Watching the KWh/m drop at speed - Chevy Bolt EV Forum
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post #1 of 15 (permalink) Old 04-23-2019, 07:27 PM Thread Starter
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Watching the KWh/m drop at speed

Vertiformed awesome data analysis notwithstanding, a single data point experience this past weekend makes me concerned about the viability of Bolt road trips. So far--and it's admittedly early in my experience--according to the display next to the GoM, I'm hitting pretty close to 4 m/KWh. Hitting that 4.0 figure as I'm driving and then trying to maintain it has because sort of a visceral goal each time I get fully charged.


So this past weekend, I got a full charge at work and then drive the 18 miles home on Friday night, arriving with a nice 4.2 m/KWh. Sunday, I had a mini road trip, a drive to Sunnyvale and back. No traffic, and with a couple of brief exceptions on the way back, I maintained around 72-75 MPH. When I got home, 75 miles later, I had dropped precipitously, to 3.4 m/KWh.



I'm used to setting the cruise control to 84 most of the drive along I-5, 58, and even parts of I-15 from here to Vegas, a 580-mile one-way trip. And both mountains and wind can do a number on mileage as well. Thing is, in my former ICE cars, I'd typically see a rise in gas mileage, even at those speeds, because of no stop-and-go. But even if I tamp down my cruise control to, say, 75, I don't feel confident in being able to go more than about 150 miles before the battery level starts to worry me, especially west of Bakersfield where charging is still sparse.



Not to mention pretty much any trip would be spent with A/C going pretty strongly, also sapping energy.



Am I being overly paranoid, or is this a legitimate concern?

Last edited by twriter; 04-24-2019 at 02:58 AM.
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post #2 of 15 (permalink) Old 04-23-2019, 08:05 PM
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In my model, driving at 75 mph I'd expect 3.49 miles/kWh in ideal conditions. With only a 1 mph headwind (effective speed 76 mph), the efficiency drops to about 3.39 miles/kWh.

In @Zoomit 's model (with graphs in this post), the expected efficiency at 25 mph is only 3 miles/kWh. Probably @Zoomit 's model is a bit too pessimistic and mine is too optimistic. Then again, neither of our models give the Bolt it's claimed range of 160 miles at maximum speed (93 mph).

Imagine this scenario, suppose you have 50% battery left and plan to drive 90 miles at 74 mph and arrive (at a charger, perhaps) with 15% battery left. If you have a 5 mph headwind for the whole trip, you'll barely make it if you keep driving at 74 mph.

In other words, it's actually pretty hard to go from ideal range to real-world range. On the positive side, in my experience the myCheverolet app's Energy Assist feature is pretty good when combined with Andrioid Auto or CarPlay for estimating final battery percentage at your destination. It's going to give you the clearest indication as to whether or not you're going to make it.
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post #3 of 15 (permalink) Old 04-23-2019, 08:17 PM
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did the 2019's update the histogram? my 2017 is very hard to extrapolate across the chart to determine the 'Present' miles / kWh as you drive.

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post #4 of 15 (permalink) Old 04-23-2019, 08:24 PM
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Yes your observations are correct. Bolt efficiency sucks at 70+ type use. You gotta either live with the suck efficiency or slow down.

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post #5 of 15 (permalink) Old 04-23-2019, 10:01 PM
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Quote:
Originally Posted by twriter View Post
I'm hitting pretty close to 4 KW/m. Hitting that 4.0 figure as I'm driving and then trying to maintain it has because sort of a visceral goal each time I get fully charged.

I maintained around 72-75 MPH. When I got home, 75 miles later, I had dropped precipitously, to 3.4 KW/m.

I'm used to setting the cruise control to 84 most of the drive along I-5, 58, and even parts of I-15 from here to Vegas, a 580-mile one-way trip. But even if I tamp down my cruise control to, say, 75, I don't feel confident in being able to go more than about 150 miles before the battery level starts to worry me, especially west of Bakersfield where charging is still sparse.

Three items: 1) You already caught your typo, that it is miles/kWh we watch, not kW/m. 2) Where I am from, driving 75-84 mph is not "real world" driving. You will never achieve even "expected range" driving 84 mph. In the summer, when my SoC is 290 max & 230 expected, I can easily get 250 miles by slowing down to 60 (from 70) on the Interstate (i.e. no stop-and-go). #3 ) is a few questions: How far back does the mi./kWh reading average? Is it by time? Is it by distance? It is not instantaneous I know. But I notice changes in <5 minutes (or miles) of driving. Why does it change when you first start the EV after a full charge has completed, but you have not driven even one foot yet?

