Assuming the clip below from Wikipedia is correct the Volt (and likely the Bolt too) appear to already use a "middle slice" of the battery. When we charge the the battery "fully" we aren't charging it to absolutely %100, but rather 100% of usable (allowed capacity) likewise when the battery is "fully" discharged it isn't really at true 0%, but rather at the lowest usable (allowed) charge level. If this is correct is there any benefit to reducing this usable slice of the battery further?
The 2011 Chevrolet Volt has a 16 kWh / 45 A·h (10.4 kWh usable) lithium-ion battery pack that can be charged by plugging the car into a 120-240 VAC residential electrical outlet using the provided SAE J1772-compliant charging cord. No external charging station is required. The Volt is propelled by an electric motor with a peak output of 111 kW (149 hp) delivering 273 lb·ft (370 N·m) of torque. Capacity of the battery pack was increased to 16.5 kWh (10.9 kWh usable) for 2013 models, which increased the all-electric range from 35 to 38 mi (56 to 61 km). Other specifications remained the same. The battery pack capacity was increased to 17.1 kWh for 2015 models. This incremental upgrade is likely to reflect in an improvement in range over previous model years, but as of July 2014, the 2015 Volt has not been re-certified with the EPA.