BMW i3 – REx To The Rescue


Those that have followed me since before I got my i3 know I toiled a bit over whether to get the range extender option or not. Having lived with pure battery electric vehicles for five years I really didn’t like the idea of hauling around an internal combustion engine if I really didn’t need to do so, even if it was a very small, efficient one.

Ultimately, the decision was easier than I had hoped. Once it became clear the i3 would have significantly less electric range than the two previous BMW-made electrics that I’ve been driving (MINI-E and ActiveE) I knew I needed the REx. As much as I love the i3, I’m still disappointed BMW moved backwards with the electric range in every EV they have produced. The MINI-E was good for a reliable 100 miles in moderate temperatures, the ActiveE about 90 miles and the BEV i3 is EPA rated at 81 miles per charge. I drive a lot and 81 miles would just be cutting it too close for me, especially in the winter when the range is negatively effected by the cold weather.

Our Equinox, Tacoma and i3 joined by my old ActiveE before I turned it back in.

Our Equinox, Tacoma and i3 joined by my old ActiveE before I turned it back in.

Plugging the tire

Plugging the tire

So i3 REx it was for me, and fortunately I’m very happy it worked out that way. Without the reassurance of a long range EV like Tesla’s offerings, there will indeed be some cases where the limited range of the sub-100 mile EVs require you to alter your plans or make compromises. I accept that because for me the advantages of driving electric far outweigh any small inconveniences that occasionally arise from the limited range or charging times.

However, the range extender option on the i3 was inticing because I knew I’d be driving on electric for 95% of the time, but still be able to take the car on days that I knew I needed to drive further than the range could accommodate. That has indeed held true because I have a little over 11,000 miles and only about 500 of those miles were with the REx running. However so far the REx was just a convenience, allowing me to take my i3 on days I needed to drive far when I otherwise would have taken one of my gas cars. I hadn’t faced a situation where I really needed the extra range, and didn’t have any other options, until last weekend.

My wife and I have two gas vehicles besides the i3. Meredith drives a Chevy Equinox and I also have a Toyota Tacoma pick up which I use when I need to haul large items like refrigerators for my restaurant, and I also use it to plow my driveway and the parking lot of the shopping plaza that I own and manage. These are our long range cars for trips like when we go to my in-laws in Vermont. So when Meredith needed to go on a company retreat which was 95 miles away in Pennsylvania she naturally planned on taking her Equinox.

About a half hour before she was ready to leave, I went out to the garage to give the Equinox a once-over. I always do this when she’s going far from home, just to be safe. The first thing I did was a quick visual inspection of the tires and wouldn’t you know it, the first tire I looked at had a large screw sticking right into it and in a perfect position for me to see it. OK, there is no time to fix this now so the Equinox is ruled out.

No problem, I’ll load up her stuff into the Tacoma and she’ll use that. It can use a good run anyway since we barely ever drive it. I then remembered that I needed to refill the washer fluid because it was empty the last time I drove it. I popped the hood, filled the fluid and when I closed the hood I heard a loud cracking noise and the hood popped back up. As I lifted the hood I could see the latch that holds it closed was rusted and cracked in half. With no way to secure the hood closed that rules out the Tacoma. There’s only one option now, she’ll be REx-ing it to the retreat.

The owners of the B&B she stayed were nice enough to let her charge up. They told her their son in law drives a Volt so they weren't totally surprised about a car that need to plug in.

The owners of the B&B she stayed were nice enough to let her charge up. They told her their son in law drives a Volt so they weren’t totally surprised about a car that need to plug in.

The owners of the B&B she stayed were nice enough to let her charge up. They told her their son in law drives a Volt so they weren’t totally surprised about a car that need to plug in.

After a quick REx briefing (I don’t think she had ever driven it in REx mode before) she was off. Since the temperature was in the 40’s and it was all highway driving, I knew she’d only get about 60-65 miles before the range extender turned on so she’d be driving about 30 miles with it running. I had a full tank of gas so she wouldn’t need to stop for gas on the way there, however we didn’t know if she could plug in at all once she got there and she may have to drive the whole 95 miles home in charge sustaining mode. I’ve driven the car enough to know this wouldn’t be a problem as long as she kept her speed under 75mph, other than the fact that she’d have to stop for gas twice on the way home. I did a quick check to see if by chance there were any level 2 charging stations near where she was going , but as I expected, there were none. Luckily, the bed and breakfast she stayed at allowed her to plug into an outlet they had on the garage, so she was able to charge at 120v overnight. Since the retreat lasted for four days, she had no problem fully charging even with using the car to run some local errands every day.

