Voltage guage erratic while idle

So I was just laying in bed, had just parked maybe 30 mins prior, with truck off, when the battery warning came on. I had just put new batteries in 2 months ago and have had no issues.

I turn the key, digitial read out is showing 11.6 volts. Starts fine. Then it goes up to 13.9-14.0 where it normally should be.

I sit there and watch the guage slowly go from 14 down to 13.3, then all the way up to 14.4 and then back down again.

I cleaned the wires when I installed the new batteries, and put them all back in the order they were in before.

Truck is a 2017 volvo with 234k miles.

Thinking the problem is a loose wire or voltage regulator in the alternator. Or maybe the belt is going bad.

I can't stand sleeping with the truck running so I'll just shut off the battery box tonight. Not much food in the fridge, probably wont go bad in 8 hours. Don't need to charge anything.

Likely culprit?

 

I think I fixed it.

When I shut the engine back off, guage read 11.6 usually it holds at 12.5-12.7 when off.

3 of the bolts holding the wires/cables on the batteries were a little bit loose. I tightened them up.

Guage then showed 12.7

Started engine and volts showing 14.0 like it normally does.
Holding steady.

I guess this post will now be buried for years, but hopefully someone else will come along and find it helpful.

Step 1 should probably be
"Check for loose wires"