The caveat of course is that battery measurements are only good if the battery is under load. I had a dead battery that was confounding me because every time I measured it the battery would show 13.5v but then the car wouldn't crank.

This is why shops have battery testers with a big resistive heater on the bottom. The good news is you can diagnose this with a pair of jumper cables easily enough.

In general though electrical problems are the hardest to track down, especially when it turns out to be a corroded or missing ground strap somewhere.