Why the BMW i3-REx

Does it make sense to do a $/mile cost of electricity vs Prius?

Bob Wilson

 

My understanding is the EV/Hybrid schemes turned out to lose more money than they took in.

The simple solution is increase all tag/registration rates. But again legislators having to vote on it.

Bob Wilson

 

I have come up with a new model to compare BMW i3-REx performance to other cars and vehicle. But it is a little tricky because it is driven by:

1. local gas price
2. local electrical rate
3. EVSE reported charging sessions
4. BMW i3-REx reports for mi/kWh.

A) Record the gas cost per gallon and electrical rate:

	local energy	$/unit
1	reg gas	$1.73
2	kWh	$0.10

According to GasBuddy, regular unleaded is $1.91/gallon. The maximum utility rate is $0.10/kWh.

B) Record BMW i3-REx driving and charging usage:

Use the tripmeter for a long period to get:

	metric	value
1	mi/kWh	4.3
2	miles	314
3	speed	28.0 mph

For now, we are not using the tripmeter miles and speed. They will be useful in the future.

Download the JuiceBox EVSE metrics and calculate the cost and miles:

	Plug-In	Energy (kWh)	Charging Time	Cost	miles
1	Yesterday 11:44 PM	4.861	01:15	$0.49	20.9
2	Yesterday 5:29 PM	7.655	01:48	$0.77	32.9
3	Yesterday 4:44 PM	2.666	00:23	$0.27	11.5
4	Sunday 11:43 PM	13.885	02:49	$1.39	59.7
5	Sunday 3:51 PM	5.084	01:11	$0.51	21.9
6	Saturday 6:05 PM	5.083	01:30	$0.51	21.9
7	Saturday 9:12 AM	3.468	01:02	$0.35	14.9
8	Friday 4:04 PM	5.08	01:19	$0.51	21.8
9	Thursday 9:29 PM	4.764	01:20	$0.48	20.5
10	Thursday 2:51 PM	9.245	02:00	$0.92	39.8
11	Wednesday 8:59 PM	5.358	01:23	$0.54	23.0
12	Wednesday 2:37 PM	3.84	01:03	$0.38	16.5
13	Wednesday 12:56 AM	5.891	01:39	$0.59	25.3

This is the JuiceBox EVSE reported power consumed at home charging the car. It does not include 'free' remote chargers but it also includes 'overhead' of battery cooling/heating and cabin cooling/heating.

C) Calculate your electrical expense and miles from the EVSE:

	Column 1	Column 2
0	total cost	$7.69
1	total miles	330.6

So this is a calculated miles the car would have driven based upon the measured miles per kWh and the owner's "out of pocket" expense.

D) Calculate the MPG equivalent:

MPG = calculated_miles / (total_electrical_cost / $_per_gallon)

	Equivalent to EV	miles
1	MPG	74.4

UTILITY

What this allows us to do is compare the gas/diesel price to generate an effective MPG for the BMW i3-REx running on electricity:

• $1.73/gal -> 74.4 MPG Regular unleaded
  
• $2.03/gal -> 87.3 MPG Diesel
• $2.15/gal -> 92.5 MPG Premium unleaded

True Miles

Another approach is to use the tripmeter mileage reading that spans a set of charging sessions. The advantage is this includes the 'free' charger miles. But it does require syncing the tripmeter span with the charger sessions. For a month-by-month record, a perfectly fine approach.

CONCLUSIONS

At 74.4 MPG, a Gen-4 Prius could be competitive with running a BMW i3-REx in electric mode. But when 'free' chargers are included, the Gen-4 loses out.

At 87.3 MPG, there are no known diesels that compete. My last analysis of user report Jetta TDI MPG showed them to be in the mid-40s, close to a factor of two.

At 92.5 MPG, here we are comparing the BMW i3-REx in EV versus gasoline mode. But the highest measured REx gas mode, 45 MPG, is off by a factor of two compared to the electrical rate.

Bob Wilson

 

Source: The True Cost of Powering an Electric Car

Because of the variety of utility rates in the U.S., a 2013 Nissan Leaf that's a bargain to drive at average electricity rates in Washington (approximately $25 for 1,000 miles) is pricey in Hawaii, where those 1,000 miles would cost about $107. A conventional car would have to be getting 140 mpg to make that trip for the same money in Washington, while in Hawaii, a 38.5-mpg gasoline vehicle would do the trick.​

This is the same kind of analysis, the model, I'm working on.

