Fried Batteries

I got a Mamba Max 5700 for my 4-tec, but sinde i dont have a front drive belt right now i tried it in my maxx. First I just went with a 6 cell pack, then i made a plug to series two together. It was running fine then it just stopped I checked it out and on the ends the battery bar was burnt into the cell, one is ruined the other still works. Can anyone tell me what happened here.