Electrolyte venting
WARNING: This page is about a different car, the 2008 GMC Yukon and 2008 Chevrolet Tahoe. However, it is still accessible from the selected car via links, so may be relevant.
Each of the 40 hybrid batteries has a vent for electrolyte gas that is located on the top of each battery. The hybrid batteries vent system connects all of the 40 hybrid battery vents to a rubber manifold. The vent exits the hybrid drive motor generator battery through a single hose in the rear and is vented through the floor panel of the vehicle. These vents are mechanical and will only open when a predetermined amount of internal battery pressure is reached.