I don't think you need fans at all, just proper venting. I have worked a bunch with hydrogen building a fuel cell robot. The problem isn't preventing build up, the problem is holding onto the hydrogen in the first place. Very slippery stuff, it just disappears in spite of your best efforts.
So a vent at the highest point and a vent at the lowest point and you are good to go, hydrogen wise. I think that heat build up is a greater issue in some cases anyway. Mt batts are in a shipping container and I put large (1.5") holes through the floor in-between each battery. Then a large vent out the top. On a warm day you can feel the air flow.
Jonathan