Real World REX mpg

On an airplane, on cruise, consumption is pretty consistent while at a constant altitude because your throttle is normally at a fixed position. The roadways traveled by a car in most parts of the country, are not. If you want controlled, repeatable benchmarks, the EPA test is valid. "Normal" for one person, can be radically different than for another.

In the winter time, the type of trips you make, and whether you precondition the battery can make a radical difference in your range. Preconditioning, and one long, constant trip will provide a very different range than multiple short trips where you have to rewarm the vehicle after each leg. Obviously, it will also differ based on the delta T you ask of the HVAC system. The REx has the disadvantage that it does not have a heat pump, only resistance heating verses the BEV, which has both. So, one would expect a bigger hit on the REx in the winter with a large delta T when you're trying to keep the temp comfortable verses with the BEV. An ICE typically has an excess of waste heat so cabin temperature is usually a minor factor, other than maybe the fan and rear window defogger.

jadnashuanh said:

On an airplane, on cruise, consumption is pretty consistent while at a constant altitude because your throttle is normally at a fixed position. The roadways traveled by a car in most parts of the country, are not. If you want controlled, repeatable benchmarks, the EPA test is valid. "Normal" for one person, can be radically different than for another.

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No problem as I've had to deal with identical road variables with the Prius. Of course folks are free to ignore my metrics ... I do them for my purposes which includes my trip planning. Feel free to use other approaches or none at all.

jadnashuanh said:

In the winter time, the type of trips you make, and whether you precondition the battery can make a radical difference in your range. Preconditioning, and one long, constant trip will provide a very different range than multiple short trips where you have to rewarm the vehicle after each leg.

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Actually preconditioning of the i3-REx is one of better aspects of the i3 family. I've also augmented it with this climate cover (*):

When parked where there is no shade, I use this cover to reduce the cabin, solar heating. For example, the trees will shade the car after 11:00 AM but before then it gets pretty brutal. Cracking the windows doesn't work because we also have random rain showers. Now if the rain-sensor would roll-up the windows, I could leave them cracked open but the steering wheel would still be too hot.

jadnashuanh said:

Obviously, it will also differ based on the delta T you ask of the HVAC system. The REx has the disadvantage that it does not have a heat pump, only resistance heating verses the BEV, which has both. So, one would expect a bigger hit on the REx in the winter with a large delta T when you're trying to keep the temp comfortable verses with the BEV. An ICE typically has an excess of waste heat so cabin temperature is usually a minor factor, other than maybe the fan and rear window defogger.

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I have great expectations on preconditioning. Also, the climate cover will work in the winter to reduce radiant cooling. I have thought about tapping the REx coolant loop for cabin heat but right now, idle speculation. Gloves, sweaters and absence of a draft should be enough. It was the technique used with a 1966 VW MicroBus that had no heater.

Bob Wilson

* - By accident, I found the cover can entrap a wiper; pull it off the arm, and; the arm can slap down and crack the windshield ... $600 and a couple of hours ... not by BMW service who seem to take a long time to do repairs.

I've been running an extension cord from a 120 VAC outlet in the kitchen to the portable EVSE that came with the BMW i3-REx. Since I used a kitchen outlet, I set the Level 1 charging to a maximum of 10A, ~1kW/hr. But I have this dryer outlet, 240 VAC in the exterior utility area.

Our house has 'single split-phase, 240 V, features a neutral and two hot legs, 240 V to each other, and 120 V each to the neutral.' So I bought an exterior box with a pair of ground-fault protected, 120 VAC sockets. I bought a dryer replacement cord for NEMA 6-50 that connects to the two hot legs and neutral but does not have an earth ground needed for the ground fault detection. I added a ground wire to a water pipe clamp and the case ties the ground-fault wiring. But how to test?

A ground fault circuit needs 120 VAC to work, to support the reset function. But if there is an unexpected short, there will be sparks, smoke, and a circuit breaker trip ... an early 4th of July.

So I bought a sacrificial, three-prong extension cord and light bulb socket. The cord provides an earth ground, neutral, and one hot leg. With an oven light wired on the hot leg, any short will light the bulb and limit the current to ~0.5 A. A practice short and it worked perfectly. Wiring the light bulb protected hot to both hot legs of the dryer plug; the neutral-to-neutral, and; ground-to-ground, the test and power box worked perfectly ... no short and no problem.

I turned on the dryer circuit breaker and plugged in my adapter and no smoke. Then I plugged in the portable EVSE ... no drama. Finally, I set the car Level 1 to 'Maximum' or 15 A and plugged in the car. So instead of showing a full charge by noon Saturday, it now shows 6:00 AM to full charge, much nicer.

I had run the SOC to under 40% as part of another test. Ordinarily I would drive the car to a shopping center with a free, high capacity charger and use the 1 kW mode (i.e., Level 1 Reduced) to top it off over night. Now I can charge at 1.5 kW which shortens the charge time significantly. I still need to properly, wall mount the box and the job is done.

