A forklift uses a battery which varies dependant on the model and brand of the forklift. The battery for each brand of forklift is usually different but some are able to run on batteries similar to car batteries.
Oxygen and hydrogen
Before charging it.
check the battery
Distilled Water
No, that may damage the battery
That's a 'hydrometer'.
Chlorine gas is produced.
Depends what is wrong with it.
Hydrogen
Overcharging by the alternator or if it is connected in reverse polarity.
Usually the battery is too low