Basically, it sounds like a minimally charging scenario. First suspects would be loose belt, and voltage regulation but it's going to be easier to determine the problem if you can provide us with some voltage measurements across the battery. ( Engine OFF, Engine RUNNING, and Both DC and AC measurements. )

Low DC = Voltage Regulator
High AC, and DC = Alternator Diodes

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Sam

84 300SD 350K+ miles ( Blue Belle )