vented battery box

With my V-mod, I'm putting two batteries under the front deck where I built a storage area. Is it necessary to vent this area?

While it certainly wouldn't hurt, it isn't completely necessary, under most circumstances. During use, batteries don't discharge many fumes. Generally, the most fumes are discharged during charging. So, if you have an onboard charger, either open the hatch to charge, or vent it. If you are like the rest of us, and either take a portable charger to the battery, with an extension cord, or take the batteries to the charger, then you don't need to, as you have to open the hatch anyway to get the charger hooked up.