State of charge

An alternative form of the same measure is the depth of discharge (DoD), calculated as 1 − SoC (100% = empty; 0% = full).

Another one is the BMW i3 REX (Range Extender version), where about 6% of SoC is reserved for PHEV-alike operations.

[1][6] To extend battery lifetime, extremes of state of charge should be avoided and reduced variations windows are also preferable.

In offline techniques, the battery desires to be charged and discharged in constant rate such as Coulomb-counting.

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.

[15] This method, also known as coulomb counting, calculates the SoC by measuring the battery current and integrating it in time.

The battery can be described with an electrical model which the Kalman filter will use to predict the over-voltage given the observed current.

The strength of this technique is that a Kalman filter adjusts its relative trust of the battery voltage and coulomb counting in real time.