On Thu, 29 Jan 2004 19:36:57 -0700 "Denny B"
wrote:

A battery that is perfectly good why when it is -30C
won't it start a vehicle.

If the battery is actually FROZEN then it is not perfectly good.
Perfectly good batteries have acid which won't freeze until well below
-30C. As the battery becomes discharged, the freezing point moves up,
so the only thing wrong with it may be that it has just not been
getting fully charged.

A FROZEN battery won't put out much current because the chemicals are
not mobile enough to get to and from the plates freely.

If the battery is just cold, then its output will be reduced simply
because the battery works by means of a chemical reaction. A simple
rule of thumb is that a typical chemical reaction takes place half as
fast for any 10 degree C reduction in temp. The reduction in battery
output does not go down this fast because there are other things also
at work here, meaning that there are effects other than thermal which
limit battery current.

All batteries are less effective at cold temps.

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Jim Adney
Madison, WI 53711 USA
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