The original Lucas alternator was sized to supply the car as it left the factory, no more, no less. As soon as you add an accessory, or the alternator starts to get tired, it is no longer able to keep up with the power draw. It is very common to reach home after a long drive in the rain, headlights on, heater on, wipers on, and listening to the game on the radio and find the car won’t crank in the morning. the battery ran down while driving. The stock alternator just could not keep up with the load.
More at …… http://bradleyrestoration.com/br_exclusives/high-output_alternator.html
MGB tips mods and maintenance 