So i bought a brand new alternator from advanced automotive and had a shop put in the alternator due to the stupid poor design of where it is. i picked up the car friday and went on a long roadtrip and put on 700miles then the battery light came on and the lights went dim and the car died, i checked the voltage on battery it was at 11.3 and the battery is less than 3 months old.. after the van died i tried to start it but it didnt do anything, i waited ten mins and tried again and it started right up and i was able to drive it with the lights off but as soon as i turned them on it died right away... please help me on what could be causing this

Thanks
Dakota