[RMett]

I am visiting Summit county, CO and drove up I70 from Denver, unfortunately not with my Volt (due to the four passenger maximum in the Volt). My minivan had trouble keeping up with traffic on the steep inclines and driving in low down the inclines stressed the van tremendously. I wondered how my Volt would have taken this and read some interesting threads on this: http://gm-volt.com/forum/showthread...y-NEED-Mountain-Mode-for-its-intended-purpose. Apparently Volt handles this well in mountain mode. My question is what happens if you start down a major summit with a full charge? Suppose your condo is at elevation 9900 feet and you start from there with a full charge and go down to 5000 ft. Does the car use friction brakes only because the battery is already charged? Has anyone ever done this? Is this not recommended?

I calculate that a 5000 ft drop of Volt has a potential energy of about 10 kWh, which at 50% efficiency should give about half of a full battery charge. PE = mgh = 2000kg x 9.8m/s/s x 1667 m = 33 MJ/(3.6MJ/kWh) = 9.2 kWh.

 

[ghostgs1]

Don't know the technical answer for this, but I know the Volt will dump excess watts (probably a shunt type system to ground?). It won't hurt the car if it is doing this, but you hate to see the power go to waste.

 

[Gary Norton]

I do it every day. Live in Colorado at 7400 feet and drive into town at 6500 feet. The expected range maxis out at 50 miles. There is one strange phenomenon which is that I seem to get more juice going down than the extra juice used to come back up. For instance, if I start with 42 miles of range and drive 20 miles into town and back and when I pull in my range will be 26 miles. I know that is merely a projection but it seems to be saying that you gain more going down than the extra you use going up.

 

[JeremyK]

Since a full charge is not 100% of the battery capacity, there is some additional cushion for regen charging even when the battery is "full". Whether GM allows this or not, I do not know. I assume they would, since they allow the car to dip in the the reserve battery occasionally when at the low end of SOC.

If they allowed you to take on an extra 5% SOC from "full" that would be around 0.8 kWh of energy.

4900 ft elevation change would yield about 0.580 kWh of potential energy (somebody check my math)

So in theory, the Volt should be fine...assuming GM allowed a small amount of "overcharging" (i.e. up to 90% SOC).

I would also assume that if you had an infinitely long hill that the Volt will simply resort to friction breaks and stop charging the battery at some predefined SOC.

 

[RMett]

Gary, that is interesting. It seems to suggest that energy is still added to the battery going down hill even though the battery was full to begin with. This could be due to the normal 10 kWh window of the 16 kWh battery being somewhere between 20% and 80%. So you might go to 90% charge on your way to town.

 

[siromega]

I do it every day. Live in Colorado at 7400 feet and drive into town at 6500 feet. The expected range maxis out at 50 miles. There is one strange phenomenon which is that I seem to get more juice going down than the extra juice used to come back up. For instance, if I start with 42 miles of range and drive 20 miles into town and back and when I pull in my range will be 26 miles. I know that is merely a projection but it seems to be saying that you gain more going down than the extra you use going up.

I have the same issue with my commute into town (3200' to 2000' over 15-16 miles). I'll start out at home with ~40 miles, by the time I'm half way to work the dash still says 40 miles, I get to work and it'll say 35 or so, with 3/4 of the charge indicator left. Its projecting that the rest of your drive will be around the same in terms of miles per kWh as your drive so far, which isn't necessarily a correct assumption. My drive to work is usually 2kWh for 16 miles, and my drive home is about 6kWh for 15 miles (though its worse now in the summertime in Las Vegas, but I still make it home on electricity).

To the OP's question, I've been told on this forum that there is a clutch and motor in the engine that will work against the force in the engine to dissipate extra energy when driving down large hills.

 

[saghost]

Since a full charge is not 100% of the battery capacity, there is some additional cushion for regen charging even when the battery is "full". Whether GM allows this or not, I do not know. I assume they would, since they allow the car to dip in the the reserve battery occasionally when at the low end of SOC.

