Can an automotive battery reverse its polaity on its own?

[cwwest]

My neighbor has a small (4 seat like a golf cart) electric car (Chrysler) that runs on six 12v deep cycle batteries. He was having trouble keeping a charge so he had a pro perform a load check on each battery after he charged them to 12v. They were suposedly OK. The onboard battery charger cannot charge the batteries and says the batteries are 'disconnected or out of conformance'. I check the batteries and each was about 11v to 12v and all connected at the charger read almost 72v. We concluded that the onboard charger was bad. After several weeks the new charger arrived from Italy and we installed it. Same problem. I started to check the batteries and they were all at about 6v except one in the middle of the series circuit of bvatteries that read -3.34v on the positive terminal. Is it possible for a battery in this unused configuration to reverse its polarity? I am now assuming that this battery is toast and probably was the original problem - except it checked good the first time we tried to troubleshoot the problem Any comments?

 

[Willbe]

read -3.34v on the positive terminal.

I've heard of cell reversal; never seen it.

 

[kchriste]

Yes, that is entirely possible and very common in a long series string of batteries. The problem can be caused by running the batteries down too far. Since all the batteries are not exactly the same, the one with the least charge will get to 0V and then be reverse charged by the other batteries in the string. You should never run a lead acid battery down that far anyway but it is far worse in a long series string like you have.

 

[Hero999]

As all the batteries are below 10V it sounds like they're all bad and need replacing which will probably be quite expensive.

 

[Diver300]

Lead acid batteries can be reversed charged and still work. I have heard of a car alternator being destroyed by being connected to a reversed battery.

It won't do lead acid batteries any good to be reversed as the construction of the electrodes is different for the anode and the cathode.

That is the only common battery chemistry that will actually work backwards. All the other ones won't hold charge when reverse charged.

 

[Hero999]

I thought that it's difficult to revive a battery if the terminal voltage drops below about 10V?

 

[Nigel Goodwin]

I've heard of cell reversal; never seen it.

It's incredibly common, but not really cell 'reversal' - just one cell in the chain going flat - this cell now has it's +ve terminal connect to the -ve of another battery, and it's -ve terminal connected to the +ve terminal of another battery. This means it's now connected the wrong way round, so it reverse charges.

This is why it's IMPORTANT - NEVER - to mix batteries of different types, ages, or degrees of charge. For normal non-rechargeable batteries this reverse charging will quickly split the battery casing, spewing the corrosive contents all over (and destroying) your equipment.

You wouldn't believe how many times I see this

For the OP - replace ALL the batteries with brand new identical ones - the reason for the problem is all the batteries are old and knackered.

 

[crutschow]

Lead acid batteries have no inherent polarity when discharged. When they are totally discharged each electrode is lead sulphate and can be charged in either direction.

 

[Hero999]

A lead acid battery should never be totally discharged as it would totally ruin the battery.