So even though I’ve used the REx a dozen or so times already, this was the first time it was absolutely necessary, and further validates my decision to shell out the additional $3,850 for it. It really makes the car so much more versatile, and allows me to not even think about the range. As others have noted, it really does allow you to drive more electric miles because you can use it for trips you otherwise couldn’t with the BEV i3. Obviously, the ultimate goal is for longer range electric vehicles to become more affordable, and infrastructure to mature to the point where public charging and DC fast charge is ubiquitous. However until then, the range extender will play an important role in the adoption of electric vehicles, as it allows for utility that is simply unavailable in the vast majority of today’s electric vehicles.

Trip stats: I was very happy to see 3.7 miles per kWh considering it was about 70% highway driving, with the cabin heat on the entire time.

Trip stats: I was very happy to see 3.7 miles per kWh considering it was about 70% highway driving, with the cabin heat on the entire time.

Stats from the trip:

Going to PA: 61 miles on battery, 34 miles on REx
While there: 49 miles all on battery
Coming home: 65 miles on battery, 30 miles on REx.
Total: 239 miles; 175 miles on battery, 64 miles using range extender
Total gas used: 1.7 gallons (.9 gal refill when she was in PA & .8 gal refill when she was 1.5 miles from home) Averaged 37.65mpg while REx was running.

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41 Comments on "BMW i3 – REx To The Rescue"

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Great post, Tom – the i3 provides a unique opportunity to compare head-to-head the trade-offs of a BEV vs an EREV on the same platform. Glad your wife could opportunity-charge at the B&B, but it sure feels good to know you don’t “need” people to be so gracious to get to and from your destination, doesn’t it?

Hopefully there will be more EREV options entering the EV market soon and the driving public will become more familiar and comfortable with the amazing versatility, economy, reliability, and fun that EREV’s such as the i3 REx and the Chevy Volt offer.

Thanks. Yes, it was definitely assuring to know that even if she couldn’t plug in there she could drive home safely.

Oh boy. Tom, as a married guy I know we all worry a bit about our spouse’s safety – either getting to and from their car at work, or just running errands. By “safety” I remind you that you stated: ” she may have to drive the whole 95 miles home in charge sustaining mode. I’ve driven the car enough to know this wouldn’t be a problem as long as she kept her speed under 75mph, other than the fact that she’d have to stop for gas twice on the way home.” That would not be the case in our Volt. It’s sure nice and all that you own several cars and can take a gas car whenever you may need one. Yet that reinforces the whole “$50,000 City Car” argument I’ve made all along about i3. EV guys justify a lot. I’ve been there. “Oh it’s not such an inconvenience because – well, because we’re EV enthusiasts!” You see, driving 45 minutes and having to find a gas station while on i3’s tiny range extender IS an inconvenience, and more than that – can pose a danger if one has to, A) find a gas station or pre-plan the… Read more »

Thank you Tom. Always informative and helpful. I’m going to get a new car next year, electric of course. BMW is high on the list. I wonder how it will do in the snow. Please share when possible. Thanks.

Big battery or a range extender, those are today’s choices for the “non urban” commute. Four years from now, it looks like there will be multiple 120-200 mile options.

Even then Tom’s backyard will look the same though to which I can relate. Lots of acres, solar panels, EVs, and two big gas trucks because I choose to live among the trees and I need a truck to haul stuff. My F150 sees only 2000-4000 miles per year but I have to have it.

As for my main vehicle, I like driving our EREV Chevy Volt to the beach and mountains opposed to renting another ICE or driving the F150. I certainly want total AER. For now, it is the decade in which we live.

Good article.

Love my EREV.

Looking forward to my new Gen 2 Chevy Volt 🙂

Wait a minute George. You’re going to get a Gen 2 volt even though the GM engineers did not heed your advice to go pure serial and they even added a second planetary gear set just for spite? Just Kidding ya;)

Looking at the patent that JeffN on gm-volt found, the gen2 Voltec drive train will be even more ingenious – more efficient and powerful and flexible – and only uses two clutches instead of three:)

Oh no HVACman.

I’ve always loved the 4ET50.

I’ve written many articles on it.

the next one is even tricker.

WOT calls it the 5ET50.

I want one 🙂

Great writeup. The REx will make many more comfy owning what is a great BEV.

Great story! I really love the Rex on the i3. I got to try it on my 3-day test drive and I was made a believer. Lots of people still say it is some sort of “limp home” mode, and it is not. I understand there are cases (like mountains) where it is less-than-perfect, but I think in most situations it will work fine. I wouldn’t hesitate to drive an i3 Rex from Dallas to Houston, for example. Wouldn’t be a problem at all, other than frequent stops for gas. I couldn’t do that in any BEV except a Tesla right now.

