Hi Ted:
> it seems that the low voltage used in the application is being
> mistaken for low energy. Your garden variety alkaline manganese cell
> has such a low internal resistance that it behaves as a little bomb when
> shorted. Same with lithium battery technology. If the solar cell can
> put out 10W it matters not at what voltage the current flows. From a
> destructive viewpoint 10W of energy in the wrong place (your car
> glovebox, a purse......) can be the triggering event for significant
> damage.
I believe 10 watts dissipation to be largely
benign in most cases, i.e., not particularly
hazardous.
Having done some empirical studies, and having
examined results of failures, fire is not
impossible, but is extremely difficult from a
power dissipation of less than 15 watts.
For low power, less than 15 watts, the local
environment (no matter what the environment
may be) provides a sufficient heat-sink such
that material ignition temperatures are not
likely.
I like to consider a 15-watt light bulb such
as that found in outdoor Christmas lighting.
15 watts gets hot enough that you don't want
to touch it for long. That is a long way
from typical plastic ignition temperatures
which are in the 350 C range and up.
The issue of temperature is largely an issue
of watts per unit volume. The greater the
volume, the lower the temperature for the
same watts.
If you imagine almost any battery dissipating
15 watts internally, then it will heat up.
But, it should not get hotter than the 15-
watt Christmas light. However, most
batteries are in a battery compartment
comprised of 2 or more cells. This allows
all the cells to contribute to heating, and
provides a poor environment for dissipating
the heat.
The battery event is largely one of a build-
up of gasses within the cell due to the heat,
sort of like a pressure cooker. It has got
to get hot enough to vaporize the internal
chemicals. Since there is no vent in most
of today's batteries, they distend and
eventually rupture, often exposively.
I believe most AA and larger batteries can
output considerably more than 10 watts for
short periods of time. Under short-circuit
conditions, the power is dissipated in the
battery, resulting in a hot battery.
Lithium batteries are made up of many different
chemistries. Some lithiums, as used in clock
and watch circuits, are very high impedance
batteries, and will not even get warm if
short-circuited all day. Others lithiums, in
the AA size and larger, are low-impedance
batteries and are equally dangerous under
short-circuit conditions as are most other AA
chemistries.
Best regards,
Rich
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