Bob Wilson

It was not all that uncommon for old 240vac dryer circuits to not have a ground wire, but did you check inside the box? The fact it had a 3-wire plug does not mean that there wasn't a ground wire in the box. FWIW, a GFCI circuit does not use the ground internally...it measures the voltage going into the circuit and that coming back from completing it (i.e., the hot and neutral) and if they are not equal, trips since some of that current must therefore had to have gone to ground, thus a ground fault. Ground wires ARE required for full safety in a circuit and provide a means to trip the circuit breaker. The supplied EVSE with the i3 is limited to either 10 or 12A, depending on how old it is.

jadnashuanh said:

It was not all that uncommon for old 240vac dryer circuits to not have a ground wire, but did you check inside the box?

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The house was built in 1968-69 so I'm pretty sure there is no earth ground.

jadnashuanh said:

Ground wires ARE required for full safety in a circuit and provide a means to trip the circuit breaker.

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Something I agree with too!

jadnashuanh said:

The supplied EVSE with the i3 is limited to either 10 or 12A, depending on how old it is.

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I checked the label and it shows 120VAC 12A so looks like I lucked out.

Good news, my Electric Motor Werks, JuiceBox Pro shipped. When it arrives, I'll make a NEMA 14-50 to NEMA 6-50 with proper ground wire. Then time for another 'smoke test.' But this is just temporary while I negotiate the wiring upgrade and Generac contract.

Bob Wilson

jadnashuanh said:

it measures the voltage going into the circuit and that coming back from completing it (i.e., the hot and neutral) and if they are not equal, trips

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You are probably well aware, but it's actually the current that the breaker (called an ELCB or earth leakage circuit breaker in the UK) measures. In practice they're generally designed simply to measure the difference, rather than each current individually and subtract them.

Yes, it is current, and in the USA, the difference of 5ma or more trips the thing, shutting off power. The EVSE spec includes that, so while if there's a plug involved, it might require protection on the receptacle since something else might be plugged in, it is redundant. If the EVSE is hardwired, since it has a similar circuit built-in, no further protection (other than the home wiring) requires any protections.

So I built a model using these assumptions:

• 15 min - refuel time, fixed since the actual pump time is so short
• 48 mpg @50 mph - estimated
• 44 mpg @55 mph - measured
• 42 mpg @60 mph - averaged
• 40 mpg @65 mph - measured, charge is sustained
• 36.1 mpg @70 mph - averaged, charge sustained (TBD)
• 32.2 mpg @75 mph (SOC -22.5%, limit 3 cycles) - measured at 90F (33C)

Using this model, there is an effective block-to-block speed as a function of the cruise control set speed (aka., trip meter mph.) It turns out there is a maximum, effective speed. Any faster and the block-to-block time increase tapers off. So far, that optimum speed appears to be 65 mph (104 km/h):

• no battery charging - leave with enough charge to handle hills
• 75% REx coding - assumes range extender is enabled at the earliest point
• 65 mph - drive 24x7, plan 50 mph, 1200 miles (1920 km) per day, recommend two drivers

Bob Wilson

Hi,

Recently a Honda fan in PriusChat claimed that the BMW i3-REx was inefficient compared to the 2014 Honda Accord Plug-In. So I picked up the roll-down coefficients from "Test Car List Data" at epa.gov and ran the drag power functions:

At 70 mph, I remapped the drag power curve to reflect the additional power provided by the traction battery. This is what drains the SOC when trying to cruise on REx at higher speeds. Little surprise, the aerodynamic drag of BMW i3 is evident relative the Prius Plug-In and Honda Accord Plug-in. If we can find ways to reduce aerodynamic drag, we can extend the EV range and raise the highest, sustainable REx cruise. But the original comment had to do with efficiency ... which depends:

• 72 EV miles - BMW i3-REx
• 13 EV miles - Honda Plug-In
• 11 EV miles - Prius Plug-In

If we set the range limit to 150 miles, the longest, common range for all three cars, the BMW i3-REx, we get:

• 76.4 MPG = ((72*117)+(78*39))/150 - BMW i3-REx EPA max range
• 53.3 MPG = ((11*95)+(139*50))/150 - Prius Plug-in
• 52.0 MPG = ((13*115)+(137*46))/150 - Honda Plug-in

If we change the distance, eventually the Prius Plug-in will overtake the BMW i3-REx. A little farther, and the Honda Plug-in overtakes the BMW i3-REx. But for the first 150 miles, the BMW i3-REx is supreme.

Bob Wilson

Hi,

I am doing an experiment that involves loaning my L2 EVSE to a local business at a shopping center I frequently visit. This means I have reverted to the L1 charger for our 2014 BMW i3-REx. So three days into the experiment, I put together this OpenSource spreadsheet:


If there is any interest, I'll try to upload the spreadsheet but hopefully the way I laid it out is self evident. Regardless, lessons learned:

• L1 EVSE - if you want to drive a science experiment, the way to go. The car spends too much downtime getting a charge. You really have to parasitize off of public L2 EVSE and hope they have not been castrated. In effect, dependent upon the kindness of strangers.
• L2 EVSE (30-31A) - if you want a car that you can drive pretty much anytime and anywhere within town, this is critical.

Bob Wilson