My experience with Mountain Mode is that it keeps (or generates) a small percentage of battery available to supplement the ICE when going up hills. If you have exhausted the battery and then turn on Mountain Mode, something I do often, the ICE will generate extra power to charge the battery to provide that supplementation when needed. In cases where I have arrived at my destination still in Mountain Mode and shut off the car, then drive it again in Normal mode (without charging), there is about 5-10% battery available that is left after using Mountain Mode. If you keep it in Normal mode, the Volt will use up that 5-10% and then switch over to the ICE. It's a great feature. I have blasted up steep mountain pass grades with plenty of power thanks to MM. Just one of many reasons I love my Volt!