Ni-MH Open Circuit Voltage vs State of Charge

Can we guess the state of charge of NiMH cells by just measuring their open circuit voltage?

According to some papers I read on internet it is possible. So let’s put it into test.

Test Method

Test object: Panasonic Eneloop for its consistency and low self-discharge
Currents: 1.0A charge, 0.5A discharge
Steps for constructing the upper voltage boundary:

Steps for constructing the lower voltage boundary:

Given more time the cell’s open circuit voltage would settle somewhere between the upper and lower boundaries.

Voc as a function of SoC

Voc as a function of SoC

SoC as a function of Voc

By swapping X and Y we get the inverse function - useful for estimating the state of charge when we know the open circuit voltage.

SoC as an inverse function of Voc


It is possible to estimate the state of charge of Ni-MH cells but the error margin seems to be too wide, almost impractical.