Charging 10 NiMh in series

Discussion in 'The Projects Forum' started by MattP, Jan 14, 2013.

  1. MattP

    Thread Starter Member

    May 21, 2012
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    Hi everyone, I'm building a portable speaker, and I want to build a rechargeable battery pack for it, using 10x 2500mah nimh batteries.

    I really don't know much about this, so I need someone to let me know if I've got things right.

    I want the speaker to be usable whilst it is charging, and ideally the charge time would be about 10 hours.

    From what I understand, all I need to do is connect the battery to a DC current, and limit it (with a single resistor?) to about 250ma, which should be safe for trickle charging. Is this literally all I need to do?

    What should the input voltage be? Thanks.
     
  2. takao21203

    Distinguished Member

    Apr 28, 2012
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    Why do you want to wait 10 hours?

    Also a resistor charge won't always give the same current.

    Better use a charging IC or a constant current source.

    The voltage has to be 1.5 volts for each cell.

    If there is one bad cell all others will be affected. Most commercial NiMH chargers only charge 2 cells in series. The modern chargers only charge single cells each.

    After a while for sure one cell will go bad first. Depends on the brand but it could be less than 100 cycles.

    You could use a 300mA LED driver if you wanted to, if it is capable to produce 15 volts. Or even a 700mA driver. I hope at least this could be used for battery charging.

    There are charging ICs for NiMH which I never tried however.
     
  3. MattP

    Thread Starter Member

    May 21, 2012
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    2
    With a 10 hour charge, I could put it on charge overnight, just like say my mobile, and it'll work out nicely. Longer charges are better for batteries and their cycles, as well (or at least experience tells me, with fast and slow chargers). It would also mean that I wouldn't be able to 'overcharge' it, as it would be within trickle charge rate, and could be left charging for a few days at a time.

    Out of interest, why won't a resistor consistently limit current? Does it depend on how charged the batteries are?

    Anyway, will this do? http://www.ebay.co.uk/itm/300mA-Con..._Measurement_Equipment_ET&hash=item1e75ea8dbc

    It says it's 12v though... if it was fed 15v would it still do the same job?

    Can you think of any way I could charge the batteries individually? I guess some fancy wiring might work, along with a multiswitch. Not looking forward to getting my head around that though, lol.

    Thanks for the help.
     
  4. takao21203

    Distinguished Member

    Apr 28, 2012
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    In order to get 15V out you need more than 15V. If it is a MOSFET chip only a little more, otherwise at least 3V more.

    Chances are the chip can withstand more than 12V, so yes, you could use higher voltage.

    I would simply try and see if the batteries do charge correctly.

    I have never tried this myself but I know SMPS (switch mode power supplies) a little.

    If you can, maybe try to identify the chip on the PCB and look up the datasheet, just to be safe. The PCB seems to be for 12V AC and that always can be a bit off.

    Multiswitch would be highly unusual, if it must be a number of cells, yes they are charged in series.
     
  5. takao21203

    Distinguished Member

    Apr 28, 2012
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    You have of course no guarantee the chip PCB will actually allow the voltage to raise as high as 15V. 300mA LEDs don't use such a high voltage even if it is not unsual to use a few of them in series.
     
  6. takao21203

    Distinguished Member

    Apr 28, 2012
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  7. Audioguru

    New Member

    Dec 20, 2007
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    Charging at 1/10th the battery cell capacity is a lot more than a "trickle charge current".
    1/10th the capacity was used for old Ni-Cad cells, not modern Ni-MH cells.
    Energizer and a few Japanese Ni-MH battery cell manufacturers say a trickle charge should not have more current than 1/40th the capacity which for these 2500mAh cells is only 63mA.

    I think you will be overcharging the cells frequently because you do not know when they are fully charged. Battery cells get hot when overcharging which reduces their life.
    A charger IC charges them properly because it detects a full charge then disconnects itself.
     
  8. takao21203

    Distinguished Member

    Apr 28, 2012
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    I use AA cells for my wireless mouse. All the noname brands are now detected as "bad" by my electronic LCD charger. It is using a SAMSUNG IC.

    Only Duracell and Energizer still work. That is a fact I know from real life.

    They were already defective before I bought the mouse. So I only use Duracell/Energizer now.

    However they still hold charge and can be charged with regular charger (which I have around buried somewhere).
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    Energizer Ni-MH cells are made in Japan for Energizer. Maybe they are made by Sanyo.
    Energizer 9V Ni-MH batteries are made in Germany for Energizer.

    I don't know where Duracell ones are made.
     
  10. takao21203

    Distinguished Member

    Apr 28, 2012
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    In China actually but I also saw some Energizers made in the USA.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    Energizer and Duracell alkaline cells are made in USA.
     
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