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.
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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.
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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.
It's extremely likely that your headphones use a dumb, trickle-type charger to charge the batteries. It probably takes like 5-10 hours to charge them, right?<BR><BR>The dumb charger consists of a wall-wart, a diode, and a resistor. There is no peak detection or end-of-charge termination - it simply charges the batteries indefinitely until you unplug the charger. The voltage of the batteries approach the input voltage, so the charge current eventually tapers off to a very low value. NiCD batteries can handle this type of charge realtively well (though proper peak-detgection charging is better).<BR><BR>NiMH batteries, on the other hand, should not be charged with this type of method. First, they do not take continuous overcharge as well as NiCDs - doing so will significantly shorten their lifespan. Plus, all things being equal, the voltage across a NiCD is slightly higher during charge than a NiMH. For instance, a NiCD may peak at around 1.6V/cell, whereas a NiMH will peak at around 1.5V/cell during charge. So if you try to float charge a NiMH like a NiCD, the charging current will be higher, and it will taper off to a higher charging current than the NiCD, overcharging the NiMH cell even more, damaging it even faster.<BR><BR>So your best bet is to get a set of NiCDs as a replacement. They really shouldn't cost you more than $1 each. Your other option is to use NiMHs, but charge the batteries outside the headphones in a proper NiMH charger.
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