[Battery Acid leaking from covers and hissing]

So I just replaced the alt, bought a new battery and drove the disco (98) for the day, came home and the battery was hissing from the negative side point and I could see acid leaking from the top cover. I looked into it and I've seen that people say it could be a faulty battery, or overcharging. I'm not sure how to check for overcharging, I do have a multimeter.

Thanks

[Can't get alternator out (Disco 1998)]

So I'm not sure what I'm doing wrong or missing. I've taken off the belt, removed the two long bolts securing the alternator and it won't budge at all. Is there any more bolts I'm missing. Rave and others just say those two.

1998 Discovery.

thankyou.