Alternator bad already?

So first cold day last week my 2014 tundra would not start. Replaced with battery from Napa.
This week, the morning I was leaving for a trip truck would not start. Tried to jump with the wife’s car, no luck. Battery test that morning showed weak battery. WTH? Guessing alternator? Truck has 51k miles

 

Have them test the alternator. If you have a voltmeter check it while the truck is running. It should be putting out between 13 and 14 V. Ideally between 13.8 and 14.2 V. At least this is my basic understanding of it.

 

Riding back from out of town now, should be driving my own truck. Caught a ton of heat when the guy in the ram had to drive me. Anyway. Plan was to charge the battery then head to advanced auto to have alternator tested.