I've had an intermittent starting problem for a couple of months. You'd turn the key and it behaved like a flat battery. It only happened every 2-3 weeks and had me flummoxed. Later on it starts just fine.
I began data logging every time I drove until I finally caught it happening. The battery voltage had dropped to 8 volts while cranking despite being well over 12 before I hit the starter. I truly doubted that it was the battery and knew the alternator was charging.
I checked the tightness of all the connections and bought a spare starter switch to carry. It happened again on Friday after a 20 mile drive and I had a brainstorm. The previous failure had happened after a 45 minute drive.
It might be a failing connection that heated up while driving. The only other thing I had inline with the battery was a cutoff switch. Before we headed out yesterday for a staycation on the other side of the island, I bypassed the switch. No problems so far. I might have found the culprit!