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Not exactly sure what you are asking.
The Volt is not a simple serial hybrid. In Charge Depleting mode it operates as a Pure EV. In that mode it depletes the battery to its minimal SOC (about 25%) and then the ICE starts for Charge-sustaing (CS) mode.
In CS mode ICE can couple in the generator and operate as a serial hybrid, and in doing that it still uses the battery as a buffer to provide bursts of power beyond what than ICE can provide, and then back filling that with excess from the generator. When there is a a ligher load and/or higher higher speed in CS mode the ICE can also couple directly to the power-split device as well as the generator and the car then operates in a parallel+serial mode that can improve efficiency. In that mode the gear ratios is similar to a 6th gear in other cars.
As someone already posted, the car also has a MountainMode, which raises the minimum SOC to about 45%, providing larger buffer for longer "bursts" of power.
If the charge is below 45% when one switches into Mountain Mode, the car will recharge the battery back up to the 45%. By doing that one can push the car into a different region of the BSFC. If the car is running at 1700-1800 RPMs, then when I put it into MM, the car will shift to 3000-3100 RPMS which is in the BSFC sweet spot. Of course recharging the battery has multiple losses, but there are times when it can be useful (e.g. to have more EV power for low-speed/city driving). Also useful for showing off the EV nature of the Volt, even after a long trip.
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