Battery polarity reversal?

[Brett Dikeman]

At 2:23 PM -0500 8/28/04, sdewitt at stx.rr.com wrote:
>Strange thing just happened... A battery of mine that I thought was 
>dead just about burned out my charger.

That's a shorted battery- or one severely discharged to the point 
where delta-V (ie, difference between the battery's voltage and the 
charger's normal output voltage) is so great that current is way, way 
too high for the charger; the battery almost acts like a short.

   I've had this happen with a 10A charger on my dead optima (left an 
interior light on over several days).  I had to use a DC bench power 
supply with current limiting to charge it at 3A for several hours to 
bring the voltage up enough that it wouldn't overload my larger 10A 
charger.  It went on the 10A charger for another hour or two, then I 
was able to start the car.  Later the car went on my maintenance 
charger.

If the battery has been severely discharged, it should be slowly 
recharged anyway- charge rate is best matched by the discharge rate. 
NEVER use those giant fast-rate chargers mechanics love...they're a 
sure fire way to kill the battery.  Charge rate should not exceed 
(Amp-Hour capacity) / 20 unless the manufacturer specifies otherwise- 
SLA's and SWLA's can often be charged at higher rates.  While some 
gassing is good for STATIONARY batteries as it stirs up the 
electrolyte (stratification happens in stationary batteries) - this 
has no advantage on car batteries which get -plenty- of stirring, and 
it will only result in breakup of the plate material (of particular 
concern on batteries with very high CCA ratings; the higher the CCA 
rating, the finer the plate grid.  The finer the grid, the weaker the 
grid!)

The little bits go down to the bottom of the cell, and if enough of 
them collect, guess what happens?  That cell shorts out, the other 
cells now get overcharged, possibly short themselves but certainly 
boil down their electrolyte- and the battery dies.

>I then flipped the positive and negative cables of the charger, ie 
>black goes to positive on the battery, red goes to negative and the 
>battery started charging? It hought it was impossible to reverse the 
>polarity on a battery????


http://www.uuhome.de/william.darden/carfaq14.htm


  14.14. Once formed, batteries will not change polarity.

If a battery is fully discharged and continues to have a load, for 
example leaving the headlights on, it is possible for one or more 
cells to reverse polarity. When the battery has been recharged with 
reversed polarity the polarity can change. This is referred to as 
"cell reversal". To change polarity, fully discharge the battery and 
recharge it with the correct polarity.

I certainly hope the battery was not connected to the vehicle when 
you did this, because if it was, you've most likely blown virtually 
every electronic unit in the car.  Electrolytic capacitors are used 
extensively in anything electronic, and they are nearly universally 
intolerant of polarity reversal- they will eject the electrolyte 
violently.

The battery will not operate at full capacity with reversed polarity.

Brett
-- 
"They that give up essential liberty to obtain temporary
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