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.
1. Measure the battery’s chemistry (specific gravity) with a hydrometer (accurate method).
2. Measure its voltage with a volt meter while open-circuit, no load (general approximation).
3. Track the current flow in and out of the battery with a ‘shunt’ and associated metering circuit (common with alt-energy systems).
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I put together the following battery state of charge 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.
The chart is listed below. But first, a few important notes and caveats…
How I Made The Battery State of Charge Chart
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.
Battery Voltage Measurements Are Just Approximation
Note: Voltage measurements are only approximate to determine SOC. Measuring battery voltage is NOT the most accurate way to do this (there are variables to consider). But a good generalization. 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.
Measure While “At Rest” “Open Circuit”
Note: For best accuracy when measuring battery voltage, the battery must be in ‘open circuit’ condition (at rest, or ‘resting’). 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.
Battery Voltage vs Temperature
Note: Battery voltages are temperature dependent. In fact, good charger systems (alt-energy 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.
Keep Above 50% State of Charge
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.
This is NOT the Charging Voltage
Note: The 100% voltage is NOT the recommended charging voltage (which will be higher, and multi-stage). See your battery manufacturer recommendations regarding charging.
BATTERY STATE OF CHARGE CHART
Open circuit voltage, or specific gravity per cell
If you know your exact battery, you might find related voltage information on their website. But the chart above will provide a general view for state of charge.
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