RV FEATURE

Measuring the SOC using battery voltage is a bad idea .

You ’ ve probably seen a chart similar to the one here showing the approximate SOC of a battery depending on the voltage measured across the terminals . While this is a reasonable indication of the SOC of a lead acid battery , it ’ s really only a guide and , for lithium batteries , it is almost completely useless .

The problem is , there are several things that can affect the resting voltage of a lead acid battery . These include ambient temperature , time since last fully charged , any load on the battery , and the general condition / age of the battery . Any one or a combination of these can throw the true SOC out the window .

Another problem can occur when a remote voltmeter is being used to measure the voltage . Depending on the length of wiring between the battery and the display , these can be affected by voltage drop along the wiring , reducing the displayed voltage by 1-2V . This may not sound like much but if you look at the chart again , 1V makes a big difference to the assumed SOC .

As for lithium batteries , they tend to give a consistent voltage right up until the last 10 per cent of their SOC . This means you can be lulled into a false sense of security , thinking your battery has a lot of charge left , only to find it goes completely flat in a very short span of time .

If you intend to do a lot of freecamping and need to rely on charged batteries , you should seriously consider fitting a shunt if your system doesn ’ t already have one .

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