Keep in mind your 2014 Volt uses the battery ALL the time. The car is propelled 100% of the time by the larger electric motor. The Gen 1 Volt is a "battery-operated device" (like a flashlight or cell phone), and the battery is not a fuel storage container, but a portable generator. Inside the cell, a chemical reaction creates the output power. When fully charged, the reaction can be quite robust. When you are just cruising down the road, only a moderate level of output is asked, but when you floor the accelerator, you may be asking the battery pack cells to create 100 kW or more of peak power (to compare, charging from the wall at 240 volts is less than 4 kW of output).
As the cell is discharged, that chemical reaction gradually slows down. The ability of the cell to create the output power at the desired strength diminishes. Eventually, the cell can’t produce enough output to meet the demand (think of a cell phone dropping a call or a flashlight beam starting to fade). GM has programmed the Volt to start the gas engine and switch automatically to Charge Sustaining Mode before the battery pack cell voltage drops this far.
Like all battery-operated devices, the Volt functions better when the battery is charged, and your Volt’s battery is a rechargeable battery. Over time the environment within the cells may deteriorate, causing the cells to lose some overall capacity (i.e., the kWh Used per full charge), but when the cells are operating within the Electric Mode limits, the battery pack should remain relatively capable of meeting the fueling needs of the motor.
The Gen 1 Volt is also unusual among these types of plug-in cars, in that once you’ve run out of Electric Mode power or you switch to Charge Sustaining Mode by using Hold Mode, the car continues to be propelled by the electric motor. There’s no "automobile engine" under the hood that takes over the task of accelerating the car and then propelling it at speed. GM’s "range extender" concept is quite intriguing. In Electric Mode, the battery supplies both acceleration power (high draw, short time) and speed maintaining power (low draw, long time). In Charge Sustaining Mode (Hold Mode or fully depleted battery), acceleration power is still provided by the battery (when you floor the accelerator, that 100 kW+ of output can only be supplied by the battery), but now it’s generator output that provides the lower level of power for maintaining speed (generator output is also used to recharge the battery so it’s ready for the next acceleration episode). As long as there’s fuel for the generator (i.e, gas), the Gen 1 Volt can continue down the road on electric power.
Granted, I’m ignoring the low-torque conditions under which the generator may also be clutched to the drivetrain to allow engine torque to contribute to propulsion torque, but when you drive your 2014 Volt beyond battery range, in effect, you are STILL using the battery to accelerate the car, and you are STILL using electricity to maintain speed (but now you also need to burn gas to create the gas-generated electricity to maintain the speed)... and if you know the trip is going to be long enough to be beyond battery range, you should use Hold Mode when there’s still, say, 2-4 green bars of grid power, so that the "range extender" is doing its thing with the battery pack cell voltage still at a "healthy chemical reaction in the cell" level. Then when you know you can finish the trip using those final 2-4 green bars, you switch back to Normal.
