start up problems.
 

hey, this week my truck went dead. for some reason. battery might be finished but anyways this is what i did. i boosted it up and went for a small drive seemd alright. turnd the truck off and tried to start it back up again and it wouldnt. i put it on a charger over night came abck started perfectly. next day.. dead. so im guessing the altenator is finished. i find someone with a tester and it turns out it is done. so i recharge the battery overnight again and change the altenator that i bought from a scrap yard for 50$ with a guarentee. i hook everything back up seems fines runs for 2 days.. and this morning go to start up and battery is dead again. the tester also said my battery was still good. so now im wondering if my altenator i bought isnt good either or if the battery really isnt good anymore and the altenator just cant recharge it. because now i cant get that tester. but does anyone know anything else what it could possibly be the problem? dodge dakota 2003 4x4 3.9L.