The battery has a finite amount of power. At some point if you are pushing the car so that it is consistently putting more power to the ground than it can get out of the generator you will get to the point where the battery is depleted.
This could be something as simply as cruising at high speeds for a while, or an incredibly long hill after you've already driven past the all electric range. While someone that actually cares about burning gas wouldn't be cruising at 85 mph you know that there are going to be people that will do it and eventually hit the point where there is absolutely nothing left in the battery.
The charging system will not allow the battery to get so low that it can't provide assist. You have the on-board generator plus regen braking to keep the battery at the "customer depletion point" of about 30% charge. So, you will always have battery assist in the Volt, barring a malfunction.
Zero indicated on the battery gauge to the driver, is actually just the customer depletion point, the battery still has plenty left if necessary.
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Below is Volt chief engineer Andrew Farah explaining how the Volt will behave at and beyond that level.
How will the vehicles 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 its supposed to. But when the engine comes on to spin the generator, it does so with the idea that were generating electric energy to drive the wheels, not to charge the battery. People say the engine comes on to charge the battery, but thats 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 dont have performance problems if youre trying to go up a hill with only basically half the power capability? Thats where the battery comes back into play. Because the customer depletion point is not full depletion, theres still energy available. Thats 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 youre done doing the merge, we had taken a little bit out and the battery has a little less in it. So what well 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 thats not needed to turn the wheels and bring the battery up to the customer depletion level.
So we dont 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.