voltage regulator. Cheap easy fix, known for failure, fails to allow alternator to charge battery. Car will run on battery alone for a while (longer with no lights or accessories on) and then suddenly...nothing..no juice in battery because it was not being charged.
this was my problem on my 1970 Challeneger.

I found a mobile mechanic. He had a tester that hooked up to alterantor and battery for diagnostics. Step one was taking alternator to parts store to test...that was good, so voltage regulator was next. never a problem after.
Steve