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I'm developing a power controller which monitors and recharges a 9.6V NiMH battery pack (8x 1.2V cells @ 3300mAh) which is used to supply power to a board when mains supply is lost. The controller is able to use a trickle charge of 75mA and eventually will be able to monitor the remaining battery capacity.

The controller has to charge the battery to an "acceptable level" so that it has enough power to supply to the board (the board draws 12V from the mains supply) for at most ~30 seconds each time mains supply is lost.

My question is what would be a good method of charging an NiMH battery without damaging it? My original idea was on power up, the controller checks the remaining capacity of the battery and if it's less than 90% (e.g starts the trickle charge until it's at ~100% and then stops). However I read that an NiMh battery shouldn't be recharged until it's nearly empty according to: NiMH or Li-ion trickle charge?

Can anyone recommend a charging method for my situation?

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I decided to start charging the battery if it's <30% charge and stop charging it once it reaches 90% charge.

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  • \$\begingroup\$ Starting a charge at 1.2V (around 10%) and stopping a charging at 1.5V per cell could do the trick. Detecting 30% remaining ( should be around 1.23V per cell) is really hard as that is flat area of discharge curve and exact voltage varies greatly with cell age and quality. \$\endgroup\$
    – Volodymyr Kalinyak
    Nov 3 at 21:47

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