After a lengthy and healthy discussion about costs of driving the Volt we needed to know at what battery state of charge (SOC) the onboard generator would kick in. GM was kind enough to let us know that was intended to be 50%.
The next question to arise was that once the generator started at what SOC would it stop again as the car continued to drive.
Once again GM has been very helpful and informative and tells us the answer is 80%.
So what do these numbers mean? Mainly it tells us they wish to be very conservative with the batteries. Although A123 Li-ion cells are very robust and can handle repetitive deep discharges to below 20% and up to 100% again, up to 7000 times, GM clearly doesn’t want to take any chances with this very expensive (and possibly leased hardware). By keeping it in this limited range, the hope will be increased reliability and longevity I would have to assume.
Lets see what happens when one drives the Volt.
For the first 40 miles, as we’ve discussed, the battery will drain from full (16 kWh energy) to 50% (8 kWh), this 8 kWh will cost you roughly 85 cents in electricity. If you drive that distance or less, NO GAS.
If you keep driving, the generator will start. It will generate 53 kW of electricity. It needs to restore the pack by 30%, which is 4.8 kW, that would take ~5.4 minutes if the generator was only charging the battery. But at the same time, as the car continues to drive, the battery would continue to drain, so the generator would have to run longer.
GM estimates that in this condition, the combustion engine would provide 50 mpg efficiency.
From what engineering experts in the PHEV/EV field tell me, the battery pack, electric engine, and generator are all on the same bus (not yellow or greyhound folks), which means that one can have current flowing into the battery from the generator at the same time it is leaving to run the powertrain.
This is great stuff, and an engineering process never before witnessed by humankind. Let’s hope it works.
In the coming weeks GM will have the Volt “mules” (cobbled-together, rough and ugly engineering experimental prototypes) up and running with the first gen A123 packs (and CPI). Then these issues will be testable under driving conditions.
Right now, as per Rob Peterson of GM, “the engineers are still working out the optimum charge cycles and control systems”. As per Bart Riley of A123, “The key will be to achieve the life target for the battery across the all operational requirements (temperature, cycling, storage, SOC range)”.
Remember folks, you just can’t get this stuff anywhere else!
This entry was posted on Friday, August 24th, 2007 at 7:21 am and is filed under Battery, Electric Motor, Engineering. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.