91 cutlass won't hold charge?

Not mechically inclined, so no laughing. In the process of selling my 91 cutlass supreme. So I washed the engine, and the next day it won't start. Its dead. I boost the car fine and it ran for 30 min. I turned it off and tried starting it again -- nothing. Do you think its just the battery, or could it be something else? It was a week ago and it still boosts fine and runs but won't hold the charge.
Thanks for any input

Welcome to AF, slow charge the battery and check the cable connections for corrosion, then have the battery load tested after the slow charge.

When you say slow charge, do you mean with a battery charger? or can this be done with another vechicle and a boost?
Thanks again

Alternators are not intended for charging batteries; they are simply designed to maintain a fully charged battery. If you don’t have a battery charger, take your battery to a parts store for charging & testing. Once you have a fully charged battery in place, let them test the alternator output. Water can kill an alternator!

Ok, so I got a hold of a battery charger and had it on until fully charged. Went to start car, nothing, the lights weren't even on in car, at all. So, the battery?? or alternator??

Gotta check those cables first.

checked cables--look good, no corrosion, tight
I think my best idea is like you said, take to parts shop and get them tested. My friend at work said they'll usually do this for free.
Thanks for the input!!