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post #6 of 15 (permalink) Old 04-24-2019, 02:21 AM
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Thing is, in my former ICE cars, I'd typically see a rise in gas mileage, even at those speeds, because of no stop-and-go.
ICE cars are incredibly inefficient in city traffic because they dump all kinds of energy overboard when they brake and when they idle. So you actually get better efficiency on the freeway simply because you eliminate all that waste. This doesn't mean that they're more efficient at higher speeds, what it tells you is how truly awful they are in stop-and-go traffic. If you could drive for a long distance at a constant 30MPH then you'd see much better fuel efficiency than you get on a freeway at high speed.

EVs are incredibly efficient in city traffic because they don't waste power idling and they recoup a lot of power when you stop. So the relationship between speed and efficiency isn't masked by the horrible inefficiencies you see in ICE vehicles. Simply put: the faster you go the less efficient the car will be because of all the extra energy needed to push the air out of the way. Going from 65MPH to 80MPH requires a LOT more power, far more than you save because the journey takes less time. If you're concerned about efficiency, give 65MPH a try and you'll probably be surprised by how much better it is. If you're worried about being an impediment then just find one of the many trucks that drive at that speed and follow so that it takes the blame. You don't have to tailgate it, just drive behind it at a good, safe distance.
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post #7 of 15 (permalink) Old 04-24-2019, 03:12 AM Thread Starter
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Yeah, I realized when I was headed to my soccer game that I meant m/KWh. I fixed everything except the title.


I think my biggest worry would be the trip back. After 45 miles on I-15, there are some biiiig long climbs. I think that right there mandates a stop in Baker. Inexplicably, there are still no DCFC in Barstow. I suppose that might be good because the trip west on 58 is almost always into a stiff, stiff headwind through the high desert. And that's before you have to make the climb to Tehachapi.



Thing is, when I plan those trips, I plan them to avoid serious traffic. For example, I go for a holiday weekend, I'll leave on Tuesday and miss all the Vegas-to-LA traffic that clogs I-15. So I can comfortable set the cruise control to 82-84 and not only have an uncluttered drive, I'll still get passed by more than a few cars.



If I were try that in my Bolt, I can't imaging wanting to set cruise control at more than 70-75. That'll not only add not insignificant time to the trip, I'll be overtaken by lots of cars.
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post #8 of 15 (permalink) Old 04-24-2019, 11:59 AM
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I've found cruising at 65mph isn't so bad. This coming from somebody who used to do 85 every opportunity to do so. Play some tunes, re adjust your mind to the 65 life for road trips.

You can always blast around the shorter trips @ 85 when range doesn't matter... Don't get me wrong the Bolt does the 85 type work fine too, its just the efficiency is crap, as you have seen.
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post #9 of 15 (permalink) Old 04-25-2019, 07:29 AM
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Originally Posted by hatchy View Post
I've found cruising at 65mph isn't so bad. This coming from somebody who used to do 85 every opportunity to do so. Play some tunes, re adjust your mind to the 65 life for road trips.

You can always blast around the shorter trips @ 85 when range doesn't matter... Don't get me wrong the Bolt does the 85 type work fine too, its just the efficiency is crap, as you have seen.
And a road trip in an EV with hour long charging stops is relaxing compared to trying to make a speed run to your destination. I used to drive 10 to 12 hours on a road trip, and arrive exhausted... now a days driving for the same distance traveled is typically 14 to 16 hours and I am not nearly as tired when I get to my overnight stop.

If I am making a drive due to family emergency or something of that nature I could use my ICE car, but for vacation trips I actually prefer the comfort of the Bolt. This may change if my ICE wasn't a Miata

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post #10 of 15 (permalink) Old 04-25-2019, 10:00 AM
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Here in California, anyone driving at just 65 mph on any freeway is likely to be rear-ended. Of course, this is assuming that traffic isn't gridlocked.

I pay little attention to the guess-o-meter or my mileage. It just doesn't mean anything to me. I plug in my Bolt at home three nights a week (whether it needs energy or not), and leave it at that. The electricity my Bolt uses is so much cheaper than the gasoline an ICE car uses. Gas is over $4 a gallon here these days.
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