The rechargeable battery in my electric toothbrush is on its last legs and needs to be replaced.
The current battery is a tagged NiCd jobbie, and gets charged by some non-contact method (presumably induction) when the brush is sitting its holder.
The only possible replacement batteries I can find are NiMH rather than NiCd. If I fit an NiMH battery of suitable capacity, is the existing charging mechanism likely to work ok with it?
TIA.
Hello Roger....
(welcome back by the way!)
Depends a bit on the crudeness of the charger. If its a slow trickle charger, then it will probably be fine. A modern fast charger with proper charging control IC will also likely be ok since most do NiCd and NiMh. The difficult ones would be the custom designed fast charge circuits that may not detect the end of charging correctly.
Thanks - it's nice to be missed! I've been in Australia for a month with no access to Usenet. I didn't give advance notice of my absence in case the info got into the wrong hands!
I guess it's a fairly crude trickle charger, so will probably be ok. The current battery says that normal charge is 14 hours at 100mA - and there's no charge light on my brush (although some of the more up-market models covered by the instruction leaflet *do* have charge lights).
I did this a while ago and there were no problems for the 12 months or so before something more terminal failed. It's a slightly tedious job because of the need to unsolder several bits to get the battery out.
Dave
Chances are its probably not even a real constant current charger. Some are so basic as to be literally a transformer, diode and resister to feed half wave rectified AC into the cell with some current limiting. Given the low charge rate you can probably charge the cell indefinitely without doing any damage.
Yes, the charge current is way below the max for the nimh, so cell treatment gets less critical. Best to only charge it when needed rather than leave it on all the time. Ensure proper waterproofness, or it can die quickly.
NT
Thanks to all who replied with useful advice.
After all that, I've discovered that you *can* still get tagged NiCd batteries in AA size - so I have bought one of those (from Maplin's new shop in Stratford-upon-Avon) in order to avoid any possible problems with NiMH batteries.
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