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Battery in reverse? esrever?


The Ents

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Hi All,

This is a story about an MGB battery. Someone (!) left the sidelamps on. For a week.

The battery was totally pancake flat, but I didn't take a voltage reading.

I put the battery on charge, Halfords cheapo charger, for a couple of days. upon testing the voltage then I found I must have connected the charger backwards. The polarity of the battery was reversed, but reading 12.7 volts!!

Oh dear I thought.

I then connected a headlamp bulb across the battery to flatten it intending to charge it correctly when flat. I charged it the correct way round with a decent garage charger. after two days it was still pulling full scale on the 15 amp meter and was a bit warm. The observed voltage at rest after charging was 12.6 but still the wrong way round!!

I have of course put the battery in the recycle bin, but my question to you is how could it remain reverse polarity?

Mike.

 

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Are you sure it was reversed?. Some early MGBs had 2 X 6V batteries in series under the rear seats and were positive earth whilst AFAIK all modern cars ( post mid 70s ish) were negative earth.

 

Cheers

 

David

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If you charged it the right way round the second time, it would be the right way round!

 

If the battery voltage was reversed, then, with the charger connected, you would have two 12V sources connected in series and shorted together. I doubt the charging cables would survive.

 

Is it possible that the charger, despite being a cheapo, senses battery polarity, and charges to suit? Or that its polarity is reversed, and you connected it up (apparently) correctly the first time?

 

Try a Voltmeter on it while it's charging, or on the charger ouput before connecting the battery.

 

As an aside, in my impecunious youth, I used to totally flatten a battery, charge it the wrong way, flatten it again and charge the correct way. It seemed to give a few months extra life to a dying starter battery, enough to get through the winter, anyway :)

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Well looking on Wiki, the physical structure of a battery is symetrical so once truly flat, you can charge it either way around.

 

Did you read that link?

 

 

For all intents and purposes, the battery will be ruined. You could technically charge it up, negatively, and continue to use it, but your plates are designed with the positive plates being lead dioxide, and the negative being composed of a sponge lead, which would now be reversed. Because the reversed battery is no longer formatted correctly, it will only work to a limited degree

 

Richard

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and I think there is one more negative plate, at least there used to be when we quoted battery "size" by the number of plates. It was always an odd number. I once knew the reason for this but as its not very useful info have forgotten.

 

 

You wait - that reason will resurface when you are doing something that has nothing to do with batteries or boats

 

Richard

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It's because lead sulphate is somewhat thicker than lead dioxide. So as the lead dioxide is transformed to lead sulphate the positive plate becomes thicker. If it only had a negative plate on one side of it then it could distort. So, a positive plate is always sandwiched between two negative plates, therefore becoming thicker on both sides and remaining more stable.

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