This is the early plan for a battery enclosure intended to house and vent the battery bank in the garage: Battery Enclosure Plans (REVISED 2/17/2011)
A brief explanation of passive ventilation and the use of conductive materials (copper) for the piping:
The metal pipes act as a heat sink, which effectively eliminates drafts, ensuring that venting only eliminates the lighter than air hydrogen — an analogy to illustrate this principle is to imagine a modern vestibule (a typical grocery door entrance with a set of inner and outer doors). A vestibule creates a space of air that is somewhere between the inside and outside temperature. By opening one door, and then the other, a draft is eliminated. But when both sets of doors are opened at the same time, a current of air is produced by the pressure gradient created between heavy cold air and light hot air. By using thermally conductive pipe, a pocket of air is created that bridges two temperature extremes, eliminating the draft. A PVC pipe would not create this thermal gradient, and would actually facilitate the creation of a draft — which is known to create swirling pockets of air capable of trapping hydrogen.
Note: hydrogen gas will not be created in massive quantities — but it is more often-than-not better to over engineer, especially when the price of copper pipe compared to PVC is insignificant compared to the overall cost of the system.
offgridcabin 
Thank you so much for you blog and detailed information. It is heads n shoulders above most of what can be found out there! I am trying to design and build 2 battery boxes. One for a friend who has only 4 each batteries similar to yours. Hers will need the passive venting system that you’ve used. Can you tell me how long the 3/4″ copper pipe is? how far into the 1-1/2″ does it go? The 2nd box is for our Relion 51.2V/300Ah LifePO4 battery so I plan to just use a 12v computer fan and passive vent primarily for the hot summers. The box will be in our insulated shop and it will be insulated as well but a little extra heat extraction would be good.
The passive venting pipes are both copper because they will transfer heat (in coming air will become the temperature of exiting air via conduction and vise versa). The large pipe opens into the very top of the box (because hydrogen is lighter than air) and the small pipe ends about two inches from the bottom of the box. I have about 24” total of pipe for heat exchange to occur. Sorry for the delay – I’m in the process of changing the system over to LiFePO4 and just solved the temperature extreme issue of the garage with respect to charging batteries when ambient air is below freezing.