Hi everyone! I'm trying to find some detailed theory / information regarding a specific phenomenon that occurs with NiMH (and likely other) batteries.
I imagine many of us are familiar with the typical "S"-shaped discharge curve that is mentioned in basic battery theory and battery datasheets: as we start discharging a fresh battery, the voltage starts off high (around 1.3-1.4V for NiMH), then drops rapidly until we reach the nominal voltage of about 1.2V, then stays quite flat throughout the middle of the discharge, and it starts falling off more and more rapidly towards the end, culminating in a sudden crash (around 1.0V) when the battery is depleted.
The thing is, the battery only behaves this way if the discharge is uninterrupted. I've noticed that if I stop the discharge partway through, let the battery rest for a few hours, then start the discharge again, the battery will have recovered some of the "lost" voltage. In effect, instead of continuing from its last reported voltage, the battery will recapitulate the start of the discharge curve, except much faster. About 15 minutes into the discharge, the voltage will return to its normal trajectory.
Clearly the battery is not actually recharging during the pause - it must be then that the voltage of the battery is not a monotonic function of the state-of-charge (however nonlinear) but also depends on some other phenomena that can temporarily boost it.
Does anybody know of some resources with in-depth theory or explanation of the latter part, i.e. why does the battery appear to recover voltage during breaks, and maybe a model predicting how this occurs? I'm not having luck searching for "NiMH voltage recovery", ther results are mostly about resurrecting broken NiMH cells.