We are often asked if it is safe to replace the original Ni-Cd batteries (also labeled Ni-Cad or NiCd) with a newer higher capacity Ni-MH (also labeled NiMH) type as an upgrade. For older toothbrushes that were previously equipped with Ni-Cd batteries, this usually results in significantly longer run times. NiMH batteries of the same physical size can have a charge capacity 2 to 4 times higher than older NiCd batteries.
The official recommendation is to use like replacement like only, as this is the safest and guarantees that the batteries will charge properly.
However, at your own risk, you can usually replace NiCd batteries with NiMH batteries because the capacity is much higher and therefore they can run longer between charges. The voltage is the same, so they will run the motor in exactly the same way. There are a few things to keep in mind about charging.
If your device typically takes 12 to 24 hours to charge the original battery, then it has a slow charging circuit. This is the best case scenario for upgrading to Ni-MH, as you can remember to unplug and stop charging after this time. Any overcharging will be minimal and overheating will not normally occur at low currents. Don't let it charge permanently between uses.
This is where the charging circuit typically takes 1 to 4 hours to charge the original battery and automatically shut down. Upgrading to NiMH batteries is more risky because the automatic shutdown of the high current charging circuit may not be triggered at the correct location on the NiMH battery. Due to the higher current, the battery may overheat and expand. This remains valid as long as you remember to manually unplug and stop charging after the usual time, then at your own risk. However, it is safer to stick with NiCd batteries in this case.
Note also that if, for example, a newly installed NiMH battery has a capacity 2 times higher than the original NiCd battery, then it is theoretically possible to increase the constant current charge time by a factor of 2 to achieve a full charge. You will need to do your own calculations and schedule the charge time appropriately.
NiCd batteries handle better and repeated overcharging does not really affect NiCd batteries. The overall life of NiMH batteries will be shortened if they are overcharged repeatedly or for long periods of time. Allowing any kind of constant charging between uses will shorten their life.
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