On Sat, 06 Aug 2005 09:35:18 +0100 "Dave Plowman (News)"
wrote:

In article ,
Jim Adney wrote:
If the battery is sulphated, most modern chargers won't deliver enough
volts to get anywhere near 5 amps due to the high cell resistance. Many
are voltage limited to perhaps 14 volts or so, and might need several
days to get any result - if ever.

That's probably for the best anyway. A sulfated battery should never
be charged at a high rate. That will just hydrolize water and can
damage the plates. IF recovery is possible, and it sometimes is, then
your best bet is with a very small charging current over a number of
days. I find that a week is typical for a car battery, but I had one
that took 2 weeks.

Yup. I was just worried someone might try using a variable bench supply to
push through the 'required' 5 amps if their charger didn't do this.

If you insist on instant gratification, you should just buy a new
battery.

Think you'll end up doing this anyway. ;-)

It's generally pretty easy to tell whether you have a good chance or
not. If you just measure the voltage while trickle charging it, the
voltage should rather quickly come up to 2 V or more per cell. If your
battery seems to be low by some multiple of 2V, then you probably have
a shorted cell, which generally can't be fixed. If you can get to the
2V per cell threshold, on a trickle charge, then you have a pretty
good chance of recovery.

Make sure the water is covering the plates.

If there is a broken connection in the battery the charging voltage
will be high, but it will drop quickly as soon as you stop charging
and put a very small load on it. You won't be able to fix a broken
connection, either.

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Jim Adney
Madison, WI 53711 USA
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