I don't have an electro setup, but charging a battery in a vehicle also releases hydrogen.
I used to have a 68 GMC. Left the battery on the charger overnight with the hood down, in the garage. When I raised the hood, and unhooked the charger, it made a spark. That was a big bulbous hood, and had trapped a lot of hydrogen. Big bang for sure. Worst part was it blew the caps off of the battery, and splashed the acid all over. Had a few red blotches on my face from it. Lucky I was wearing glasses. :-[