# of batteries to cause polarity reversal

Update to above:

I noticed that my son's motorized bubble blower, which uses six AA batteries (I use NiMHs), frequently causes one or two cells to reverse polarity.

A monstrosity I built (16 NiMH AAs in series) to provide power to my laptop's 18.5VDC input, *always* fails by causing one or more cells (sometimes four at a time!) to reverse polarity.

So, would it be better to power devices with multiple AAs wired in PARALLEL, with a step-up converter to raise the voltage, to prevent cell reversal altogether?

Michael

Actually, it would be better to pay attention to how far down you're drawing the batteries. Polarity reversal happens *EXCLUSIVELY* as a result of discharging the cell too deep - there is no other cause. Avoid droppng a cell below the critical point (I've forgotten what that value is for an NiMH cell), and you'll *NEVER* reverse one.

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Are there circuits which exist which periodically poll the voltage across each cell, and can signal the load to stop drawing current?

Michael

Yes.

-- Don Bruder - snipped-for-privacy@sonic.net - If your "From:" address isn't on my whitelist, or the subject of the message doesn't contain the exact text "PopperAndShadow" somewhere, any message sent to this address will go in the garbage without my ever knowing it arrived. Sorry... for more info

But I don't know of any you can buy after-market.

They're usually designed into LiIon charge control systems that check all cells and restrict both charging and discharging.

If you'll recall a post of mine some time back that showed how to control current flow direction using back-to-back PMOS power-FETs... that, plus a controller that sequentially samples each cell and compares to a reference, and you have a charge/discharge controller.

I designed my first one maybe 15 years ago ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     | 
|  Analog Innovations, Inc.                         |     et      | 
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
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"Jim Thompson" schrieb im Newsbeitrag news: snipped-for-privacy@4ax.com... | On Tue, 16 Jan 2007 16:09:10 -0800, Don Bruder | wrote: | | >In article , | > snipped-for-privacy@gmail.com wrote: | > | >> Are there circuits which exist which periodically poll the voltage | >> across each cell, and can signal the load to stop drawing current? | > | >Yes. | | But I don't know of any you can buy after-market.

Once I designed one for lead-battery electrical vehicle. Patented and even works.

- Henry

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www.ehydra.dyndns.info

here's a plan for something that detects the voltage of the weakest cell.

  | | | | gnd -+---||----+---||----+---||----+---||----+-- | | | | | | | | | | | | | | +->|-+->|-+->|-+->|-+->|-+->|-+->|-+->|-+ | | | | ___ === === === === | | | | | | |OSC|---[R]--+---+---------+---------+---------+ |___| | .--------. | `-|envelope| gnd |detector| `--------'

when the envelope drops to two diode drops pk-pk one of the cells is depleted to 0V. so if you stop at around 3 diode drops... you'll have got about 99% of the available energy. Bye. Jasen