polarity reversal

[artv]

Hi all,... I searched this topic  found  three related threads. After reading, didn't really answer as to why a battery reverses in polarity.Just curious what causes this??......I had it happen while charging little battery bank, I was watching the numbers and everything was good, next day checked the readings ,and one was -.4volts,the rest were ~1.2volts , I can't seem to find out why. I read in the (wind) forum Chris Olson mentioned about reverse dc being a welder ,is this the principle they work on?? ...I was using a little motor out of a cd player for charging,the output reads around ten volts ,but is this ac??.....maybe the cause for reversal??......thanx for reading.......artv

[Bruce S]

ARTV;
 What Chris is talking about and "cell" reversal are completely different. Chris, if I correctly sped-red it right is Neg voltage welding.
Cell reversal is a reaction taking place within a NiCd or even in extreme cases NiMh batteries.
This happens when you take the individual cells below the 0.9V threshold of those types of batteries. That cell is now no good to anyone other than recyclers.
Take it out of the battery pack before it damages the rest of the cells.

Bruce S
PS>> Google is your friend " http://en.wikipedia.org/wiki/Nickel-cadmium_battery#Cell_reversal"
 

[artv]

Hi Bruce S.........are you saying the cell that went negative, should be discarded??,..........I re-incorperated into the charge cycle and it now reads 1.35volts........How can it go from -.4v.......to 1.35v............this is what I don't understand.............not trying to confuse just trying to understand.............artv

[Bruce S]

Hi Bruce S.........are you saying the cell that went negative, should be discarded??,..........I re-incorperated into the charge cycle and it now reads 1.35volts........How can it go from -.4v.......to 1.35v............this is what I don't understand.............not trying to confuse just trying to understand.............artv

NOT trying to be offensive here, but the google link I attached explains the how & why much better than I could.
This is another very informative site,http://batteryuniversity.com/ , they are the people our company bought the battery tester from.

Have a good read.
Cheers
Bruce S

[Ungrounded Lightning Rod]

I had it happen while charging little battery bank, I was watching the numbers and everything was good, next day checked the readings ,and one was -.4volts,the rest were ~1.2volts , I can't seem to find out why.

If it happened while CHARGING I have no clue.

If it happened while DISCHARGING it's because the cells were not "equalized" or the one in question was high leakage and lost a bunch of charge early during the discharge.  After the number of amphours that the weakest cell contains are pulled from the battery, pulling more causes the cells with a charge still on them to force current through the fully discharged cell, "charging it backward".  This is usually fatal to it - if it wasn't already essentially dead.  (This is why you stop discharging your battery when its voltage is down to where the weakest cell still has a charge on it.)

If these are lead-acids and recently failed, give 'em a good equalizing charge or two, just for grins and giggles.  But don't expect it to stay happy, now that it's spent a nontrivial amount of time reverse-charged.  Don't bother even trying with other chemistries.

[artv]

Hi folks thanx for the replies ....Bruce S,..I read the link that says if you reverse charge it breaks the electrolyte down into hydrogen and oxygen,....is that correct??..........ULR ...I was charging  5  1.2v batteries in series (from solar lights) using a little cd player motor in conjunction with a doide and  2  12000uf caps in parallel,   the caps were draining, then going negative that way reverse charging the batteries , but they reached full charge but in the negative  1.35v negative,.....then I hooked them backwards ,drained them and recharged them back to positive voltage. Is there any way to capture the gases produced by the break down of the electrolyte??.......if that is correct.............artv

[ghurd]

Salt breaks a car fender down into rust.
Adding pepper does not bring it back.

[Tritium]

Salt breaks a car fender down into rust.
Adding pepper does not bring it back.

Makes it taste better though. 

I use Hawaiian sea salt that contains iron oxide.

Yes off topic, sorry. As you were.

Thurmond

[Madscientist267]

ok ok, i expected that... but I did have some valid points...

So I reiterate... hopefully edited to the point of satisfaction... 

Artv -

The questions in there are not entirely clear, and I think you may be mixing a few things together and around that might be confusing you.

Quote
reverse charge it breaks the electrolyte down into hydrogen and oxygen

This is true for any chemistry that has a water based electrolyte. Not only for reverse charging, but 'normal' charging as well. NiCd and NiMH generally have some method of dealing with the gasses so that the pressure does not build inside of the cell to the point of danger. There are various mechanisms for this. Some use recombinant technology, others simply vent it.

There are also design considerations taken to prevent more of one gas than the other by doing things such as making one plate a different size than the other, and so on.

I'm not sure what you're getting at with your diode, cd player motor, and capacitors. The good news is that chances are, you're not dealing with enough energy to get yourself into trouble. Yet... Smiley

As for other things...

Quote
Don't bother even trying with other chemistries.

I'd take it a step beyond that. Not only don't bother, don't attempt it with some. Lithium in particular, should you find yourself ever inclined to do so. They strike back with a vengeance when you pi$$ them off.

And topics...

<cut that part out>

Steve