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Last night I pulled the battery holder out of a toy RC car to test the voltage with my meter. The pack has 4 AA batteries. The combined voltage at the terminals of the pack was well under 1 volt, and then I tested each battery individually. These are ordinary AA alkalines (not rechargables). They were in good condition, with no leakage.

The first two batteries showed about 1.2V; the third had a small negative voltage, about -0.2, which I thought was pretty interesting, and the last showed about -1.2v. I have never come across a negative voltage on a battery.

Now before you ask, yes, I double-checked, and then triple-checked, that my leads were plugged in correctly. And I was not holding the batteries upside down. I had all four right in front of me, all pointing the same way. Two showed a positive voltage, one slightly negative, and one -1.2v. I did this repeatedly,

So my question is, what the heck? How does a battery get a negative voltage on it? The pack had been in the RC car for a couple of weeks, with the car switched on. I think, but cannot prove now, that they were all inserted the right way in the holder. But even if one or two were in backward, how could this happen?

Now the batteries are Canadian, and my meter was made in the USA, but I don't think that explains it. [That's just a joke.]

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    \$\begingroup\$ I have also seen and wondered about this phenomenon. I have seen it occur in alkaline batteries that have experienced a deep discharge. \$\endgroup\$
    – mjh2007
    Commented Feb 25, 2011 at 15:47
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    \$\begingroup\$ I've noticed it too with an alkaline AA. I tested the battery with a 50mA load and it was actually able to supply significant reverse current. \$\endgroup\$
    – supercat
    Commented Mar 10, 2011 at 3:55
  • \$\begingroup\$ I had this happen with three alkaline LR6-AA batteries in series, only it happened to TWO of the three. The device ran with new batteries for over a year. It last reported a battery level of about 30%, and then it just died. When I removed the batteries, one had +1.6v (way more than I expected) and the other two read -0.44 and -0.34. I had no idea this was possible, much less that one battery could do this to two others. \$\endgroup\$
    – ngreen
    Commented Jul 18, 2023 at 15:17

4 Answers 4

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Batteries when are fully discharged they can reverse their polarity. Sometimes you can carefully discharge this reverse voltage on a single cell and the battery will then successfully charge back up. Other times the cell is ruined and needs to be replaced.

I used to see this on the large batteries used on aircraft.

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    \$\begingroup\$ Any clue as to the physics/chemistry of this effect? \$\endgroup\$
    – drxzcl
    Commented Feb 24, 2011 at 21:43
  • \$\begingroup\$ Wikipedia: "If the battery drain current is high enough, the weak cell's internal resistance can experience a reverse voltage that is greater than the cell's remaining internal forward voltage." Seems that it has something to do with the chemistry still being at 1.2V, but the ESR creates the negative voltage somehow. And no, I don't understand either... \$\endgroup\$
    – Thomas O
    Commented Feb 25, 2011 at 0:01
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    \$\begingroup\$ @Thomas - That's ridiculous! No internal resistance can ever cause the voltage to drop so much that the output voltage becomes negative. Nor external resistance, by the way. -1 for Wikipedia! \$\endgroup\$
    – stevenvh
    Commented Jun 22, 2011 at 11:21
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    \$\begingroup\$ @stevenvh - That comment is taken out of context, and is actually accurate. Basically what they mean is that if you have two good batteries in series with a bad battery, and you load the battery stack with a very large load, The ESR of the bad battery can dominate. As such, the ESR of the bad battery causes the good batteries to force a reverse-voltage across the bad cell. \$\endgroup\$
    – Connor Wolf
    Commented May 16, 2012 at 9:41
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It happens if one cell is somehow "weaker" and gets charged by the other cells.

http://en.wikipedia.org/wiki/Rechargeable_battery#Reverse_charging

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    \$\begingroup\$ I'd like to note that this is far more common with NiMH and NiCd batteries than other types. Lithium ion/polymer batteries do not like it one bit and will probably be ruined - if they don't burst or explode first. \$\endgroup\$
    – Thomas O
    Commented Feb 25, 2011 at 0:00
  • \$\begingroup\$ @Thomas O: I had an alkaline battery do that once--the amazing thing was that it could actually supply a few milliamps of reverse voltage. I doubt it could supply much power for very long, but I was amazed that it could supply non-trivial current at all. \$\endgroup\$
    – supercat
    Commented Sep 18, 2011 at 1:46
  • \$\begingroup\$ Ya - except I just came across this same phenomenon on a regular non-rechargeable AA Energizer alkaline battery. \$\endgroup\$
    – CreationTribe
    Commented Apr 21, 2019 at 18:16
  • \$\begingroup\$ I've had this happen to me too. This is why its important to use all new and same brand batteries in series. \$\endgroup\$
    – sfscs
    Commented Sep 2, 2019 at 2:13
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Alkaline and other batteries can switch polarity in a series configuration. The battery doesnt actually have a negative charge, the positive terminal became the negative end and will meter -V when tested normally. A common occurrence, although rare that someone volt checks 'dead' batteries. HOW IT HAPPENS: a single cell depletes before the other batteries drop below half power and is deep cycled to 0.00v. This zero voltage state makes the + & - field unstable. Most batteries,on their own, will rebound to a low voltage when drained too far. But at that critical moment the positive tip is against a negative end of a charged battery and the electromagnetic field re-stabalizes backwards and starts taking positive charge from the negative terminal.

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I don't understand the chemistry of lithium batteries, but I experienced this with a pack of 4 new disposable lithiums in an Amazon wall clock. After just over a month the clock stopped. I checked all the batteries on a VM and found three at +1.65V, and one at -.87V on the positive terminal. Presuming that this was a defective from the factory...

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