BTW, I used the roll-down coefficients to look again at the BMW i3-REx:

The REx power equals the drag power at ~70 mph. To go faster, the car has to draw on traction battery power. Then it reaches the limit at ~93 mph.

Reducing aerodynamic drag with a tapered, after-body, should move the i3 drag power to the right. Even a 5 mph improvement would make a significant improvement in highway mileage but also benefits in urban driving.

Bob Wilson

 

In Huntsville there are two "Blink" EV charging stations 3 miles from home. The one with the working screen charges something like $0.02 per 2 minutes and I used it to test my Blink card a couple of weeks ago. The other one has a dead screen ... and works ... and does not (because of the broken screen) ask for a card.

So I loaded up my wife's dogs to take them for a walk. I connected to the 'dead' charger and walked them about ~0.3 mile (~0.5 km) to a well lit, non-smoking, local bar that has sushi and is open to 1:00 AM. I tied the dogs to a handicap reserved sign out front and went in to enjoy the delights. Did you know small dogs are a chick magnet?

So I ordered a club sandwich for my wife and the 'dog pack' power walked back towards the car where I of course was now perfectly safe to drive back to the bar for the sandwich . . . one large enough that everyone got some.

Near as I can tell, my future, daily commutes will cost about 1.6 kW assuming I top off at this 'broken' station on the way home. Or I could just consider 98% SOC not just good enough but perfect! (Thank you Dick Hicks.)

Bob Wilson

ps. Many years ago I worked at General Electric and Dick Hicks was a classic GE program manager. We were in the 'war room' giving our status and Hank, the software manager was reporting progress proportional to the schedule until about a week or so before his delivery was due. Then it was 'we had a set back and the software is only 89% done.' The next week, '92%', and the next, '94%'. The fourth week he reported '96%' and Dick Hick announced in voice that needed no amplification:

"Ninety Six Percent is not good enough! It's ... PERFECT! Ship it!"​

Actually once we got the software in the hands of the integration and test team supported by operations we were finally able to get a working version.

 

Now we have some electrical use metrics:

	Plug-In	Cumulative $	miles
1	16/08/05 12:17	$20.06	327 (~$0.16 / 10 mile)
2	16/08/04 21:51	$19.61
3	16/08/04 14:42	$19.54
4	16/08/03 23:53	$18.62
5	16/08/03 14:50	$18.27
6	16/08/03 12:21	$17.54
7	16/08/01 23:32	$17.37
8	16/08/01 15:57	$17.24
9	16/08/01 01:54	$17.10
10	16/07/31 17:43	$16.66
11	16/07/30 18:20	$16.35
12	16/07/30 18:06	$15.85
13	16/07/29 15:27	$15.69
14	16/07/29 01:31	$14.91	0
15	16/07/28 16:01	$14.63
16	16/07/27 23:03	$14.05
17	16/07/27 22:38	$13.97
18	16/07/27 14:59	$13.97
19	16/07/26 14:52	$12.18
20	16/07/25 23:44	$11.27
21	16/07/25 17:29	$10.78
22	16/07/25 16:44	$10.02
23	16/07/24 23:43	$9.75
24	16/07/24 15:51	$8.36
25	16/07/23 18:05	$7.85
26	16/07/23 09:12	$7.34
27	16/07/22 16:04	$7.00
28	16/07/21 21:29	$6.49
29	16/07/21 14:51	$6.01
30	16/07/20 20:59	$5.09
31	16/07/20 16:20	$4.55
32	16/07/20 14:37	$4.55
33	16/07/20 00:56	$4.17
34	16/07/19 15:44	$3.58
35	16/07/19 03:08	$2.98
36	16/07/18 15:34	$2.46
37	16/07/17 20:17	$1.88
38	16/07/17 20:16	$1.54
39	16/07/17 20:15	$1.54
40	16/07/17 19:23	$1.53
41	16/07/17 09:58	$0.91

For the next week, I intend to vampire "free" chargers. One week at $0.00/mile.

Bob Wilson

 

A 2" receiver hitch installed on Monday:

Unfortunately it does not come with trailer light adapter which according to the vendor, I'll have to do myself.


For my our wheelchair carrier, this will do. It is a folding "transport" chair, not those huge, heavy powered units. I'm still assembling the carrier but need some parts to pickup on Wednesday. Once installed, I'll put the wheelchair on and take some metrics.

Bob Wilson