[jakkes]

probably a good idea to explain why it can be dangerous.

nothing in a lead acid battery is explosive... but the battery will give off hydrogen when charging and depleting..

hydrogen is volatile.
it's also lighter than air so will not hang around the battery in an open air environment for very long.

this is why you need to fill the battery up with distilled water.

If its not full you get an air gap under the battery's lid and hydrogen will build up & accumulate in the top part of the battery's case/lid. This is the same for indoors closed environment.

This in a non-ventilated area with an ignition source (connecting leads to the battery or surrounding ignition sources ie:electrical equipment "stationary motor" or "fork lift" etc etc can be enough to ignite it.

I would mitigate the these risks like this:
.have hose/water ready for clean up and emergency
.eye protection / glasses are a must!
.do this on a day with a little bit of wind and out doors
.fill battery with distilled water leave caps loose
.have welder unplugged from wall
.have welder's switch turned off as well
.connect welder to cleaned terminals
.insure welder is set to lowest power setting
.plug in to wall
.then turn on
.then adjust the power to suit


whynot is correct it can be very dangerous if the person is not informed

another way to mitigate this would be to not be a tightass and just buy a new battery pmsl.

we always keep an old car battery in the shed for testing stuff, pumping diff oil etc etc if you want to revive an old battery this can work well.....l and then you have people who do this to keep there currant car battery going and they have good success