have a 94 ranger and the altinater is good it starts and runs untill the battery goes dead then it wont start why does my altinater not keep it char

If you are sure the alternator and charging system is working properly, It may be that you need a new serpentine belt and belt tensioner. Check all the idler pulleys also when you have the belt off.

Also if you have any accessories drawing a lot of juice, such as a big stereo or a lot of add lights, your alternator has to have the proper amperage output rating to keep up.