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.