Thanks Tim, I didn't know about that. But I imagine that it has a fairly low time-constant and would be unlikely to read peaks of 42V or so when the starter is released. The alternator should keep its output voltage to between 13.75 and 14.5V so a weak battery may not show up with a normal average-reading voltmeter. But a duff alternator would show up. My second automotive electronics project was putting a green 10-bar LED inside the instrument panel of my Austin Allegro. It displayed the voltage such that each bar was a volt over 10, so 3 bars was normal = 13V.
My TSM2 voltmeter would be a peak-detector, and be active ONLY for say 10 seconds after ignition is switched on. The highest voltage detected in that period could be due to weakening battery plus starter motor back-emf.
But if the TSM2 is immune from 42V spikes then a weak battery simply means having to get a push to start the car!
