# 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:

• discharge the cell to 0.9V
• charge a certain amount of mAh into the cell (in 100mAh increments), use as X coorditate
• measure the voltage of the cell after it rested 24 hours, use as Y coordinate
• plot the [X,Y] data points into graph

Steps for constructing the lower voltage boundary:

• charge the cell, terminate on 0dV
• discharge a certain amount of mAh from the cell (in 100mAh increments)
• measure the voltage of the cell after it rested 24 hours, use as Y coordinate
• distarche the cell to 0.9V, use the measured capacity as X coordinate
• plot the [X,Y] data points into graph

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

## 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. ## Conclusion

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