You need power capacity not just proper voltage. BTW same thing happened to my Toyota pickup (twice).
Check the voltage with the engine running, if its above 13V the alternator is working. Assuming that checks okay and no obvious load is draining the battery, pull the battery and have it tested.
First time for me it was a bad battery, six months later the alt went bad. Thus began an ongoing repair saga on the truck...and the reason for the new Titan in my garage
