Tuesday, May 12, 2015

Freeway efficiency


Previously, I was going from San Diego to OC, but issues aggravated me to not take data. The way back was different. I knew it had to go up almost 2000ft, so I conserved energy by keeping it 55 MPH at the right most lane. I do this when I tow a trailer with my van, so it's not so unusual. Traffic wasn't so bad so people were able to pass me when necessary. Few were following me, probably hyper-milers or just curious about the slow EV.

Car was fully charged at start. Indicated range was 94 miles. The next stop would be Del Norte Plaza of Escondido, CA, 84.7 miles away. This is more than advertised range of Spark EV, but less than indicated miles. I wanted to see if 80+ miles on a charge was actually possible.

94 - 84.7 = 9.3 miles estimated margin

On straight roads, it was using roughly 11 kW of energy at 55 MPH.

(55MPH * 1 hour) / (11kW * 1 hour) = 5 miles/kWh

5 miles/kWh use is better than my dog beach trip (4.8 mi/kWh) where speed was up and down through local roads and stop and go traffic. This goes to show that it's still better to drive at constant freeway speed than using regenerative braking in slower speeds, at least at 55 MPH.

As the miles passed, estimated miles got better. At some point, indicated miles left was 30 miles more than the actual miles left according to GPS, leaving plenty of margins. After about half battery left, the numbers started to converge again. It's difficult to tell if this was due to going slight uphill or not. OC elevation is 125 ft, Escondido is 680 ft.

As I pulled into the parking lot, indicated 9 miles were left and it was asking to turn off the radio. I probably could've driven to another L2 charger 5+ miles away if needed. Driving 55 MPH, this gave very good estimate of the range even with going uphill for 555 ft. Of course, this is all with new battery.

There was only one Mitsubishi i-MiEV using L2 charger. Great! No waiting! I pulled up and connected to CCS and called eVgo. Coincidentally, the same rep from my first call on Saturday picked up the call (his name is Kevin). He got me going in short time, and away it went with fast charge. Thank you Kevin! I took the dogs for a walk around the shopping area and returned when I saw 3 (!) BMW i3 mulling about the charger. Well, someone is going to wait over 1.5 hours to charge; better you than me.

It took 13.2 kWh in 20 minutes 20 seconds (1220 seconds, 20.33 minutes) to bring to 82% battery level. eVgo charges $0.10/minute on DCFC with my plan. 9 miles is roughly 10% starting battery level; this is very crude estimate, but then so is the 82% number.

84.7 miles / 13.2 kWh * (82%-10%) = 4.62 miles/kWh
20.33min * $0.1/min = $2.03
$2.03 / 13.2 kWh = $0.15/kWh

What? It's cheaper to fast charge than home charge? If the numbers are correct, then yes! It is cheaper to fast charge. Of course, this is for almost empty battery. As the battery accumulates charge, the charging rate should slow while money per time remains the same; this will be the next experiment on how slow it will get. DCFC charging rate is

1220 seconds / 13.2 kWh = 92.4 sec/kWh (round to 100 sec/kWh)

Now that I have baseline charge rate of about 100 seconds per kWh, I can stop charging when it gets much less than 0.01 kWh in one Mississippi if I want to optimize. It's too bad there aren't any fast chargers near my home. I'd be using them all the time.

Assuming 5 mi/kWh discharge number is close enough for extended 55 MPH drive, we can figure out the charging efficiency.

1 - (5 - 4.62) / 4.62 = 92%

DCFC charging efficiency is pretty damn good! But then state of charge of 82% and 10% are very wild estimates, so efficiency number is also wild. But this gives some idea on battery charging efficiency alone without the on-board charger.

When I got home 24 miles later, I connected to home charging. It took 12 hours to charge 10.64 kWh to bring it back to full. Because the SoC was only 82% at start of my trip home from DCFC, not 100%, we have to figure out how much energy would have been needed to bring it to full. Then we can find mile/kWh. Assuming 19 kWh battery and ignoring charger efficiency (as I said, numbers are wild but in ballpark),

19 kWh * (1-0.82) =  3.42 kWh
24 miles / (10.64 - 3.42) = 3.32 miles/kWh

I went uphill about 1300 ft, so miles/kWh suffered as expected. As sanity check, this is also roughly what was found from dog beach to home trip where initial charge was 100%. As with that trip, there's also the on-board charger inefficiency that reduced miles/kWh compared to DCFC. The overall for the trip was

(84.7 + 24) miles / (13.2 + 10.64) kWh = 4.56 miles/kWh

This is lot better than 4 miles/kWh I had from dog beach trip. As I mentioned before, driving constant 55 MPH is lot better than slower speed with regenerative braking. But typical of my drive has traffic lights, stops, and so on. Therefore, 4 miles/kWh is more realistic use case. But it's nice to know I'm getting 10% more MPGe when driving 55 MPH.


  1. Does the Spark not give you a state of charge %? What about an efficiency meter?

    1. It does not give state of charge %, but miles remaining is roughly analogous: 9 miles remining ~ 10%. If I use fast charger, I can get SoC% by reading from the charger.

      Once the drive is finished, it gives miles driven and miles/kWh (battery to wheels).