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I have an old lead-acid car battery that got taken out of my car by AAA 5-10 years ago when it died during a cold winter. I stashed the battery away and forgot about it.

Recently I dusted it off and measured it with a multimeter. It registered -0.189 volts. Kind of weird.

I hooked up a battery charger to it and the battery charger generated an error signal: reversed polarity, even though the leads were hooked up correctly. So, apparently the charger kind of did the same thing as me: test the voltage. When it found out it was backwards it refused to charge it the right direction.

Why does the battery have a reverse charge? Does it just drift negative when it gets sulphonated or something? Or is it an effect of extreme cold?

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  • \$\begingroup\$ Does THIS answer your question? \$\endgroup\$
    – evildemonic
    Jun 22, 2020 at 21:07
  • \$\begingroup\$ @evildemonic No, because the battery in that case was obviously charged backwards (because it had -11V). My battery has a reverse polarity but was never charged backwards, at least with a charger. My question specifically says right in the title OTHER THAN BY BEING CHARGED BACKWARDS. \$\endgroup\$
    – Tyler Durden
    Jun 22, 2020 at 21:09
  • \$\begingroup\$ It is reversed, but at a pretty small voltage. The cells are in series, so it is possible if they become imbalanced for some to get reversed charged by the others. As the cells continue to deteriorate, you can end up with a net negative charge across them. \$\endgroup\$
    – evildemonic
    Jun 22, 2020 at 21:19
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    \$\begingroup\$ Tyler, the answer for a lead-acid battery depends a great deal on the type of construction (it has changed substantially over the years so that they can make much, much cheaper ones) and the condition of what you have on hand. Are you able to get at and visualize some of the plates in the battery? More modern ones simply need to be replaced if they've been left to deteriorate for a long time. Yours is completely discharged, possibly dried out inside for all we know, and in bad shape. Older car batteries or those used for fork lifts are salvageable, But newer ones, not so much. \$\endgroup\$
    – jonk
    Jun 22, 2020 at 21:25

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Specifically, when cells are in series, the one(s) with the least current capacity (due to imbalances during manufacture, or uneven deterioration) will be reverse charged by the remaining cells as the last few coulombs are withdrawn. In this state, the battery as a whole still would have a small net charge, as opposed to reverse charge... but then, over time, all the cells degrade. Perhaps those with a bit of remaining charge degraded faster, leaving a net reverse charge.

The interesting thing about Planté lead-acid cells is that technically, the chemistry is reversible, though modern cells are physically designed with pockets to hold lead oxide, so provide full current only in the forward direction.

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