a while back my 99 5th gen had a prob starting...just clicking and then id hold the key down and it would click click then start to turn over. turned out there was just acid and corrosion on the terminals cleaned em up and it worked fine..that was 3 months ago now theres 2 x as much corrosion and its doing it again...whats causing the build up on my battery..is it the battery or is it something worse/else

thanks