why did my battcaps melt?

Okay guys, so three days ago, I added two Battcaps by Xstatic to my system. Everything worked fine for two days until I replaced my battery night before last. All seemed okay until I went to start my car this morning and it was completely dead. The car smelled bad of chemicals, so I immediately thought about the Battcaps. I went to check them and the casings were starting to melt. They weren't hot to the touch. Then I metered the battery and it read 3.7 volts DC. I'm drawing a blank as to why this happened. I haven't been able to do much more because I had to go to work in my other vehicle. Can anybody tell me why this happened!? Diagram of Wiring: If you have any further questions please ask! Thanks! Zack

I see your diagram and how the grounds are set up this does work but dedicated grounds for each amp and the same for the caps at the same point on the chassis, as for your problem it could be poor grounding or one of those caps were defective from the start. Check the voltage with a multimeter using your ground you have now with the negative probe then use negative probe on cap grounds, see if  you have a difference, let me know.Chadillac

You have 3 batteries connected in parallel. Welcome to the world of "it is not okay to parallel batteries".

The only thing that I can think of is overcharging, have you used a volt meter to see what voltage your alternator is putting out?

https://www.the12volt.com/installbay/forum_posts.asp?tid=113502&KW=kinetic

Battery overheating is also caused by collapsed cells etc - it isn't always a system over-voltage (but became the same when charging a 6-cell 12-volt battery that only has 5 or 4 cells operational).    And yes - those AGMs can get too hot to touch. So can flooded cells - after the electrolyte has boiled off. And again - that's why caution is required if paralleling batteries - if a cell collapses and the other battery(s) charge the collapsed battery...... Wait till you see a room full of distorted AGMs! (twin 200 or 400V strings of 12V AGMs)