If they allowed you to take on an extra 5% SOC from "full" that would be around 0.8 kWh of energy.

4900 ft elevation change would yield about 0.580 kWh of potential energy (somebody check my math)

So in theory, the Volt should be fine...assuming GM allowed a small amount of "overcharging" (i.e. up to 90% SOC).

I would also assume that if you had an infinitely long hill that the Volt will simply resort to friction breaks and stop charging the battery at some predefined SOC.

There is some headroom for regen on a full battery, but not that much (I think it is around 2-300 Wh.) We have a couple members who blow through it regularly.

When that happens, the Volt starts disappating it as heat in the transmission and power electronics (it goes into two-motor, and pull a bunch of power out with one motor and put most of it back with the other, leaving a net zero charge but losing kinetic energy into the cycle inefficiencies.)

 

[baragona]

I do it every day. Live in Colorado at 7400 feet and drive into town at 6500 feet. The expected range maxis out at 50 miles. There is one strange phenomenon which is that I seem to get more juice going down than the extra juice used to come back up. For instance, if I start with 42 miles of range and drive 20 miles into town and back and when I pull in my range will be 26 miles. I know that is merely a projection but it seems to be saying that you gain more going down than the extra you use going up.

I have a similar commute that goes downhill from home to work.

I find the total range used up when I get home to be consistent with the actual number of miles I drove that day...

 

[Chevrolet Customer Svc]

@RMett,

The Volt is designed for mountainous driving, and in many ways is better than a conventional vehicle because it does not suffer "altitude sickness". If the battery is "full" on the display, there is still some room for added regen energy into the pack. If the upper limit for the battery is reached, then the system will use both electric motors to resist one another to help slow the vehicle. This many not be enough resistance on a very steep decent to maintain what you normally feel in Low range. So in the rarest of cases, you may need to use the brake pedal to engage the friction brakes in this situation. Simply having the shift lever in Low will not automatically apply the brakes. This has added exposure with the new Hold mode feature in 2013, where it is not necessary to charge at the top of a hill to experience this functionality.

 

[Chevrolet Customer Svc]

Gary,

Relative to the displayed range, remember that this is an estimate tuned for typical driving. Extreme elevation changes over a short distance may alter this estimate. Thanks for your feedback. How has your experience been with the Volt in the Durango area?

 

[Bzzzt]

Using your delay start charging timer in the car, you could 'short charge' the battery (say 80% or whatever) by telling the car to be ready later than when you actually leave. Therefore, maximizing your energy recovery....

 

[Gary Norton]

Gary,

Relative to the displayed range, remember that this is an estimate tuned for typical driving. Extreme elevation changes over a short distance may alter this estimate. Thanks for your feedback. How has your experience been with the Volt in the Durango area?

Wonderful. The Volt has been fantastic to drive in town, in the country and in the mountains. And the economy is better than expected. My electric rate is 4.6 cents/KWH and my cost of driving off the battery is about 1 cent/mile. My first 1,000 miles cost me $10.08 for electricity and $5.30 for gasoline. But beyond that it is just so cool.

 

[jaksecret]

Me too, 7400 feet to about 5300 feet every day in about 7 miles. car starts full at 44 miles, increases as I go downhill to 50 miles, then near the bottom of the hill, i lose the deceleration of regen and the car feels like its coasting. I don't know that it dumps that power to ground, I think it just stops regen mode.

 

[saghost]

Me too, 7400 feet to about 5300 feet every day in about 7 miles. car starts full at 44 miles, increases as I go downhill to 50 miles, then near the bottom of the hill, i lose the deceleration of regen and the car feels like its coasting. I don't know that it dumps that power to ground, I think it just stops regen mode.

Put it in neutral after you think it's coasting, and you'll see. The power dump isn't nearly as strong as L usually is, but it's definitely there...