The last time I removed the battery pack there was a split in a weld at the rear of the battery pack. I decided to quickly stick weld it up but due to the unlucky position of the insulation there was a gap and one cell got a hole in it. I had to remove the whole pack then cut the battery out and weld the battery case up with a glue gun (my favorite tool).
When working on the battery pack I really must remember to keep the vent pipes on the vacuum pump otherwise a Hydrogen explosion is going to happen.
The batteries are constantly producing Hydrogen and Oxygen gas, especially after a water refill. This is because when the water level is raised the acid connects two regions of lead in the battery in different states of charge. (the top of the cell is at lower voltage than the bottom) This produces current from top to bottom of the plates and makes a lot of gas. I discovered this last time when I switched off the vacuum pump while waving a blowtorch around. It ignited the gas and blasted the tops off 18 cells! I had to use the glue gun for about 3 hours that evening to get the van running for the next morning.
In fact I am looking for a vacuum pump that can run continuously to clear the gas.
It has to be able to cope with acid vapour.This video is of an early method of filling the batteries with demin water. I have switched to an Edwards EDM12 with a vacuum regulator.