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The Natural Philosopher

wrote:
dcbwhaley wrote:

But the main bus voltage can fall below 6 volts when cranking the
engine, especially if it is very cold.

If the terminal voltage of a lead acid car battery falls below about
9V, the effective internal resistance (for high currents) will also
have become so high as well that they're no longer capable of supplying
starter-levels of power. Try it and you'll find that voltage at the
starter might be 6V when off, but drops to approximately zilch when you
pull starting current.

Not quite true, as those of us who built go karts powered by starter
motors can attest. Its a bit of a race as to whether the battery
explodes, the start current in the motor burns the windings out, or the
kart actually gets up to speed, but for a split second, the voltage is
pretty low..

We estimated that from te Kart duration on a stock battery, peak
currents were up to a Kiloamp, and normal running was a couple of
hundred plus.. About 2-3KW

...of coourse these days you can buy model aircraft motors of less weight
and similar power. I think I juts read that someone has a 15KW one in
pre production. Bang in a thousand quids worth of lithium batteries and
you have something that will take a 20lb plane vertically upwards. Takes
all the fun out of it.