I've got a good idea. It's called engineering meets packaging.
The engineers who designed the batteries knew these batteries
needed this or that amount of air flow or whatever heat transfer
method is relied upon to cool the batteries. Other design
engineers came up with a manufacturing solution that was pretty
close to the desired specification but then the real world
stepped in and maybe at certain times, the load on the batteries
was a little more than expected. When you run motors, for
example, they draw more current when having to overcome
mechanical resistance. That makes the batteries get hotter than
expected and something's going to pop eventually because parts
of the system are being stressed beyond what was expected or to
put it another way, you thought that camel's back had a lot
less straw weighing it down than it really had.

	I was once a repair technician with our Audio Visual
center and we repaired film projectors of all kinds. Film
projectors present interesting design issues because they
contain lamps that generate huge amounts of heat to produce the
light that shines through the film and ultimately puts the
picture on the silver, actually glass bead, screen.

	The projection lamps or bulbs as we usually called them
are usually amazingly small for the amount of light and heat
they give off. The filaments inside those bulbs often-times
look like metal springs and carry lots of current as they heat
to temperatures hotter than the surface of the Sun.

	As with any incandescent bulb, 90% or so of the energy
is simply wasted as heat. The melting temperature of glass is
around 1000 degrees Fahrenheit or 537 degrees Celsius. The
filament wire is close to eight-thousand degrees Fahrenheit but
stiff metal posts hold it free and clear of the glass and the
glass is extremely clear so it doesn't absorb enough energy to
overheat. That is until somebody happens to touch the bulb when
it is cool and harmless.

	That starts a chain of events. No matter how often we
wash our hands, our fingers secrete skin oils which leave a print
composed of oil, a little salt and a few skin cells on the
glass. It doesn't look like much but the next time somebody
turns it on, the deposit scorches on the glass and burns a
little black spot. Carbon absorbs light/heat beautifully and now
the glass absorbs energy instead of passing it through. It
easily reaches the melting temperature and forms what looks for
all the world like a pimple or insect bite. If things get really
bad, the glass will sag out until it touches some part of the
projector. One day, the bulb will explode or help melt things
that shouldn't have gotten as hot as they did. Anyway, somebody
is going to have an expensive repair or even worse a fire or
injuries resulting from flying hot glass.

	All these parts were designed by engineers who knew what
they were doing. Unfortunately, they are frequently used by
people who either don't know or don't care and then wonder why
things went so wrong.

	My philosophy about engineering design is that when it
comes to heat, one should always assume the worst and then
you're ready for a little trouble. If you do that, things may
initially cost a bit more but keep running longer in the long
run.

Martin WB5AGZ
David W Wood writes:
> All so true and basic.
> 
> I wonder how Boe-ing got it wrong with the batteries on  the Dreamliner 
> when
> they overheated in use from insufficient ventilation?