De
the alternator is what charges the battery and it works by being driven by a Belt over the alternator pully and then sends the electrical current to the battery...
If the batetry is not being charged while driving the car from the alternator, the battery will go dead.
The Alternator could be stretched and not taught enough causing it to slip and that would do same thing. Fail to charge the battery when driving...
So the problem may well NOT BE the battery. The battery is a SYMPTOM of a problem with whatever is supposed to be charging it, the alternator or that belt that spins the alternator pulley.