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Thread: Replace old Ni-MH battery with a higher capacity Li-ion one...

  1. #1

    Default Replace old Ni-MH battery with a higher capacity Li-ion one...

    Hi

    I have a manual crank dynamo rechargeable LED flashlight (no brand or anything) bought 3 years ago which uses a 3 series cell 3.6V 300 mAh NiMH battery (each cell is a bit larger than LR44 button cells, wrapped in delta shape) but it is old and doesn't last more than 5 minutes without the LED dimming considerably (I crank it every time). I want to replace it with a 750 mAh 3.7V Li-ion prismatic battery from a dead cell phone but the battery is good and would make that flashlight last longer even when it was new.

    Could that battery be safely used (and recharged) in that flashlight? I know that Ni-MH and Li-ion chargers use different cell voltage, but being the flashlight's original battery pack nominal 3.6V, being very close to Li-ion ones (3.7V), it might be somewhat compatible I think, but I would like to hear more opinions.

    Many thanks.

  2. #2
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    Default Re: Replace old Ni-MH battery with a higher capacity Li-ion one...

    Hello Derek10,

    Go for it. The only question is how your charger terminates its charge. You could end up overcharging the Li-Ion cell.

    In general 1 Li-Ion cell replaces 3 NiMh cells in series, so in theory it should work. The details may be a problem because while there is a self discharge rate associated with NiMh cells, and they can handle some degree of overcharging, Li-Ion cells can't. If you can figure out how the charge terminates, you will have a better idea if the replacement will work properly.

    Tom
    Behind every Great man there's always a woman rolling her eyes...

    Most batteries don't die - they are tortured to near death, then murdered...

  3. #3

    Default Re: Replace old Ni-MH battery with a higher capacity Li-ion one...

    Hi and many thanks Silver fox!

    First of all the flashlight's NiMH battery is 80 mAh and not 300 as I erroneously stated, so the difference would be even greater (80 vs 750 mAh), also the Li-ion battery is a bit larger and seems it wouldn't fit but that is secondary at the moment and I have some solutions like gluing it to the flashlight body or moving all the inside to another shell. Also it was branded but it is peeled due to the hand's wear and couldn't figure it, and it was expensive and uses a relatively big and bright LED.

    I don't think it will terminate the charge, maybe only limit the alternator voltage and current up to 4.2 V/80 mA(?) if I crank it too fast, but I will check it anyway.

    Also could the protection PCB of the Li-ion battery (it is protected) stop the charge even if current still flows from the charge circuitry when it reaches 4.2V?

    So, could the solution be, if not termination is present, to make partial charges, taking account the bigger battery would need more crank turns to fully charge it, or, to watch the battery voltage regularly when charging and stop turning the crank when it reaches 4.2 V?

    Many thanks

  4. #4
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    Default Re: Replace old Ni-MH battery with a higher capacity Li-ion one...

    Cheap-ish crank flashlights 'feed' battery with non-filtered rectified AC, with huge ripple therefore, and have no voltage limits. So they can severely damage (potentially - leading to physical damage, such as venting/explosion) a Li-Ion

  5. #5

    Default Re: Replace old Ni-MH battery with a higher capacity Li-ion one...

    That's what I was thinking too. Sounds like a bomb in the making. Here you go buddy, crank this up, lol. But I only know a little about li-ion and most of it was read here on CPF.
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    Flashaholic* srfreddy's Avatar
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    Default Re: Replace old Ni-MH battery with a higher capacity Li-ion one...

    Hm... I have a cheap crank flashlight that uses a li-ion CR3025 battery... is that bad?

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    Default Re: Replace old Ni-MH battery with a higher capacity Li-ion one...

    If it was made with Li-Ion's in mind, it most likely has proper (in li-ion terms) charging circuitry. So don't worry.

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    Flashaholic* fivemega's Avatar
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    Default Re: Replace old Ni-MH battery with a higher capacity Li-ion one...

    I won't believe that any low price NiMH crank flashlight has any kind of charging controll. So I would go with protected Li-Ion cell.
    This way, at least your battery won't overcharge but possibly may overdischarge because it drains by low consumption LED.
    So, I suggest to run time test the full charged protected cell down to safe side then crank to recharge before voltage goes down to unsafe side.
    Last edited by fivemega; 07-10-2012 at 03:21 PM.

  9. #9
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    Default Re: Replace old Ni-MH battery with a higher capacity Li-ion one...

    I would just buy another cheap dynamo torch and scrap the old one , rather than mess about ...

    Unless you really actually want to mess about with the oldie , in which case I would still buy another new one anyway.
    .

  10. #10

    Default Re: Replace old Ni-MH battery with a higher capacity Li-ion one...

    It shouldn't be a big problem, because dynamo couldn't give a very strong current. It is like charging the Li-ion battery with a constant current.

    I think that something made to not damage a 80mAh NiCd battery can't damage a 750mAh Li-ion. The battery will likely work under-charged, with a long life.
    Last edited by Datman; 07-16-2012 at 02:21 AM.

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