The way the Volt is designed, the first 40 miles of driving are powered by the electric energy stored in the battery. After that distance from full charge, somewhere around 30% state of charge, the on-board generator kicks in. The generator's job is to keep the battery at that 30% level all the way until the driver can get to where he or she will begin charging again. That level is called the "customer depletion point". Below is Volt chief engineer Andrew Farah explaining how the Volt will behave at and beyond that level.
How will the vehicle's propulsion system work when you get to the customer depletion point?
When you get to the customer depletion point, the engine will come on seamlessly as it's supposed to. But when the engine comes on to spin the generator, it does so with the idea that we're generating electric energy to drive the wheels, not to charge the battery. People say the engine comes on to charge the battery, but that's not what really goes on. The engine comes on to make enough electric energy to turn the wheels, because the wheels are always turning electrically.
Now comes the fun part. Remember the electric generator is about half the size of the motor. So you say, how come you don't have performance problems if you're trying to go up a hill with only basically half the power capability? That's where the battery comes back into play. Because the customer depletion point is not full depletion, there's still energy available. That's by design. The idea is during certain other peak situations such as climbing a hill or merging into traffic, you will actually take some more energy out of the battery. So you may actually come down a little bit below customer depletion level.
And then when you take your foot off the gas, as an example when you're done doing the merge, we had taken a little bit out and the battery has a little less in it. So what we'll do then is we will opportunistically put that energy back into the battery either through regenerative braking or if we have to we will take some of the energy that's not needed to turn the wheels and bring the battery up to the customer depletion level.
So we don't recharge the battery. The customer wont actually see any of this, as their electric range indicator in the car will only say zero.
We are actually using that battery at that point as a peak buffer and we will keep trying to recapture energy as the opportunities allow.
Is the customer depletion point going to be exactly 30% state of charge (SOC)?
We are continuing to tweak and tune and develop exactly what that number is.