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I'm trying to estimate the initial state of charge for my battery (NiMh 9.6V @ 3300mAh) using the open-circuit voltage - state of charge discharge curve. I use an IC that measures the battery voltage/current and has coloumb counting capability.

The problem I'm having is estimating the inital SOC e.g when the SOC is 42% (~9.2V) and I charge the battery for 1 hour at 75mA, the actual capacity increases by 2% to 44% SOC however the voltage jumps to 9.6V which is considered fully charged. If I now estimate the SOC using the ocv-soc discharge curve, the SOC will be 100% which is wrong.

The situation I have requires me to use a voltage-based estimate to get the initial state of charge when the system starts up so I cannot use coloumb counting here.

How do battery fuel gauges use a measured battery voltage after it was discharged/charged to get an initial estimate of SOC? Do they plug the voltage into some kind of "resting" model?

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