Efficiency Testing and Long Range Driving
3 min read

Efficiency Testing and Long Range Driving

I plan to eventually make some long trips in my Bolt.  I found a program called "A Better Route Planner" (ABRP) that seems to be the bees knees for planning a route for an electric car. The problem is that to work right it needs a good estimate of the vehicle efficiency in terms of miles per kWh.  

So, the Bolt is sold as having a 65kWh battery and a 259 mile range.  So that works out to just a hair less than 4 miles per kWh.  ABRP assumes 3.5 miles per kWh for a 2021 bolt (which has ended up matching my experience fairly closely).  But that is only 228 miles of range.  A little bit of a disappointment, but not too bad.  However, I felt that was a long way from the 4 Mi/kWh that were advertised.  So I decided to do my own testing.

Now, I would prefer to do this testing on an Interstate highway.  Be none is really close at hand that does not have so much traffic that I could not maintain a constant speed.  And it occurred to me much of the trip I was planning to make most often (to Galveston) would be made on Texas highways and U.S. highways.   So I chose a stretch of US 290 from Henly Texas to the intersection with US 281.  This is about 11 miles or 22 round trip.  You do have to slow and stop for the turn around, but its not a very long period of time.  For the rest of the trip the speed limit is 70 MPH and its 4 lanes.  Generally you can pick your speed and not be interfered with.

So, I ran this test loop four times.  Twice at 60 MPH and twice at 75 MPH, once each with the AC on and the AC off.  When the AC was on I had it set to 60 degrees so it would run constantly and with it off I ran the fan on full.

For each run I started at a stop in Henly, reset the odometer (which resets the Mi/kWh indicator) and then pulled into the traffic lanes and accelerated to the test speed.  When I got to 281 I looped around the crazy intersection there and when back to Henly where I stopped and recorded data.

So here are the raw results:

Speed AC Mi/kWh
60 No 4.5
60 Yes 4.1
75 No 3.2
75 Yes 3.2

So, this is about what I expected.  The one surprise is that A/C made so little difference at 75 MPH.  I assume that efficiency is so low at 75 that the AC usage is in the noise; but, I have no way of knowing for certain.

Other that that, this shows just about what you would expect: 60 is much better than 75 for efficiency and the AC hurts quite a bit at 60.

I wish I had repeated the test at 65 and 70, but four runs was enough for one day.

Other observations I made were about the efficiency indication.  When I first reset the odometer, it would start with a value of 3.9.  I would then accelerate and the Mi/kWh would drop, which is expected, but it would continue to drop down to about 2.1 and then come back up and finally settle after about 20 miles.  I have noticed that whenever I reset it it takes about 20 miles to have a stable indication.  

If I reset the measurement at the start of a drive to and from work (about 100 miles in mixed traffic) the meter settles at 3.5, though occasionally 3.4 or 3.6.  I have run it continuously for two trips to and from Austin at which time it set at 3.5 and refused to move.

(All of this behavior matches my experience with EWM averaging when the initial estimate is off but not too far.  It makes a lot of sense for them to use EWM for this since it avoids storing a lot of samples. So, I am just going to assume that is how it works.)

So, my thought is that I need to figure around 3.8 for the trip to and from Galveston (4 hours 10 minute diving time 254 miles). Using these numbers ABRP says I can make one stop in Katy for 20 minutes, but I arrive in Galveston with 20% charge.  If I say I want to arrive with 50% charge it adds a stop in League City with the two stops totaling 49 minutes.

The only problem with these plans is that I doubt I can stay in the car for three hours to get to Katy without stopping at a rest room.

For the longer trip to North Carolina, there will be more Interstate, so I figure about 3.4 Mi/kWh is the best I can do and that will get me 7 hours and 20 minutes of charging for a 22 hour drive (1300 miles).  But again that assumes that my wife and I can stay in the car a lot longer than we usually do.