> I don't know how frequent it is that a battery goes below 5V during
> starting. I'm not sure, but I gather that there isn't much starting torque
> anymore at this voltage, so maybe you can design with a workable
> minimum voltage

For ordinary 12V automotive I doubt you'd get any turnover from a
battery at that voltage. Below 10.6V they're considered flat

Diode isolation between the battery and a reservoir cap should be enough

Someone did post an automotive 5V PSU a while ago. IIRC it included
a pi filter comprising a couple of coils, with additional diodes and caps,
and perhaps a zener too, all before a 5V regulator

I have an idea it was one of the Bobs

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