Re: shorted (?) lead-acid battery

From: Dave Platt (dplatt_at_radagast.org)
Date: 01/27/05 

Date: Thu, 27 Jan 2005 19:52:01 -0000

>Yesterday I started to charge a car battery that was sitting unused
>for quite a while with a 1A charger. (my other one is broke)
>Initially, the battery was around 6V, that jumped to over
>15 when I connected the charger, but dropped in about 30
>seconds to around 10V after the oxide layer broke down.
>A couple of hour later I measured the battery, and it was around
>10.40V, low enough that I was wondering if it had a shorted cell.
>Then I measured it again before going to bed -- 8.50V!!.
>This morning -- 6.50V, and the charger was too hot to touch,
>and I disconnected the charger at this point.
>
>I've never seen anything like this. I figured since the charger can
>supply so little current (was 850 mA @ 8.5V) there's no way it
>can overcharge or damage a car battery. I know something isn't
>right since after a day, even with a wimpy charger like that it should
>read more than 6.5V. Any idea what's happening? How did the
>cells become shorted, if that's the case? (the battery wasn't moved
>or jarred since charging began)

>From what I've read of lead-acid battery design, my understanding is
that one not-uncommon failure mode is that deep discharging (whether
due to excessive load or due to failure-to-keep-charged-while-in-
storage) can result in a physical deterioration of the plates in each
cell. The plates weaken, disintegrate, and the rubble falls
to the bottom of the cell. You can end up with chunks of the plate
material forming bridges across/around/through whatever is normally
used to keep the plates separated, and this shorts out the individual
cells.

If this is what happened to yours, then the charger wasn't at fault.
The battery was fatally rotted even before you hooked up the charger,
and the electrochemical changes which occurred during the charging
process (and perhaps converted the rubble back to a conductive form)
simply made the existing damage apparent.

As far as I know there's no practical way to recover from this... take
the battery to a dealer for recycling, buy a new one, and treat the
new one a bit more gently :-) 

-- 
Dave Platt <dplatt@radagast.org>                                   AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
  I do _not_ wish to receive unsolicited commercial email, and I will
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