State of Charge in Electric Vehicles

State of charge (SOC) is the equivalent of a fuel gauge for the battery pack in a battery electric vehicle (BEV), hybrid vehicle (HEV), or plug-in hybrid electric vehicle (PHEV). The units of SOC are percent points (0% = empty; 100% = full).

Determining SOC

SOC cannot usually be determined directly. In general there are four methods to determine SOC indirectly:

• chemical
• voltage
• current integration
• pressure
• measuring impedance

Chemical method

This method works only with batteries that offer access to their liquid electrolyte, such as non-sealed lead acid batteries. The specific gravity or pH of the electrolyte can be used to indicate the SOC of the battery.

Voltage method

This method converts a reading of the battery voltage to SOC, using the known discharge curve (voltage vs. SOC) of the battery. However, the voltage is more significantly affected by the battery current (due to the battery's electrochemical kinetics) and temperature. This method can be made more accurate by compensating the voltage reading by a correction term proportional to the battery current, and by using a look-up table of battery's Open Circuit Voltage vs. Temperature.

In fact, it is a stated goal of battery design to provide a voltage as constant as possible no matter the SOC, which makes this method difficult to apply.

Current integration method

This method, also known as "coulomb counting", calculates the SOC by measuring the battery current and integrating it in time. Since no measurement can be perfect, this method suffers from long-term drift and lack of a reference point: therefore, the SOC must be re-calibrated on a regular basis, such as by resetting the SOC to 100 % when a charger determines that the battery is fully charged (using one of the other 3 methods described here).

Pressure method

This method can be used with certain NiMH batteries, whose internal pressure increases rapidly when the battery is charged. More commonly, a pressure switch indicates if the battery is fully charged. This method may be improved by taking into account Peukert's law which is a function of charge/discharge rate or ampere.

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