Does a dead 1.2V rechargeable AA NiCad battery act as an open or

  1. Does a dead 1.2V rechargeable AA NiCad battery act as an open or closed circuit?

    My electric toothbrush's rechargeable battery recently died and the manufacturer expects me to replace the entire unit. Personally, I think that's a waste of money so I've already opened it up and I can see the battery. However, instead of simply replacing the battery, I'd rather hook it up to an external power supply so that I will never have to worry about the battery again. It is easy for me to hook it up in an external power supply but the problem is that the battery is housed pretty tightly inside the unit and therefore, I am trying to avoid removing the battery because I might break the housing.

    Once again, I am planning to leave the battery where it is, drain the battery so that it is completely dead, and hooking up the external power supply. This will only work if the battery acts as an open circuit when it is dead. Does a dead 1.2V rechargeable AA NiCad battery act as an open or closed circuit?
     
  2. jcsd
  3. jim hardy

    jim hardy 4,511
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    IF you are considering connecting something that goes into your mouth to a wall outlet,,
    That's placing an awful lot of faith in the insulating ability of your "external power supply" .

    I'd not be a party to that adventure.

    Find a placement battery .

    http://batteryuniversity.com/learn/article/how_to_repair_a_battery_pack

    A quick search on "AA Nicad Replacment " turns up places that sell them, some for for as little as eighty five cents. Going rate looks to be two bucks.
     
    Last edited: May 17, 2014
  4. No way you want to use a power supply. Danger danger danger. For a bunch of reasons.
     
  5. Try it. Come back and tell us what you see when your eyes light up. Ok, maybe that was a bad joke but posters previous to my post have very valid points. This is a safety concern for sure.
     
  6. OmCheeto

    OmCheeto 1,944
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    Not really relevant, but yes, definitely try it, and have a video recorder running, and give someone a key to your room, so when your toothbrush device explodes, and/or electrocutes you to death, it will get lots of shares on You-Tube, and you will surely be the Darwin Award winner for 2014.*

    (ref)

    ps. The answer to your question depends on how your Ni-Cad battery failed. Some just get old and become a load, and some become open.

    For a full explanation of why your question is difficult to answer, please see the Electropaedia document on Why Batteries Fail.

    -------------------------
    * Though you have some already accomplished competition. Ouch.
     
  7. The external power supply is just a wall wart. The only way I can see that I will be harmed is if water somehow ends up near the electric socket. You take the same risk with any battery-powered toothbrush anyway because you need the charger connected to an electric socket.

    Anyways, it looks like I can't risk leaving the battery in there so I removed it.
     
  8. jim hardy

    jim hardy 4,511
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    not if it disconnects from the charger when you pick it up to brush your teeth.
     
  9. Why so?

    Is it because the circuit is open when it is disconnected? My model actually uses inductive charging so as far as I know, the circuit is always closed.
     
  10. sophiecentaur

    sophiecentaur 13,401
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    I think you will find (it happened with three of mine) that, when you place the brush on the charger, the motor will not run; the battery is disconnected.
    Have some fun fitting a replacement battery (chop it out with a dremmel or similar and pot another one in with araldite) or just use a regular tooth brush.
     
  11. jim hardy

    jim hardy 4,511
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    Yes. When you lift the toothbrush out of the charger you disconnect it, removing path for conduction from wall outlet into toothbrush..




    Over what distance do you suppose that inductive charging will work? My guess is about 1/4 inch.
    So removing the toothbrush from the charger removes the path for transmission of energy.

    But what has that to do with anything ? You're planning to replace the inductive charger and the toothbrush battery with an external power supply, presumably connected to a wall outlet.

    You've not demonstrated any understanding of electrical safety. Try a search on "medical electronics isolation"
     
    Last edited: May 20, 2014
  12. sophiecentaur

    sophiecentaur 13,401
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    Electronic toothbrushes have no electrical connection to the charger. They use magnetic induction, via a short stub which goes into the bottom of the toothbrush. The charger is designed to be waterproof and safe for operation in Zone 2 at least, I am sure (on the shelf above the basin, for instance) - possibly not actually IN the bath, though haha.

    The idea of using a direct electrical connection, with home made construction techniques, is, quite frankly, barmy! All this to save the price of a manual toothbrush (a very obvious safe alternative).
     
  13. jim hardy

    jim hardy 4,511
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    Thanks - i never owned one so didn't know.. That's with good reason i'm sure. So such a connection shouldn't be added.
     
  14. sophiecentaur

    sophiecentaur 13,401
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    So a gummy smile back at you Jim!!!
     
  15. NascentOxygen

    Staff: Mentor

    As a NiCd ages it usually develops a needle of metal bridging its electrodes, and it becomes a short circuit. You can confirm this by noting how it died. It was working okay one day, then next day it was dead and now can't be recharged?
     
  16. sophiecentaur

    sophiecentaur 13,401
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    I remember reading somewhere (Wireless World??) that you could try a burst of very high current charging to burn up those crystals and almost restore an ageing Nicad. Just reduces the capacity.
     
  17. NascentOxygen

    Staff: Mentor

    Yes. In fact I typed that in my post, then erased it, not wishing to encourage the poster to try something that could explode the cell. ("Short duration" is wide open to interpretation.)

    I have used it successfully. But where there is one needle of nickel, there are likely to be more, so it's at best a temporary fix.
     
  18. Alright, I think I'll just use a manual brush. I figured the idea of this project was pretty cool.
     
  19. jim hardy

    jim hardy 4,511
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    I for one am relieved to hear that.

    Thanks !

    If you're really interested in "beating the system"
    The junkshops are chock full of cordless drills that've outlived their expensive batteries. A 12 or 14 volt one modified by adding a nice long, heavy cord (big speaker wire?) attached to a car lighter plug or battery clamps can be mighty handy for yard projects. It'll run from your riding lawnmower battery.
     
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