The battery itself could be bad, the top could be dirty and a small current if flowing across the top or the alternator internals could be bad.

Check those first then remove each fuse and put your DMM across the contacts. Measure the current in each circuit.

Maybe your trunk light is on.

__________________
Regards

Warren

Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL

ENTER > = (HP RPN)

Not part of the in-crowd since 1952.