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Reverse polarity of battery?

  1. May 9, 2009 #1
    This isn't an engineering question, but I don't know which forum is better suited to ask.

    I pulled two AA batteries out of my daughter's toy this morning after it had been left running all night. One of the batteries read 0.80 volts, and the other read -0.24 volts! That's right, one was reading negative voltage and the leads were on right (as evidenced by the positive voltage on the other battery). The negative-voltage battery had some powder (sulfur?) on the negative terminal and a small stain (leaking acid?) on the sleeve. They were both Energizer batteries, new just a few months ago, though they may have been subject to trauma in the toy.

    What gives for the reverse polarity? This just seems weird to me.
  2. jcsd
  3. May 9, 2009 #2
    If the batteries were connected in series and one battery discharged before the other then a reverse charging current would flow through the discharged battery.
  4. May 9, 2009 #3
    They were in series. I didn't think of that, but could the a chemical battery such as a AA Energizer maintain a negative charge even after it is disconnected from such as system?
  5. May 9, 2009 #4
    Yes. Charging with polarity reversed can cause a reversal in battery polarity causing gas pressure inside the battery to rise, which can activate the safety vent, lead to alkaline electrolyte leakage, rapid deterioration in battery performance, battery swelling or battery rupture.
  6. May 9, 2009 #5
    I never "charged" the batteries in a charging device, they were only ever used in a small children's toy connected to a small electric motor.
  7. May 9, 2009 #6
    Yes, I know. The battery was reverse charged while it was in the toy. Since it discharged before the other battery, it was reverse charged by the other battery. This is a common problem when batteries are not properly matched. When using multiple batteries they should have the same amount of charge. Otherwise one will go dead before the other and it will be reverse charged by the other battery (when connected in series).
  8. May 9, 2009 #7
    Thanks. I appreciate the explanations.

    I would just like to point out that these were two brand new batteries, out of the same package. But with manufacturing differences and even a possible defect in one of the batteries I could see houw this could be the case.

  9. Sep 6, 2009 #8
    how can we avoid reverse charging current would flow through the discharged battery.
  10. Sep 6, 2009 #9


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    Since the direction of the "reverse" charging current is identical to that of the devices normal forward operating current it is not easy to do with a passive circuit, though a parallel schottky diode could limit it to perhaps about -0.25 to -0.3 volts.
    A good active method is to have the devices electronics auto power off the device when the total series battery voltage falls below some pre-determined level. This prevents the problem very nicely but is only really applicable to devices that have built in electronics capable enough of "doing the smarts" here. Obviously a simple device with just a switch and a small motor or lights etc is not so well suited to implement this method.

    The usual operating assumption in this case is simply that the user will switch off the device when the batteries run down to the point where it is no longer operating satisfactorily. Of course kids being kids they will often just leave the device switched on even after the batteries are too flat to run it. I guess if this happens and it destroys a battery and leaks corrosive material into the battery compartment which ruins the device then you just have to chalk that up to one of the 1,000,001 ways that kids can wreck something. :)
    Last edited: Sep 6, 2009
  11. Sep 7, 2009 #10
    I have actually come to the conclusion that Enegizer sells type B batteries in my country. Every time I have seen this effect it has been with Enegizer batteries and every time, one of the pair is in good condition (>1.2 volts).
  12. Sep 7, 2009 #11


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    An alkaline battery with >1.2 volts isn't necessarily a good battery. It should read about 1.6 volts open circuit to be considered good.

    Both batteries could have been used up by being in the toy for too long. The fact one of them went negative doesn't matter. They both have to be thrown out.

    Maybe the toy would run on rechargeable batteries?

    Alkaline batteries usually have a "use-by" date on them and it is often many years in the future. Check this on your next set of batteries.
    Also check your daughter's toy is turned off before you turn the lights out at night.
  13. Sep 7, 2009 #12


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    Thanks to those of you who explained the reversal. When I bought my first cell phone, back in '79, I was warned that if the battery was left too long, the charging unit itself could drive the thing into reverse-charge. No further explanation was offered. Now that I've read more details here, I'm curious as to how a hard-wired polarized charger could do that. Or was I being snowed?
  14. Sep 7, 2009 #13


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    No, I don't think it could happen during charge.

    It is only at the discharge time it happens.
    If one battery gets really flat so its internal resistance voltage drop is higher than its EMF, then you have a voltage reversal if there are other cells still pushing current through the flat one.

    I have never seen it happen, but it would be tempting to try recharging it in the right direction, even if it was not rechargable, and see if the polarity reversed.
    I have tried recharging flat alkaline batteries but never got them to hold much charge.
  15. Sep 8, 2009 #14


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    Sorry... didn't state the situation clearly. I was told that the battery at full discharge would begin to reverse-charge, and that connecting it to the charger would then accelerate that process.
  16. Sep 8, 2009 #15


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    I have never seen a proper account of the Chemistry involved in this, however, when a battery is being charged, the current flow is in the direction that would charge it in the proper polarity.

    When it is being discharged, the current flow is in the direction that would tend to reverse the polarity of the battery if there were the Chemistry in the battery for this to happen.

    However, a battery that has already become reverse charged is presumably permanently damaged and won't take a charge in either direction.
  17. Oct 3, 2011 #16
    I have seen several batteries with reverse polarity under the conditions described by multiple posts; device left on, fully discharge. In one intercom with 6 AA batteries in series there were four batteries in the -.03 to -.06 range, one was -.243 and one was +.833.
    The most interesting case was an actual factory defect with the caps put on the wrong ends. The device had two D cell batteries--one was fully charged (1.58+) and one was fully negative (-1.56). The power switch was off. It was a Teddy Ruxpin doll returned to the store. I was going through a tote of returned toys for a project at a sheltered workshop I was doing an internship at. There were several with partially discharged batteries and several with some negative polarity batteries.
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