Go to the parts store and have them check the battery and see if the alternator is charging correctly. This will only take a couple minutes and they do it for free, Go from there.
It's almost garanteed to be the battery, unless it's a brand new battery (make sure you charge the new battery first). From the Info given it's the battery, and I don't know why they would say it isn't the battery and not tell you what it actually is.
The Alternator is still good because you haven't reported anything else dieing in the interior (Ie AC, Heater, fans ect). It could be defective wires but I find that unlikely.
alternator/generator not charging the battery correctly
Here is how you can test your alternator and see if it is bad:
it started about a year ago. My car wouldnt start if i didnt drive it for a day. the battery would be dead. it never dies while im driving but it dies when i didnt drive it for a good amount of time. fast forward a year and now even after driving a good amount, it wont be able to start after an hour. i asked places before and they said it wasnt the battery, what could it be?
