# Reversing polarity on a battery?

1. Aug 26, 2011

### Evil Bunny

A colleague of mine (who I trust is telling me the truth) just told me that he has seen a completely dead battery (a car battery) recharged so that the polarity was reversed from its original configuration.

I'm not well versed on the inner workings of a battery, but I thought the chemistry inside the cells determined the polarity...

Can someone explain this?

2. Aug 26, 2011

### chrisbaird

You are right. Your friend is wrong. To change polarity of a traditional electrochemical voltaic battery cell, you would have to change the internal materials around. This is kind of pointless, as it is easier to just switch the orientation of the battery if you want to switch polarity.

3. Aug 26, 2011

### Evil Bunny

A completely dead battery that has been dead for years... If you put the charger on it backwards, would it not hold the charge for a short time? Is it impossible?

I'm asking because I don't know. He insists he's seen it and he usually knows what he's talking about. I would be surprised if he was making it up.

4. Aug 26, 2011

### Philip Wood

I think he may be right. When I was a boy I made a primitive rechargeable cell simply by immersing two lead plates in sulphuric acid. I charged the cell by connecting across the plates a pd of more than 2 V from an external source. One of the plates acquired a brownish tinge from the lead dioxide deposited on it, and the cell did store a little energy.

Since the plates started out identical, which one became the positive depended solely on which way round I connected the charging pd. Now in a proper commercial lead-acid battery, the plates are different, though, I think both are lead-based. One is prefilled with lead dioxide, I believe. So the battery only stores a decent amount of energy if it's charged one way round, but I wouldn't be surprised if it stored a little when completely discharged and then charged the wrong way round.

[I wouldn't do experiments on a proper lead-acid battery: its internal resistance would be so low that accidental short-circuits could be disastrous.]

5. Aug 26, 2011

### cjameshuff

His friend is correct. The plates of a discharged lead acid battery (like the car battery his friend referred to) are both lead sulfate, there is nothing to switch around. A fully discharged lead acid battery can indeed be charged in either polarity.

It won't necessarily work as well, however. When charged, one plate becomes spongy lead, while the other becomes lead oxide, with different reactions occurring on each as the cell charges and discharges. Some batteries may have the plates of each cell be differently shaped to accommodate the changes in volume or give one side more surface area, leading to reduced performance when charged backwards. They might also use different alloys or additives for the anode and cathode plates. And this is all for lead acid batteries, not all battery chemistries allow this.

6. Aug 26, 2011

### Drakkith

Staff Emeritus
I don't know that much about batteries, but what I have read shows that cjameshuff is correct. Once discharged, both plates in a cell of a lead acid battery are Lead Sulfate, PbSO4. Unless there is something very different between the plates, then reversing the polarity should work.

7. Aug 29, 2011

### chrisbaird

Thank you for the correction. I did not know that car batteries had identical plates when totally uncharged. It looks like I will have to read up on it more. :grumpy:

So I think the best summary is that batteries with different-material electrodes cannot be reversed in polarity upon recharging, but batteries with same-material electrodes can be.