Design problem for combining AA NiMH batteries

Discussion in 'The Projects Forum' started by sleepyMonad, Aug 18, 2010.

  1. sleepyMonad

    Thread Starter New Member

    Aug 18, 2010
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    0
    Hi,

    I have a device 5V/1A that I want to power with AA batteries (because I think these offer an interesting combination price/capacity/weight, and both batteries and chargers are available everywhere).

    If I want to have an autonomy of 6 hours, I need 30,000 mAh, or 12 AAs of 2500 mAh each.

    I am considering the following 2 configurations:
    A: have all 12 AAs in series, resulting in a voltage of 12..17V, followed by a buck converter
    B: have 4 sets in parallel of 3 AAs in series, resulting in 3..4.2V, followed by a boost converter

    Solution A suffers, I think, from the reverse charging effect: the weakest battery will run out of juice and be reverse charged by the other ones, with damage as result. With 12 batteries in series, the capacity of the individual cells may be quite different.
    Solution B suffers much less from this effect, but I think it's a bad idea to have multiple battery sets in parallel, as the voltage will never be perfectly the same, and one set will be charged while draining the others, wasting energy even when not in use.

    Any opinions one way or the other? Or should I abandon this idea completely, and go for a single battery with sufficient capacity (e.g. a lead-acid battery)?
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,694
    904
    Both solutions suffer from imbalance due to differences in batteries. Solution B is potentially more efficient and even has a name in LiPo battery packs, 3S4P (or is it 4P3S?). Also, NiMH do not tolerate high discharge or charge rates as well as some other battery chemistries. The ones I use by the brand name "eneloop" (Sanyo) should not be charged at more than 1A. I charge 4 in series at 0.8A with no problems so far (1.5 years). You may want to consider bringing leads out from the pack so the charging configuration can be different from the discharge configuration. I would have to do some reading to know what to recommend for the configuration you are proposing.

    John
     
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  3. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
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    I'm not quite sure which you want:

    My question lies in your definition of autonomy and citing a 30 amp hour source.

    5V at 1A for 6 hours = a 6 amp hour battery but you'd want to get something of larger capacity for the sake of battery life

    In either case if you use NiCd or NiMH batteries 5 will give you 6V but to get that amount of amp hour rating you've got to mix them all up and batteires don't like to be hooked up in parallel or series because, as noted earlier, small differences in output voltage, series resistance or charge/discharge current variations can mess things up.

    If size isn't a limitation (probably not much of a difference anyway) I'd use a 6V Gel Cell battery hooked up to a 5V regulator of proper rating. Due to the amount of voltage dropped in a common 7805 IC I'd use a circuit with an additional output transistor, often added to increase the current handling capacity but they also lower the Vin to Vout drop.

    Be sure you're selecting a battery used to power things such as a UPS supply instead of a typical alarm system battery that isn't used to being charged and discharged at any sort of rate.

    Doesn't even have to be a gel cell, there are inexpensive 6V motorcycle batteries out there too.
     
    Last edited: Aug 18, 2010
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  4. sleepyMonad

    Thread Starter New Member

    Aug 18, 2010
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    0
    Thank you both very much.

    I understand now that managing 12 batteries (without even considering that I should count them only for 70% of their capacity) will be too much of a hassle, and the solution with 1 gel cell will be much simpler (both for efficiency and logistics).

    Due to the amount of voltage dropped in a common 7805 IC I'd use a circuit with an additional output transistor, often added to increase the current handling capacity but they also lower the Vin to Vout drop.

    That would indeed be most desirable. Do you have a particular circuit in mind -- I'm a bit at a loss here about how to search for such a thing.
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    higher current from 7805 IC

    higher current from 7805 regulator circuit

    higher current from 7800 series ICs

    increase output current of

    .... you get the picture. many of the spec sheets include an app note on doing this as well.
     
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