Friday, November 28, 2008

Inner Workings of the Chevy Volt

Below is an animation from GM, presented by CarDataVideo, that shows how the Chevy Volt works. Plug-in series hybrids or Extended Range Electric Vehicles (EREV) have not been used for non-industrial retail vehicles so the concept is new to many of us. This is a good intro.

I was recently trying to explain to someone how the generator does not completely recharge the car, rather it keeps the batteries in a buffer zone around 25-30% charged. This animation shows that quite clearly.

So why do they keep the batteries in a buffer zone? Why not deplete the batteries completely and then run from the generator? A couple reasons. One to avoid fully cycling the batteries. Fully depleting the pack (or fully charging it) will reduce the lifespan of the pack. If you want the batteries to last for 100k miles, then need to avoid the 0-15% and 85-100% SOC range. So when you plug-in a Volt and it reports "full" it will really be at ~85% SOC. As long as it can deliver the promised range, this is the right thing to do for the life of the battery pack.

The second reason is that you may need more power than the generator alone can provide. The Volt has a 54kW generator and a 111kW motor. At first glance it mat seem like the generator is only about half the size it should be. However, it is rare that the engine will be at full capacity. Consider normal driving, how often do you have it red-lined? Even for aggressive drivers it is a very small percentage of the time. In the times that you do need the full power of the motor, such as passing on a hill, it is available to you because the batteries still have plenty of juice when the generator kicks in. Then as you go down the other side of the hill, gravity provides the acceleration and all of the generator's power can go to the batteries. And maybe more if you do some regenerative breaking. This means that the generator only has to provide more than the average amount of power used (even by an aggressive driver) in order to maintain the charge level in the buffer zone.

Another question, the video shows the generator turn off when the batteries hit the top of the buffer zone; why not have the generator charge the batteries all the way up to 85%? It could, but that would be using gasoline unnecessarily. There would be no advantage to the car's performance. The idea of a plug-in vehicle is to get as much of your mileage as possible from the plug-in power. By keeping the charge low, you get to fill it up with grid (or solar) provided electrons when you get home or to any charging station. This makes it cheaper to operate and reduces your number of trips to the pump.


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