Mate I reckon I know your problem, If it isn't a mini one (I probably is but just incase) as I use a non-mini alternator. You see the alternator may have an 'excite line' which is permanent 12v, which helps the alternator charge at the correct rate(withou this it may charge up to 17v

). The lucas one gets this through the alternator light but many other alternators work different. If you don't connect the 'excite line' through igntion but through battery the alternator is kept magnestised when the car is off (there's a word for it, but I can't remeber as that was last semester's physics!

). And you are continually loses 12v. Which will drain your battery. At the moment my alternator light doesn't work with the alternator I have now. I know why though but it's a bit more comlicated so I won't confuse you anymore (uses an LED and a resistor). I hope this helps, check that all lines heading to the alternator on from +ve do actually come from igntion power, NOT battery. But by all means see an auto electrician an learn about it all.
[EDIT] I just re-read your posts, you, most likely have a Lucas Mini one I reckon so what I said above doesn't apply to you, but I will leave it there for other people to read.
What is the number on it? What brand? Etc...
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1964 Morris 850, 1330 Supercharged - 81.8hp atws.
1975 Leyland Mini S 1100S powered - Nice and reliable.
1977 Leyland Mini LS - Project LS-T
