If the terminal crudded up like you say it did, there is a leak at the terminal that's allowing the battery acid to escape.
An old method of fixing this sort of leak is to buy a bottle of Karo Syrup. As I recall, we used the "dark" kind.
For those outside the US, Karo Syrup is a moderately thick corn syrup used for baking bread, cakes and other good to eat sweet things. You probably have something like it where you live.
Anyway, to fix the battery leak you pour a bit of the syrup around the joint where the lead terminal comes out of the plastic case.
In a few days it will harden and totally seal the joint. (No, the syrup is not bothered by the acid but it is an insulator so you don't want to get it on the surfaces where the wires/cables attach.)