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Charging 2x NiMH 9v batteries in series

Discussion in 'Power Electronics' started by Builder_Of_Bots, Dec 31, 2016.

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  1. Builder_Of_Bots


    Dec 24, 2016
    Hi, I need 18v for a project I am working on. I have 2 NiMH batteries that will be wired in a series. So how do I charge them? My B6AC pro charger doesn't have 18v 2 cell setting. Can I modify a 9v wall battery charger to safely charge to 2 cell 18v battery? If not is there any other way to charge them? Thanks for any help.
  2. Alec_t


    Jul 7, 2015
    Can't you charge them separately?
    Whether or not a 9V charger could be modified would depend on the charger construction.
  3. Builder_Of_Bots


    Dec 24, 2016
    I was planning on putting them deep in my project. Isn't there someway to charge them both at the same time as a 2 cell battery?
  4. Rayregula


    Dec 20, 2016
    It would probably be harder to do then what your looking for. You would probably have to replace most of your 9v charger with new components and maybe do a little rewiring.
    Builder_Of_Bots likes this.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    It is theoretically no more difficult to charge two batteries in series than just one.

    NiMH chargers were all the rage in electronics magazines a decade or so ago. Modifying one to work at a higher voltage should be possible.

    On the other hand it might be easier to just use 2 cheap chargers (presuming they charge correctly using the delta-V method) or provide two charging sockets and charge them one at a time.

    Edit: I probably need to add that there are practical problems but they concern the batteries themselves. When charging multiple cells in series a weaker cell will often be both over charged and over discharged during a single cycle. Your 9V NiMH is already a battery, so you're stuck with this problem. If you make sure your 2 batteries are the same type and have the same initial state of charge (and cannot be discharged independently of each other) then you've done as much as I'd practically possible.

    Oh, that brings up a question. Are the batteries in series to provide a single supply voltage, or do they supply a double ended (+/-9V) power supply? In the latter case there exists the potential for the batteries to be discharged unevenly which would be a big no-no for series charging.
    Last edited: Dec 31, 2016
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