could just be the brush pack in the alternator with the zener diode arrangement in, used to replace loads of them in my last job , 9 times out of 10 with alternators its the brush pack that fails... failing that next port of call would be the fuse and relay , then i would consider an alternator, but would be a good idea to charge the battery up to full and take it to a garage to drop test it for you . might be worth finding what the ampage draw upon the battery is when ignition is fully of also and if there is something drawing your precious volts disconnecting the offending items one by one... electrical gremlins can be a nightmare and take time to sort out properly, but just stick with it, you can bet your life on it if it isnt the alternator it will only be a 5 min job IF you knew what it were .... all diagnosis im afraid ..