Batteries - reverse charging

[awol]

A sailor of my acquaintance (no, it wasn't me, honest!) had 2 flat batteries on his boat on the hard so went out and bought a cheapy charger. When he connected it, the clever wee thing flashed a light for "polarity error" so he swapped the leads round and left the thing connected until the charging lights went out. He did the same to the second battery. When he connected the batteries back to the boat all was well until he noticed a burning smell from the engine and he now has a fried alternator. Both batteries actually show >12.5v but with the poles reversed.

I didn't think it was possible, but my only explanation is that the totally flat (he admits to about 1v on the meter!) batteries somehow managed to produce a residual -ve charge sufficient to convince the charger that they were the wrong way round. Ah well, with a new alternator, 2 new batteries (and he doesn't know about the radio and instruments yet) to buy, it was an expensive cock-up, but he does drive a Merc ........

 

[nigelmercier]

Could have been worse, he could have reverse charged just one of them!

 

[Plevier]

Not common but not unknown.
There's no fundamental difference between + and - plates as manufactured the difference is in the forming process. Less easy to reverse after use though.
The batteries will probably not perform too well now reversed! The plate balance will be wrong and probably in poor condition anyway after such deep discharge unless it was very short term.
Would be fun to try to re-reverse them.

 

[nigelmercier]

There's no fundamental difference between + and - plates as manufactured

... Although the physical structure may be different.

 

[Plevier]

... Although the physical structure may be different.

Please note I said no fundamental difference. I did not imply that the pos and neg plates in a particular battery are the same. They may differ in e.g. thickness and grid layout, and the paste formulation may be a bit different.
However the same mass produced plates for conventional flat plate flooded batteries may well be used as a +ve plate in some battery configurations and a -ve plate in others, and formed (i.e. electrically processed) appropriately.
Plates for AGMs are more differentiated but the fact is whether you take a car battery, a Lifeline, an Optima, a Trojan, an SBS, an Odyssey or an OPZs tubular, in all cases you could process the intended +ve plates to be -ves and vice versa. Not suggesting they would work optimally.
Does not apply to NiCd etc of course, only lead acid!

 

[Plevier]

Could have been worse, he could have reverse charged just one of them!

Oh yes good fun!

 

[nigelmercier]

Please note I said no fundamental difference. I did not imply that the pos and neg plates in a particular battery are the same...

I was agreeing with you!

 

[Plevier]

I was agreeing with you!

Well it seemed worth clarifying