I have seen technician that by neglecting some procedures, batteries had exploded on their faces. this is not a joke. Please handle with cautions.

When you apply DC current to a battery, your batteries are being recharged. but by electrolysis it starts to separate the hydrogen from the water which is very flammable

This gas is explosive in certain concentrations in air (explosive limits are 4.1 to 72 percent hydrogen in air).

Hydrogen gas is colorless and odorless, and is lighter than air, causing the gas to rise to the top of a building. The concentration of hydrogen in the air should be kept below 1% to reduce risk of explosion.

Some recommendation when you are doing regular maintenance on your batteries

This is essential to prevent an explosion:

Don’t put metal recipient on top of your batteries when you are refilling them.
Don’t come near batteries with cigarettes or matches.
If you need lighting. NEVER get near to the batteries with any candles of any type of flame.
You should (I always do it) vent the area when you are going to service your batteries. With a fan or just waving your hands with some carton board. Specially if I will remove any jumper or connecting any other equipment. (Capacitor charge always cause a spark when you connect equipment for the first time)

And if your batteries are gassing too much, here some tips:

Always verify the level of water of your cells.
reduce the amps of your charger and Bulk voltage.
It might be one or a few batteries that are gassing so you should check battery per battery.
If your batteries are AGM or Gel look for any leak of Gas because is not your batteries that are flammable. LOL