MSHA - Code of Federal Regulations - 30 CFR 56.4502
30 CFR § 56.4502
(a) Battery-charging stations shall be ventilated with a sufficient
volume of air to prevent the accumulation of hydrogen gas.
(b) Smoking, use of open flames, or other activities that could
create an ignition source shall be prohibited at the battery charging station
during battery charging.
(c) Readily visible signs prohibiting smoking or open flames shall be
posted at battery-charging stations during battery charging.