Battery polarity reversal?
brett at cloud9.net
Mon Aug 30 02:50:43 EDT 2004
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
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
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????
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
The battery will not operate at full capacity with reversed polarity.
"They that give up essential liberty to obtain temporary
safety deserve neither liberty nor safety." - Ben Franklin
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