Last summer I had just bought a brand new deep cycle Interstate battery for my boat. My kid was carrying it to the dock and accidently dropped it on the sidewalk in its plastic battery box. He didn't tell me. I soon found out. Small crack. Battery was no good. I had to trade the core in for a new one.
I wouldn't recommend plastic welding a battery either but here's a easy tip on plastic welding.
I used a hand torch and black zip ties to plastic weld stuff together. It works like stick welding. Heat your material up and feed and drip the zip tie. The piece your welding and the zip tie doesn't have to be the same type of plastic, just somewhat similar.
Now with this new info, I still recommend not repairing the battery.