Exactly David. So far for me the range extender has been able to do anything I needed it to. However I realize that isn’t the case for others that need to make long, sustained climbs in REx mode. As long as I keep it under 75 mph (which isn’t a problem for me) I can drive it as far as I need to without any issue. You just keep an eye on the SOC and if it looks like it’s getting down to 2% or so, you just slow down for a bit and it regains back up to the ~6% hold level.

I now have 11,500 miles and have bought a total of 14 gallons of gas. However a few of those gallons were when I first got it and purposely didn’t charge it so I could do some extended REx testing. I probably really needed about 10 gallons if you exclude the REx testing I did. Having it there when you need it like last week was really useful though.

Tom – you didn’t apply the firmware fix, or does SOC still fall when rex is trying to climb steep hills?

I must have misunderstood, I thought it was the fact that SOC started so low that the rex had trouble keeping it sufficient.

I have not coded my car to allow for a higher SOC hold if that’s what you’re asking. I really don’t have any need for it. I thin it would be cool if it had the higher SOC hold mode, but for me it not necessary at all. The way it works is the range extender turns on when the SOC hits 6.5%. The range extender then tries to hold the SOC at 6.5%. It can do so under most all conditions, except for prolonged high energy demands like high speed driving and climbing long, steep inclines. On flat ground, the REx can supply enough energy to hold the 6.5% buffer at speeds up to 75 mph. Higher than that and you’ll slowly eat away at the 6.5 % buffer until the car goes into reduced power mode. What that does is forces you to slow down to a speed where the energy consumption is low enough to allow the REx to replenish the battery, or at least allow you to continue to drive at that reduced speed. It also can happen going up long steep hills. However if you drive along at 70 mph or so, and encounter a… Read more »

“On flat ground, the REx can supply enough energy to hold the 6.5% buffer at speeds up to 75 mph.”

I’m assuming this is with one or two people in the i3. I’m curious as to how the extra weight in a fully loaded i3 affects the maximum speed at which the REx can hold the 6.5% buffer on flat ground. Have you tested the Rex with the equivalent of four “larger” passengers and luggage/cargo? That’s about 800 lbs above the i3’s curb weight.

“I have not coded my car to allow for a higher SOC hold if that’s what you’re asking.”

Yes, that is what I was trying to narrow down – I -was- understanding that if the SOC was not allowed to hit such a low number in the First place before rex kicked in that the steep hill effect nearly disappeared, i.e. it was ‘easier’ for the rex to keep up, but maybe the reference was simply that you -could- climb to the ‘peak’ and never hit ‘limp mode’ because you had extra juice to begin with. (early French translation, if memory serves).
The reason I asked is that (if I misunderstood) I thought that That was the reason to apply the correct EU firmware ‘fix’ as shutting down the rex is easy, correct? (if it starts and you Don’t want it to run)

So please explain how more stops for gas is safer for a woman traveling alone.

Please explain how i3’s tiny range-extender plus it’s miniscule gas tank should make i3 more than a very very expensive city car.

You know we can’t all have gas cars at the ready – and i3 just isn’t a car that will, or could become a family’s only car. Sure BMW offers expensive ICE lending schemes – not sure if they’ve implemented that setup in the ‘States yet, but even so – very complicated, expensive and not convenient.

I’m not being contentious here, just a realist in an EV world where we sometimes act as though something that is – isn’t.

I’d really appreciate you read my 3 above posts and give me your take. You usually do – so I really appreciate that.

Again – I’m no naysayer, my points are well taken. 🙂

James: Relax dude I haven’t looked at this thread since you posted your comments and I haven’t commented here since either. I usually only keep an eye on my posts for the day they are up and then check in sporadically after. Your question about safety seems to indicate I’d be concerned that my wife had to stop for gas two times on the way home, with the last stop within ten miles from our home. Maybe that makes me a bad husband in your opinion, but I’m certainly not worried about my wife stopping for gas. (By the way, she studied Taekwondo for a few years and is pretty formidable should she need to be!) You bring up how you wouldn’t have to stop for gas in a Volt and yes, you’re correct about that. In fact, the volt would be a better choice for me if these kind of trips were common, but they aren’t. If I need to take the car on a trip like this three or four times a year it’s not an inconvenience stopping a couple times for gas, and I still use much less gas than I would have with a Volt for… Read more »
Thanks for your response, Tom. I know you always respond to my queries. 🙂 Thing is, I know there is not a ton of options for prospective EV buyers out there at this time. We can start at the budget basement – the iMiev, and work upwards in price, but the capabilities of more expensive machines rise with that cost increase. I live in a moderate climate zone ( Seattle ) and we do experience range reduction in winter, but nothing compared to the frosty northeastern USA or Canada. With our current crop of 80-90 mile BEVs ( the compliance car herd is beginning to make that space crowded )giving us 55-65 miles EV range in winter, the lesser iMiev turns into pretty much a grocery-getter under 32degrees. This brings me to why I comment on i3. BMW Blog, and it’s near mirror,, runs nearly every i3 “rah rah” article, video and new wrinkle it can. I don’t think material for EVs is that scarce, personally. You know, for an auto site to run fan pages for a particular car. This site has been in contact with me in the past, informing me that Tesla articles are by far,… Read more »


