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Best way to charge lots of NiMH AAs?

  • ebpDoug Thursday, October 5, 2017 10:40 PM Reply

    I agree with desolder.


    NiMH is probably the most "difficult" to charge of common cell types.


    Constant current charging is standard, which makes any kind of balancing difficult and makes parallel charging risky.

    They are normally deliberately overcharged, which is useful for small series packs but a problem if you want to connect random cells in series.

    They don't like to be continuously trickle charged.

    End of charge detection is not simple. The most common method is negative delta V - the cell voltage actually drops at end of charge. This is another factor working against series charging of random cells.


    With random cells (as opposed to packs or sets of "identical" cells always used together) the only realistic charging method is to treat each cell individually.

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  • sheepish Top 10 Forum Poster Friday, October 6, 2017 11:48 AM Reply


    On that other site, a lot of the vapers give Xtar and
    Liitokala chargers very high marks. I've never really checked any of
    them though.


    I sometimes wonder why I've never got excited about Xtar chargers. That place has 40 listed but only 3 in stock, two of which are only 2 slot and the other (6 slot) is Li-ion only. If I was looking for a new entry-level analyser the Liitokala Lii-500 might be on my short list.


    They are normally deliberately overcharged, which is useful for small
    series packs but a problem if you want to connect random cells in
    series.


    Another idea, then. How about I charge the two 10S batteries in their 10S holders during the day with a solar panel, using a Zener diode to limit the charging voltage to ~14V (or to just switch off the current when the voltage gets that high) then top them up with a 4 slot smart charger? Doing them in 5 batches shouldn't take too long. They should stay in balance in the longer term.

    Remembering 30 years.
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  • desolder Top 10 Forum Poster Saturday, October 7, 2017 4:59 AM Reply

    Slow charging with a solar panel is OK as long as the current is below C/10. NiMH / NiCD can tolerate short term overcharging by converting the excess energy to heat. This is actually how Ni-xx packs stay in balance. The cells which reach 100% charge early burn off the excess energy while the other cells catch up. You may not need a zener diode since solar cells have a naturally sharp voltage rolloff that mates well with a battery's low internal resistance. If the solar cell's short circuit current never exceeds C/10, then you can directly connect them to the battey without worries, although you may want to at least have a blocking diode to prevent the current from flowing backward into the solar cell when the sun doesn't shine.


    Although continuous trickle charging is not recommended, maufacturers kinda sorta accept that people are going to do it anyway, and recommend using no more than C/20 to C/30 if you must do it.

    What's up with the scrubbed LED driver IC markings?
    What are they trying to hide?

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  • sheepish Top 10 Forum Poster Saturday, October 7, 2017 5:07 PM Reply

    I've put diodes on the solar panels, so discharging through them isn't a problem.

    C/10 for a 1P Eneloop pack is 200mA, so just a single panel should do the job, freeing up the other panels to charge my SLA batteries. Sounds like a plan.

    Remembering 30 years.
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