Sleepy;
Of course I could reverse the flow to only have fresh air flow over the fan. But, by pulling air through, not pressurizing the box and forcing air out the far end, the risk of a box leak allowing gas to enter the electrical areas is removed.
The fan only turns on when the AC charger is powered up. At that time, there isn't any Hydrogen gas in there yet. The fan keeps gas from building up to a dangerous level by running early enough in the charging cycle. The fan isn't needed when the camper is on the truck since then the batteries are charged off the truck's alternator and the truck is moving, allowing for air circulation. I have a sensor I picked up from work that is like a Geiger counter, but for explosive environment instead of radiation. It ticks when an explosive gas is present. When first turned on, the ticks are dialed out to off set any ambient gases. I do that outside the camper. Then the sensor is moved over and around what is suspected as gassing. If it starts ticking, there's gas present. It it starts ticking a lot, it's probably a heavy enough concentration to be hazardous. I used this sensor to first; discover there was a serious problem, then to determine if my efforts to vent were successful.
The fan is inductive, so there isn't any chance of a spark for ignition as well. An inductive fan is a magnetic winding with permanent magnets in the fan blade portion. The fan is intrinsically safe.
