pick up a multi-meter if you don't have one already.
how old is the battery? If the voltage regulator misses a step even once it can damage the battery very fast.

to check:
w/ engine & ign off read across the terminals. Should be 12.5
w/ starter turning, shouldn't drop much less than 11.5. If it did, you probably have a damaged cell in the batt.

then run it & read across the batt again:
at 3500rpm or so, it shouldn't top 13.5 volts
if it goes higher than 14 or 14.5, you have an overcharge situation which is damaging the cells & depleting the electrolyte. You might be able to wing it a while by topping off the batt w/ distilled water in the cells & adjusting the VR, or replacing it entirely.

If there's condensation & fluffy white stuff around it; that indicates the same.

otherwise, you'll have to check your connections, primarily the grounds or check for something that's making a ground that shouldn't be. Whenever I leave a car for more than a couple days I remove the neg terminal to prevent a slow drain or a quick steal by someone.
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1966 122s, 1970 142s, 1974 142e... Blue is beautiful