Originally Posted by dznf0g
I don't think it's as critical for the tongue boxes, as they are not in a living area
I agree, that's the first thing that must be pointed out. There are two reasons to vent the gases. First for safety of inhalation due to toxic gases, and second for ventilation of the discharge gases in a cell-overpressuring scenario.
Modern batteries have enhanced seals--yes, I know they have a vent, but it is not the same vent as 15 years ago. The purpose of the vent was previously for venting any gases as they were formed during the electrolysis reaction of each charging cycle, and this happened constantly. However, these reactions are facilitated differently now in the days of lead-calcium grids becoming the norm as opposed to lead-antimony grids. The self-discharge rate is significantly reduced as is the venting.
If the battery is being mounted externally, I wouldn't worry at all about the venting as long as the box is not 100% waterproof. If you are really concerned, tap in a piece of 3/8" tubing just to equalize the pressure and allow some diffusion of the gases. If the batter is being mounted internally, I would say that your level of venting depends on your level of use. If you are frequently running the battery system down to minimum charge and then recharging, yes, you should certainly put some giant vents in, and what was mentioned about having 2 for proper flow in and out is also very important to remove all gases away from the battery.
In the event that you have some sort of actual vent line on the battery, you can actually just hook this up directly to a tubing line that exits the trailer. This is how some modern sports cars and european vehicles are built when they have the battery inside the passenger area. For instance, on the Mini Coopers, some models have the battery in the trunk compartment, and you will note that the batteries have a single 1/4" tubing line off of a battery vent connection that leads to a spot high up in the engine compartment.
Why high up you may ask? Because H2 gas is light, lighter than anything else, and if you install a vent down, then you will always retain some amount of H2 in the vent tube. Modern batteries vent when they reach a couple psig of pressure, so they will initially have enough force to push the gas out, but the remaining H2 in the line will stay. For optimum operation, you should have one vent going up and out, and one vent coming in low with the air, to keep the purges separate.
Just for fun, let's look at the extreme effects of liquid-gas differences with a visit to high school chemistry:
Let's say you have to put 1 fl. oz of water in your batteries. This is 30 grams of water, and at 18 g/mol, thats 1.7 gm-moles of water. The simplified electrolysis reaction is 2H2O --> 2H2 + O2. This means that 1.7 gm-moles of H2 gas are produced, and at 22.4 L/mole, this is 38 liters, or 10 gallons. That worked out pretty well.
To recap, for every 1 fl. oz of water that you have to replace (or in sealed batteries, let's assume 1 fl. oz per month), you will have 10 gallons of H2 gas.
Now we know, and knowing is half the battle! G I Joooooooooeeeee....