To prolong the life of the battery, the car is never allowed to fully charge or to fully discharge the battery. According to the Wikipedia article on the Chevrolet Volt, for the concept car, "the Volt team decided to use only half of the 16 kWh capacity to reduce the rate of capacity degradation, limiting the state of charge (SOC) up to 80% of capacity and never depleting the battery below 30%... According to GM, as of August 2016, no batteries have been changed due to degradation..."
That SOC window was expanded to 65% for the Gen 1 Volt, and appears to have been expanded to ~77% for the Gen 2.
2011/2012: 16.0 kWh x 65% = 10.4 kWh usable
2013/2014: 16.5 kWh x 66% = 10.9 kWh usable
2015: 17.1 kWh x 65-66% = 11.1-11.3 kWh usable
This ~0.7 kWh increase in usable power from 2011 to 2015 provided a noticeable, but not dramatic increase in range.
2016/2017 18.4 kWh x 77% = 14.2 kWh usable
Although the exact settings for the max/min numbers are proprietary, from OBD readings on my 2012 Volt, the Gen 1 appears to use a 65% SOC window of ~20%-85% or ~22%-87%. I’ve seen no numbers on the Gen 2 window. I think I’ve read of 15% as a hard floor setting for the window. The amount of energy remaining in a fully depleted battery is thus likely to be a consistent % of the battery capacity, which means the actual amount varies with the battery capacity.
Note that the 53 ev mile rated range of the Gen 2's 14.1 kWh usable battery = 3.76 mi/kWh, whereas the 35 ev mile rated range of the 2011/2012's 10.4 kWh usable battery = 3.37 mi/kWh, an improvement in efficiency. As many of us know, actual numbers cycle as the season warm up and cool down.
In practice, the size of the SOC window can depend on operating conditions. The Volt’s Battery State Estimate Algorithm calculates SOC readings "on the fly," and when it estimates the "minimum SOC point" has been reached, will transition the car from Electric Mode to Extended Range Mode. If before that point is reached, the car is stopped for a sufficient time, additional data is gathered and the SOC estimate may be revised. If this adjustment revises the SOC upward, additional energy can be used before reaching the minimum. If you don’t stop or the computer doesn’t revise that reading, you switch to ICE sooner. Exactly when you switch to ICE can vary depending on a number of factors. The faster you drive, for example, the less lead time the computer has to estimate accurately when the minimum SOC number has been reached. Many factors may affect the kWh Used number for a full depletion.
It’s also said that frequent short trips followed by immediately plugging in to recharge can, over time, corrupt the Battery State Estimate Algorithm, causing the computer to overestimate power consumption rates, shrinking the usable SOC window (reducing your kWh Used numbers for a full depletion). One method of recalibrating the algorithm is by doing a number of full charge/full depletion cycles, instead of immediately recharging a partially depleted battery.
