Lithium cells discharge in a very different way to older battery technologies which means that the State of Charge (SOC) cannot be calculated by simply measuring the terminal voltage.
To get an accurate State of Charge different technologies have to be employed which when configured correctly are highly effective.
Read on to see how they work.
Why would you need a State of Charge Gauge?
State of Charge indication is really important for a few reasons but mainly to give the user an indication of when the battery is about to stop supplying power.
They can also be used to monitor the health of the cells by giving an indication of the time it takes to charge and discharge for a given load.
Lithium Cells do not like being run past their minimum cell voltage so a SOC gauge gives warning when the battery is approaching this point although the BMS should disconnect the load before this happens for protection.
How does a State of Charge work?
State of Charge Gauges work in 1 of 2 ways.
1: Simple Voltage Measurement
2: Coulomb Counting
Simple Voltage Measurement can only accurately work if the output voltage profile of the battery is linear in relation to the amount of charge left in the battery.
Lead Acid has a profile similar to this but it is still not truly accurate.
Coulomb Counting is a far more accurate method with accuracies typically of around 1%.
The Coulomb counting circuit actually “counts” the amount of energy leaving and entering the battery or cell, and based on the known capacity it is then able to accurately calculate the amount of charge remaining.
Altertek produce a range of State of Charge Gauges all based on highly accurate Coulomb Counting Technology. These can be user configured for a known range of common battery sizes and can be used with an external shunt for higher currents.
They also incorporate other user friendly features such as low voltage indication, power saving modes and fault indication.