Try these things:
Get a good ammeter which reads down to either milliamps or even microamps.
With the key nbot even in the ignition, remove the +ve battery lead from
the battery and connect the ammeter between the two. You should read NO
current. If you get a reading then your battery is discharging as you allow
the car to sit!! There is a short somewhere. The ammeter on multimeters,
if it just reads in amps (not milli or micro) is not sensitive to pick
up very small current leaks.
Is the fan belt in good shape, ie not going "dry" and cracked?? Is it tight?
Just to be safe, why not fit a new one?
I had this dying battery problem on a 73 MGBGT, I had a small current leak
and the fan belt, although appearing tight was old. I ran a brand new battery
flat in about 3 weeks. I cured the problem with a new fan belt. The current
leak was too small to worry about as the car was a daily driver and the
charge lost over afew days was very small.
I also had the exact same problem on a 80 VW Vanagon, this time it was the
diods in the altenator. The altenator was kicking out enough power to
run the car but was not putting any charge into the battery.
I cured this by getting a new altenator.
Good luck.
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