We often get asked if it is safe to replace original Ni-Cd batteries (also labelled Ni-Cad or NiCd) with the newer, higher capacity Ni-MH (also labelled NiMH) types as an upgrade. This can often yield significantly longer run times for older Braun Oral-B toothbrushes which were previously fitted with Ni-Cd batteries such as the Vitality, Sonic Complete and Advance models. Charge capacities of Ni-MH batteries of the same physical size can be 2 to 4 times higher than the older Ni-Cd type.
The official advice is to only replace like with like as this is safest and the batteries are guaranteed to charge correctly.
However, at your own risk, you can often replace Ni-Cd batteries with Ni-MH ones as the capacity is much higher so they will run for longer between charge-ups. The voltages are the same so they will run the motor in exactly the same way. There are a few points about charging to bear in mind:
Ni-Cd batteries handle this much better, and repeated over-charging will not really affect Ni-Cd batteries. Ni-MH batteries will have their overall lifespan shortened if they are repeatedly or long-term over-charged. Leaving either type on continuous charge between uses will shorten their lifespans.
If your equipment normally takes 12 to 24 hours to charge the original batteries then it has a slow charge circuit. This is the best situation for upgrading to Ni-MH as you can remember to unplug and stop the charge after this length of time. Any over-charging will be minimal and at a low current so overheating would not normally occur. Don’t leave it on permanent charge between uses.
This is where the charging circuit normally takes 1 to 4 hours to charge the original battery and automatically shuts off. It is riskier to upgrade to Ni-MH as the automatic shutoff for the high current charging circuitry might not trigger at the right point on the Ni-MH battery. Because of the higher current the battery can then overheat and swell up. As long as you remember to unplug and stop the charge manually after the usual time then, at your own risk, this will still work. However, it is safer to stick with Ni-Cd batteries in this situation.
Also note that if the newly fitted Ni-MH battery is, for example, 2 times higher capacity than the original Ni-Cd battery then the constant-current charge time can theoretically be increased by 2 times to allow for a full charge. You will need to calculate this for yourself and time the charging appropriately.