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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 Feb 25 '11 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 Mar 10 '11 at 3:55
  • \$\begingroup\$ So, this question still has no answer... bad. \$\endgroup\$ – Ale..chenski Feb 13 '18 at 23:35
  • \$\begingroup\$ heh - stupid joke, but it still made me chuckle :P \$\endgroup\$ – CreationTribe Apr 21 at 18:12
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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 Feb 24 '11 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 Feb 25 '11 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 Jun 22 '11 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 May 16 '12 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 Feb 25 '11 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 Sep 18 '11 at 1:46
  • \$\begingroup\$ Ya - except I just came across this same phenomenon on a regular non-rechargeable AA Energizer alkaline battery. \$\endgroup\$ – CreationTribe Apr 21 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 Sep 2 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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