Hi Barry,

It depends on battery chemistry and whether you are charging or
discharging and what the current is.

I don't know enough about Lithium technologies to answer in their
case, but I can tell you that for lead acid, slow discharges actually
COOL the battery a little below ambient. This is because a small part
of the heat generated during charging is from a reversible reaction
rather than simple ohmic losses. In other words, there is more heat
given off during charging than can be explained by losses and that
extra heat is from an exothermic reaction which happens during
charging. Discharging reverses this reaction and so is slightly
endothermic.

Anything more than a slow discharge, though, causes ohmic heating to
exceed this cooling effect and the battery will warm slightly.
However, it is mainly charging which warms up lead acid batteries
quickly. This is especially true if (A) you charge very fast or (B)
you continue to charge beyond 100%, as all of the energy beyond that
goes into heat production and hydrolyzation of the water into H2 and
O2.

I think that the currents involved in the Mars Rover were probably
fairly small and therefore there was an opportunity for the cells to
get very cold. On the other hand, with no atmosphere to help with
cooling, it wouldn't take much heat to really warm things up.

Sean


On Fri, Dec 11, 2009 at 4:00 PM, Barry Gershenfeld <gbarry42@gmail.com> wro=
te:
> I read the last page to see what the answers were :) =A0But I'm reminded =
of a
> question I've had for a long time. =A0Wouldn't a cold battery tend to warm
> itself up when a fair sized load is applied? =A0Cranking an engine on a c=
old
> morning made me ponder this.
> --
> http://www.piclist.com PIC/SX FAQ & list archive
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