Flooded lead acid batteries generate Hydrogen gas when charging, and if the concentration of the H2 reaches 4% or more, it becomes a flammable mixture that could be set off by an ignition source.
The question is: Is there enough natural air circulation in a camper van to keep the concentration of Hydrogen to a safe level?
So, it comes down to a question of how much H2 is generated as the batteries charge vs how much natural air exchange occurs in the van.
Storage Battery Systems offers this calculator to estimate how much H2 is produced during heavy charging…
My van uses two 6 volt golf cart batteries that have a 20 hr discharge rating of 220 amp-hrs and a 6 hour rating (which is what the calculator wants) of about 185 amp-hrs. The calculator says that for those inputs, 0.82 cubic feet of H2 will be generated per hour during heavy charging.
If you then plug in the height, width and length of the van, it tells you how often you must exchange the air in the van to keep the H2 concentration to no more than 1%. For our van (6.5′ by 6.4′ by 14′) the volume is 580 cubic ft.
To keep this 580 cubic ft volume to less than 1% H2 with the 0.82 cubic ft per hour of H2 being generated, the van air must be exchanged every 427 minutes (7.1 hours). So, as long as the natural air exchange rate for the van is every 7.1 hours or less, the H2 concentration will be less than 1% that is deemed safe.
I really doubt that anyone could build a van that was so tight that it would not exchange the air inside over 7 hours. In homes, a natural air change rate of one air change per hour is considered quite good.
Hydrogen is 14 times lighter than air and a pocket of H2 in air will zip upward at 45 mph. So, if you have the typical fan in the ceiling of your van, the H2 will likely rise very rapidly up to the fan area and leak out though the small gaps that exist around the fan even when its closed.
In one respect, this calculator has a large margin of safety in that h2 concentrations of less than 4% are not flammable, but the calculator uses 1% as the maximum.
I guess that something like a charger malfunction might lead to higher rates of H2 production, but if you assume the van air change rate is one air change per hour (quite tight), the margin of safety is about a factor of 7 against a 1% concentration. I don’t see how alternator or solar charging could come anywhere close to this?
I don’t want to encourage anyone to do anything that is unsafe, so if you have a reason why venting is needed, please speak up (comment section below).
In the end, you have to look at the data here and elsewhere and make your own decision on which way you want to go with the venting. If you just feel safer with the venting, then its not a big deal to just put it in.
One thing I would not recommend is making the battery compartment air tight, as this could build a high concentration of H2 within the battery compartment.
If you do a vent system, I think an air inlet to the battery compartment from the van that is low in the compartment and an outlet to the sidewall of the van high in the battery compartment would be good. This is the setup I used: Our ProMaster DIY Camper Van Conversion — Electrical and Solar – Build A Green RV
Gary May 6, 2020