“I’ve never ignored you.”

a Lot of people do, though, particularly when reasoned response falls on seemingly deaf ears, and that the questioner has described all BMW owner-driver as ‘typical’, clearly meaning someone that they drive in a manner not fully sanctioned by the perfectly-informed questioner.

I personally save Much time when reading the comments section by quickly scrolling past the commenter that feels endless caps and bold-type to be necessary. Keeps my blood pressure lower as well.

Incidentally, good article and kudos to your continuing patient replies. It would be a shame if you hesitated to write an article due to having to respond -within a given time frame- to One, and only One commenter.

Real men plug their flat tires themselves!

Great writeup on the practicality of range extenders.

It still just seems crazy that they didn’t provide the OPTION to get a larger battery pack. Having a choice between regular battery, a larger battery, and a regular battery with ICE would have been nice.

If BMW would have offered an i3 with a 25% larger battery, say 27-28kWh I would have bought that instead of the REx. I’m sure I would be very happy with it, but I still wouldn’t have been able to let my wife use it for this trip. I would have rushed to plug the tire, and then the entire 240 miles would have been done on gas.

I wish the i3 didn’t come with a $10,000 badge on the front of it. It’s just too far out of my reach, even to lease. It’s a shame because it’s the perfect car. I hope cheaper alternatives present themselves soon, but I doubt they will.

Good writeup.

This just confirms that all those “stuck up” BEV purists need to get off their high horses and embrace the transitional technology and their benefits.

Until the car are ready (more than 200 miles range) and infrastructures are ready (DCFC everywhere), EREV is the best solution for now.


I couldn’t have said it better myself.

Nice story.
And now you can understand our love of the Volt. I have over 30000 miles, 91% electric. And it’s that low because of a 700 mile road trip.

Tom: I am seeing I3’s around Philly…not a whole lot, but some. A co-worker even aproached me about it as he had gotten an invite to BMW’s ‘come in and test drive’ event a few months ago. He didn’t buy, but admitted he liked the car. It just didn’t fit his lifestyle, but if BMW adds larger battery sizes, an EV will work for him. I switched from a Mitsu I-MiEV to a Volt and have completely driven away from that fear of not making a trip because of range. Actually, though, I have been able to use the heater more than I ever did in my “I”. Funny too that the Volt has less driving range on the battery, but because of the on board generator, I have the freedom to drive the battery to zero and let the REX kick in. As has been said above, until the battery costs come down, and we can afford those 200 mile batteries, I can see the REX type car being very popular.

Thanks, Tom. Looking forward to perhaps a casual winter comparison. B-class is available in NY, at least, though not sure anyone here has one.

My wife has been driving the Volt, of late, and heat has become a conundrum. For a 45 mile day, its whether to run the engine first, or simply to go to electric heat and end up with fewer electric miles. Then, there’s using ‘Hold’ for the “free” engine heat, but forgetting to go back to electric.

Getting away from feeling like efficiency has to be actively managed, over medium trips, is to be envied with the i3. The Volt doesn’t begin to do this (w/ERDTLT) until <36 degrees.

Everyone talks like BMW invented the Rex, and this is all new territory. We Volt owners have been doing this, and doing it better, with no power limitations and no need to stop for gas every 70 miles, since 2011.

Nevertheless, I do wish I had the i3 battery. I’ve said it before, but what we need is the Volt’s Rex and the i3’s battery. Hopefully the Gen2 will come close, for half the cost and a MUCH bigger gas tank.

This comment is totally passé. I forgot to update my browser for the latest comments. Sorry.

Why so much hatred for the i3 in here. I’m aVolt owner and I used to own a PIP. The Volt does some things better than the i3 and the i3 does some things better than the Volt. The story was about how Tom’s decision to get the REx option was proven a good one in this case. That’s all. Some of you need to get a life and look in the mirror before you start calling other folks fanboy. lol