Battery: Relation between voltage and state of charge

How to convert voltage of a Li ion battery to soc

To convert the voltage of a Li-ion battery to state of charge (SoC), you need to use a battery model that takes into account the relationship between voltage and SoC. One common battery model used for Li-ion batteries is the open-circuit voltage (OCV) method.

The OCV method uses the battery’s voltage and its known open-circuit voltage to estimate its SoC. Here are the general steps for using the OCV method:

Measure the battery’s voltage (V).
Determine the battery’s open-circuit voltage (OCV) at full charge and at empty. The OCV can be obtained from the battery manufacturer’s data sheet or by measuring the battery’s voltage when it is not connected to any load after a period of rest.
Calculate the difference between the measured voltage and the OCV at empty, and divide by the difference between the OCV at full charge and the OCV at empty. This gives you the fraction of charge (F) that the battery is currently holding: F = (V – OCV_empty) / (OCV_full – OCV_empty)
Convert the fraction of charge to SoC by multiplying it by 100: SoC = F x 100

Now lets see an example

Measure the battery’s voltage: V = 3.65 V
Determine the OCV at full charge: OCV_full = 4.20 V and OCV at empty: OCV_empty = 3.00 V (values obtained from the battery manufacturer’s data sheet).
Calculate the fraction of charge: F = (V – OCV_empty) / (OCV_full – OCV_empty) = (3.65 – 3.00) / (4.20 – 3.00) = 0.71
Convert the fraction of charge to SoC: SoC = F x 100 = 0.71 x 100 = 71%
So in this example, the Li-ion battery’s voltage of 3.65V corresponds to a state of charge of 71%.