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.

There's also the problem that trying to start at low voltages can burn
the starter windings out. The engine is often cold and reluctant to
start anyway, so the cranking time goes up. As the efficiency of a DC
motor (mechanical power out / heat wasted) drops dramatically at lower
voltages, then the temperature rise in the starter is significantly
worse when trying to start from depleted batteries - even if they're
not even delivering the same power. This is one reason why the Swedes
were so keen to switch to permanent magnet starters. In their cold
winters it was one less source of failure - it's not that they work
significantly better, it's that they don't kill themselves when trying
to.