This means that 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 more accurate measurement you should wait 6 hours up to 24 hours.
Note: Battery voltages are temperature dependent. In fact, good charger systems have temperature compensation built-in. The voltage data in the chart below is that of manufacturer spec. sheet listings (vicinity of room temperature).
Note: If testing specific gravity (deep-cycle flooded/wet batteries), when drawing a sample from the battery, fill and drain the hydrometer several times first before settling upon a measurement.
Note: For longer battery life, batteries should remain in the green zone (40% or more SOC). Occasional dips into the yellow may not be harmful, but continual discharges to those levels will shorten battery life considerably. Generally speaking, the less you discharge the battery before recharge, the longer the battery will last. Most alternative-energy systems are designed to keep the battery bank at least 50% or higher.
Note: The 100% voltage is NOT the recommended charging voltage (which will be higher, and multi-stage). See your battery manufacturer recommendations regarding charging.
STATE OF CHARGE
Open circuit voltage, or specific gravity per cell
Here’s a highly rated little ‘trickle’ charger for maintaining a 12-volt battery while not in use:
(winter storage of riding mower or tractor battery, etc..)
‘Battery Tender Junior’