So I stumbled on an interesting HEI phenomenon

There are likely dips in voltage with a weak battery, that go well below what is measured with voltmeter. A scope shows the real picture. Automotive electronic designs often incorporate a cutout for low voltage. While in your case, the engine misses, there can be more severe problems without the cut out, including damaged electronics. The gate drive for power semiconductors need to operate at specific levels, too low and the driver partially activates and may destroy the junction.

I design with logic level drivers, so my EMC unit will work down to 7 volts. The circuit runs on 5V but the regulator needs 2V of head room.