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I recently acquired a used riding mower. I figured it was dead, so hooked it up to a trickle charge overnight. Next morning, still totally dead.

I stuck my voltmeter on it, and measured -11.8V. Check the leads on the meter, and checked the meter against a AAA alkaline. No, the meter is working fine...the battery has a negative voltage.

To confuse matters somewhat, this battery has a red sticker next to the "-" molded into the plastic case. I'm not sure what to make of that.

So the question is this: is it possible through some mechanism to reverse the voltage polarity on a lead acid battery? Or, is this a case of cheap Chinese manufacturing?

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    \$\begingroup\$ Yes, this is possible but I have never seen such a 'high' negative voltage on a lead acid cell. I highly suspect that the charger has been hooked up in reverse. There is nothing in the chemistry of lead+zinc that doesn't allow them to be reverse charged (and still work, though with very low performance). It is afaik not possible that any chemistry spontaneously reverses its polarity, you need to reverse charge it for that to happen. \$\endgroup\$ – user36129 Oct 20 '13 at 14:13
  • \$\begingroup\$ @user36129 yeah, this battery came to me used, so there's no knowing what's happened to it in the past. Maybe someone was confused by the red sticker and indeed did charge it backwards. \$\endgroup\$ – Phil Frost Oct 20 '13 at 15:42
  • \$\begingroup\$ Can the battery be installed backwards? \$\endgroup\$ – user30080 Oct 20 '13 at 18:27
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    \$\begingroup\$ Whatever you end up doing with this thing, I'd recommend eye protection! \$\endgroup\$ – Scott Seidman Oct 20 '13 at 19:50
  • \$\begingroup\$ 2 batteries in series? I wonder if Phil still has the mower? '13 \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 17 '18 at 0:04
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Actually it is possible, but this will ruin the battery irreversibly. The trick is that the fully discharged Pb-acid battery has both electrodes the same composition \$PbSO_4\$. When the battery is charged, the negative plate contains pure \$Pb\$ and the positive - \$PbO_2\$ .

This way, if you fully discharge the battery, it can be charged in reverse way. The big problem here is that the positive and negative electrodes have different design, because of their destination. Even the full discharge of the battery is pretty destructive and reverse charge will cause fast plates degradation and in result big decreasing of the capacity and the possibly little bit later internal short circuits in some of the cells.

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    \$\begingroup\$ Several years ago lead-acid batteries where mote bipolar and if you had the time and a water bath, you could drain it to zero and start reverse charging it. More recent installments are polarized by early steps in the manufacturing and I have not been able to do this trick anymore. \$\endgroup\$ – winny Jun 28 '17 at 19:26
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All the chemistry/physics here is right, but yes it is possible, when somebody at the factory welds the top on the battery the wrong way round, which is possible with certain geometries. I had two identical batteries, new and dry. Added the H2SO4 and one powers up reversed polarity. Identical capacity to the other, but the +ve is negative. Only conclusion, the top was rotated by 180 degrees before welding.

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