1. When the battery is fully charged, each charging ampere supplied to the
cell produces about 0.016 cubic feet of hydrogen per hour from each cell.
This rate of production applies at sea level, when the ambient
temperature is about 77 EF, and when the electrolyte is "gassing or
2. Number of battery cells and maximum charging rate (not float rate) can
be obtained from specifications or field inspection.
3. Hydrogen gas lower explosive limit is 4 percent by volume. Good practice
dictates a safety factor of 5, which reduces the critical concentration to
0.8 percent by volume. This large safety factor is to allow for hydrogen
production variations with changes in temperature, battery room
elevation, and barometric pressure and also allows for deterioration in
4. Examples of calculations for determining adequate battery room
ventilation appear below:
A fully-charged, 60-cell, lead-acid battery, located in a room having a volume
of 60 cubic meters, is being charged at 50 amperes. The ventilation system
is designed to provide three air changes each hour. Determine the rate of
hydrogen production, the critical volume of the battery room, and the
adequacy of the air exchanges required for ventilation.
) production in cubic meters per hour is:
(50 amps)(60 cells)(0.000453 m
/cell/hour) = 1.359 m
Critical volume, with safety factor based on 0.8 percent by volume is:
) = 0.48 m
Hours to produce critical level of 0.8 percent hydrogen (0.48 cubic meters)
in the 60-cubic-meter battery room is:
0.48 ÷1.359 = 0.35 hour (21 minutes)
The ventilation system must move 60 cubic meters (the room volume), with
the 0.48 cubic meters of hydrogen contained within, before the 0.35 hour
(21 minutes) elapses.
Three air changes each hour provide one air change in 20 minutes, which is
quicker than the 21 minutes required. Critical hydrogen concentration will
not be reached with continuous operation of the ventilation system.