Back when I was a Service manager for an industrial battery shop we had a "Poly Welder" for welding the polypropylene 'jars' that contain the electrolyte in huge forklift battery cells. leaks weren't uncommon, and a leak usually would create a small but obvious dead short in the leaking cell- not to mention the corrosion that would start way down in the bottom of the tray. We'd drill off the lead intercell connectors with a 1" hollow drill, and lift the 200# cell out with an overhead hoist, drain it, and remove the assembled plates from the jar, and repair. The jar was also polywelded to the plastic cell cover on top.
The polywelder wasn't cheap- about $600.00 in 1990's dollars as I recall, but it got used every day. It used a heated tip like a large soldering iron, a piece of 3/16" polypropylene rod, and an air compressor to transfer the heat to the point where the weld is happening.