I have a 36 volt 600 watt wilderness energy system and my batteries (3 12v 12Ah gell cells) are getting hat and the terminals are corroding during charge. I've been using this same setup for about five years and never had this problem before.
Any ideas whats going on?
Five years? I think that is rather impressive. I get three years and it drops off fast and make it limp till dead DEAD. When batteries get HOT, it is usually because it is dried out and tired. Sulphated and if terminals are corroded that is major tell tale sign of dead of Pb Acid battery. My batteries get hot for a thing called field day and I kept using them. Heck they were dead and I knew it. I just got my last charge cycle out of it because it finally popped a top charging. Swelled up big time. Either turn them in for Lead cost or turn them in for core charge.
If you want to test it for capacity, then discharge it for appropriate time and check capacity. Interstate battery has good prices now. Try a good quality brand and go for another 4 or 5 years. You must be charging it pretty regular? I have a lot of MK batteries that get hot charging and discharging but at 60 AH new they still have 15 or 20 AH in them. A waste of space but that is all I need at the moment for solar application. Also be careful about charging a dead battery. It can put a stress on charger some times. In other words - burn out. If the battery can not get up to 2.4 v/c the charger will keep charging and the weakest cell will pop like my CSB finally did. I think CSB is a nice battery with a real 20 AH. Do you have room to put a larger Gel Cell battery like 20 AH vs your 12 AH? Corroding during charge means there is still acid in liquid form in it.