How to measure LiFePO4 battery state-of-charge (SoC)?

Mon Feb 26

Written by: Casey O.

Here are several methods to measure the SoC (state-of-charge) of a LiFePO4 battery. Each has trade-offs.

Voltage / Multimeter

If you have a tool to measure voltage, such as a multimeter, you can measure the voltage of a LiFePO4 battery. However, this it not the most accurate method. If the battery is currently being charged or discharged, this can affect the voltage you measure. So it’s best to disconnect the battery and let it rest for a while for a more accurate measurement. This isn’t ideal if you want to use the battery while also knowing how charged it is.

In addition, LiFePO4 batteries have a ‘flat voltage curve’, this means that unlike other battery chemistries, voltage doesn’t always vary much at different charge levels.

As seen in the above graph, a LiFePO4 battery with a 40% or 60% SoC will have basically same voltage. Temperature and battery age can also affect the voltage and throw off measurements.

Measuring voltage alone is not enough to get an accurate idea of the SoC of a LiFePO4 battery, especially with consumer tools. It is however useful to get a general idea if a battery is empty, full, or somewhere in between.

Dedicated battery monitor / Coulombmeter

A more accurate way to measure the SoC of a LiFePO4 battery is with a dedicated battery monitor. Specifically a device that includes a ‘coulombmeter’, which measures electrical charge passing through a conductor over time.

Battery monitors are electronic devices with components that you wire into your system. They allow you to set an initial SoC (usually 0 or 100%) and a capacity (usually in Ah), and going forward they will measure the current flowing in and out of the battery. They use this flow to calculate the SoC, adding to the SoC when charging, and decreasing when drawing current.

There are two main ways of measuring current that battery monitors tend to use. Some use a shunt, which you wire in series into your system (follow any instructions that come with the monitor). Shunts are resistors, and these monitors work by measuring the voltage drop across the resistor and doing a little math to calculate the flow of current. The other type use hall-effect sensors, which you thread your wire through, no need to cut any conductors. These work by measuring the magnetic field and using some math to calculate the current. Make sure any monitor you choose has the correct specs for your use case.

If a monitor doesn’t have a shunt or a hall-effect sensor, it probably just a voltmeter with a look-up table that reads the voltage and displays a SoC based on that voltage. Many sellers will happily sell you one of these for a few bucks and tell you they work for LiFePO4 batteries, however, as we’ve discussed voltage is not the most accurate way to check the SoC of a LiFePO4 battery.

There are a wide variety of battery monitors available with different interfaces and features such as Bluetooth or other connection protocols. I have a few, but none I like enough to recommend. If that changes some day I’ll update this article.