That's an SLA (sealed lead acid) battery typically used in a UPS.
It doesn't create explosive gas (hydrogen IIRC) like automotive batteries do.
Maybe get an old UPS, preferably a UPS that uses an equivalent battery, and use it to charge the battery.
The simple solution would be to get a 120VAC-12VDC transformer/power brick and run the lights directly, but you must have a reason not to do that.
Honey, does this tool belt make me look FAT?