One easy test my Dad used was start the engine and while it is running unhook one battery cable.
If the engine dies it's the charging system, if it stays running it's the battery. That will at least give you a start on chasing down the gremlin that may be draining the system.
Like Jerry said, an internal regulator is the way to go.

Don


1967 GMC 9500 Fire Ladder Truck
"The Flag Pole"
In the Stovebolt Gallery
1946 2-Ton grain truck
1946 3/4-ton pickup,
1953 1-ton pickup
1957 1-ton flatbed dually,
1954 Pontiac Straight Eight
Of all the things I've lost in my life, I miss my mind the most!