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Replacing Nickel metal hydride by Lithium ion.

Discussion in 'Troubleshooting and Repair' started by dimitristephan, May 21, 2012.

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  1. dimitristephan


    May 14, 2012
    Hello people,

    My old black and decker electric screwdriver had lost a lot of torque and speed after a

    couple of years of use, so I decided to replace the 3 x AA batteries that were connected in

    series by one 18650 lithium Ion battery of the same voltage rating but higher

    capacity. However, since the battery is not protected against overcharging/discharging,

    leaving it on the charging base has actually caused the battery to get overcharged to

    around 4.6 V. The nominal voltage of such a cell is 3.6 V, but are generally charged to 4.2

    V. For some reason, the charger is not stopping the charge at 4.2 V. Could it be that the

    battery has a lower internal resistance than 3 AA's in series? There is a serious risk of fire

    and/or explosion if I do not find a way to limit the terminal voltage to 4.2 V. Is there any

    way I can do so? Your input is much appreciated.

    Thank you!
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    It is not charging properly because the charger is not designed for LiPo cells.

    You're lucky you didn't start a fire.

    Get a LiPo charger.

    Note that the protection of LiPo cells doesn't protect the cells, it protects you. The cells are still likely to be damaged, but they probably won't burst into flames.
  3. CocaCola


    Apr 7, 2012
    Keyword "probably" :cool: They could still have a major meltdown even if flames never materialize...
  4. dimitristephan


    May 14, 2012
    I already have a Li-ion charger, but the thing is, the base has its own circuitry that converts 12

    V AC into 12 V DC since the external AC adapter cannot charge the battery on its own.

    Also, I cannot get any voltage reading when placing the leads of my DMM on the charging

    pins of the screwdriver.

    Should I remove all the inner components and re-wire everything so that i can directly plug

    in a 4.2 V DC charger?

  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    That would probably be the safest thing to do.
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