There has been a fair bit of discussion and controversy in the last couple of weeks on the interwebs about the exact nature of the powerflow and battery charging in the Chevy Volt. Back in July GM's Larry Nitz discussed how the charge sustaining mode would work. As with parallel hybrids the battery is generally kept between about 35 and 85 percent charge. That means when the battery reaches its "depletion" point it still has a significant amount of charge left. The range extender has less power output than the the electric drive motor is capable of (100 hp vs 150 hp) but it has enough output to work in most day-to-day driving conditions. The range extender doesn't normally try to charge the battery fully as that is deemed to be less efficient and more costly than holding the battery at the minimum until the car can be plugged in. For the times when more power is needed such as acceleration or hill climbing some of the reserve battery power is available to be used. The battery can be drawn down below the "depletion" point to about 25-30 percent. When this happens, a combination of regenerative braking and some surplus power from the range extender will bring the battery back up to the minimum. A video after the jump shows the various modes and the power flow.
[Source: General Motors]