Sean
The volume of hydrogen produced by a lead acid battery =
No of cells x rated ampere hour capacity x 35
Divided by 1000000
The safe limit for hydrogen is below 1%. so the volume of the room containing the battery and charger is required to work out the safe number of batteries per room.
As for the cost, this will vary as to how dischagred the battery is but as a guide the current draw will be on the side of a particular unit x length of charge, 8 or 12 hours.
Hello,
Check this out its in italian but you can use it to compare cost electrical versus combustion forklifts
http://www.carer.it/bench/bench_it.pdf