By George S. Bower
Note: The data acquisition done for this article pertains primarily to a 2011-2012 spec Volt with 16.0-kwh battery, but loosely applies to the very similar 16.5-kwh 2013 also.
How low will the battery go?
We know that the Volt’s 1.4-liter internal combustion engine (ICE) makes less horsepower than is required to climb a steep grade so we need battery power to maintain speed on some hills depending on grade and vehicle speed.
Normally the Volt uses 21.9 percent to 86 percent state of charge (SOC) of its 16 kwh battery. In EV mode, when the battery gets to the bottom of its usable range (21.9 percent SOC), the ICE starts and immediately tries to build a small buffer to use as back up. This buffer is .25-.40 kwh which is a fairly small amount.
What happens if we start climbing a hill that requires more HP than the Volt’s ICE can supply and we are already at 21.9 percent? Obviously we start draining the battery below 21.9 percent.
How far down will it drain and how long will it last?
In order to investigate, I took data using my DashDAQ up a hill that I drive routinely between my winter house in Tonto Basin, Ariz. to Payson, Ariz. For this test, I timed it so that the HV battery ran out and Volt switched to extended range mode at the bottom of the hill. My cruise control was set at 65 mph for the entire climb of 5.87 miles and 5.33 minutes. The resulting data is shown in figures 1 and 2.
We can see at the bottom of the hill right after going into extended-range mode (power split) and before the hill gets very steep that the Volt starts to build its .25-.400-kwh buffer as a reserve but soon the battery starts to deplete because of the steepness of the hill.
Initially the ICE is at 2,700 RPM but as the battery SOC drops lower and lower the Volt raises engine rpm to keep the buffer from running out. By the end of the climb, the buffer has gone to -0.7 kwh and the ICE speed has increased to 4,300 RPM. This -0.7 kwh is equivalent to a 17.5 percent SOC. I was not able to fully deplete the battery in this test. If I had, the Volt would have gone into “Propulsion Power Reduced” mode and would have slowed down. According to Walter Crowe (saghost on the GM-Volt forum) this battery “floor” occurs at 15 percent SOC.
At the rate I was depleting the battery, Propulsion Power Reduced mode would have occurred in another 3 minutes (3.33 miles) which would have given a total endurance of 8.37 minutes and 9.2 miles before the battery hit the floor.
Volt pulls a surprising shift toward the end of the climb
Usually when the Volt shifts planetary gear set configurations, it is so subtle that the driver does not even notice that the shift has occurred. However, not in this case as I remember feeling the shift when I was taking the data and did not know what had happened until I reduced the data.
The shift is shown in figures 1 and 2 at 300 seconds. At this point in time the Volt shifted from power split to series mode. A detailed look at the planetary gear set configurations before and after the shift is shown in figure 3.
In power split, note the high NEGATIVE speed of MGB (-3,597 rpm). MGB is making power and feeding it to MGA. MGA is also taking power from the battery and ICE and MGA power is being fed back into the ring gear.
In series mode, MGB is at even higher POSITIVE speed (6,078 rpm) driving thru the sun gear to the wheels while the ICE and MGA are independently making electrical power and sending it to MGB. MGB is also taking power from the battery.
But why the shift?
I do not know. I would have expected the Volt to stay in power split all the way up the hill but it did not. I can only speculate as to why the Volt shifted into series:
1) Volt wants to be in series mode when “Propulsion Power Reduced “ sets in.
2) Volt wants to be in series mode when climbing hills because of the numerically higher gear ratio of 3.21 sun to carrier vs. 1.45 ring to carrier in power split.
If reason 1 is why it shifted, why didn’t it wait to shift until it was closer to the battery floor?
If reason 2 is why it shifted, then why did it wait so long to shift?
This entry was posted on Friday, February 22nd, 2013 at 5:55 am and is filed under General. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.