BMW i3 REX – Some Initial Thoughts on the REX

Last week I was invited to the launch event at my local BMW dealer for the new BMW i3. It was an evening of presentations, the car was there for us to see and there were people around to talk to, not to mention canapes! I was booked in for a test drive the next day. Unfortunately, my other half was poorly and I didn’t make it to the event and consequently I haven’t yet driven the BMW i3.

This is a very new car. Only a very few have driven it and although it is now officially launched there aren’t many, if any, out there being driven by real owners. Therefore, everything in this blog post is either taken from reports of people that have driven it or derived from information published by BMW. Consequently, I make no claims as to the accuracy of what is here except that given the information available today this is my interpretation of what I would expect from the BMW i3 with REX.

Firstly then, what is the range extender or REX?

Let us consider a 100% B-EV (battery electric vehicle). You charge it up from the grid, say at home or via public charging, and you drive. You drive until you get to another charging location, again, either home, work, friend or family or public charging.Let us imagine that the destination is close to the range of the B-EV. You may experience anxiety or “range anxiety” on the way… will you arrive before you run out of battery power? This is the “range anxiety” the media is so keen on talking about and in most situations it just does not exist. It does not exist because the vast majority of the time the next charging location is well within the range of the battery and you know for certain you will not run out. Even on long trips you would be foolish to set out on a journey if you were not sure you would be able to find somewhere to charge. Nonetheless, the media always focus on this “range anxiety” and so it seems now so has BMW.

The BMW i3 is a 100% B-EV with a range of about 95 miles and a choice of charging options from fast AC charging to rapid DC charging using the CCS connection. In this respect it is very similar in range and capability as the Nissan Leaf. The biggest difference between them though is that the BMW i3 has the option to get it with a  REX (range extender) petrol generator. This is a little 647cc petrol engine which drives a 25kw generator. It is not connected to the wheels at all. It can be used to charge the battery or to run the car when the battery is flat. So, now, with the REX, if you do find you cannot make your next charging location on battery the REX fires up and provides the electricity to get you to your destination.

On the surface this sounds like the perfect scenario. A full, 100% B-EV that has a petrol backup should I not be able to make it to the next charger on battery. In fact, the public charging infrastructure is quite flaky at the moment and it breaks down quite often so a REX would allow you to still drive to another public charging location just using petrol.

Sounds great doesn’t it? Best of both worlds? Well, from what I have heard from BMW and from reports from those that have driven the BMW i3 REX, it is a really good car but there seems to be a huge potential for misunderstanding as to what the REX is designed for and what it can deliver in terms of performance.

The misunderstanding seems to come about because of the small size of the REX. It is only 647cc and develops about 25kw but the electric motor is 130kw. Now, I am no genius, but if the battery is flat and the power is all coming from the REX it is clear that the BMW i3 when running on the REX cannot possibly have anything like the performance as when on battery. In fact, given that my Nissan Leaf needs about 20-25kw to maintain 70mph on a level road it is quite likely that the BMW i3 would not be able to even maintain that and given any kind of a hill it could slow down to an even slower speed.

So, to try to understand more what 25kw might be like to drive, I did a test in my Nissan Leaf. I used the gauge in the car to limit my power use to just 20-25kw and drove from Saltash to Liskeard. There are 1:10 hills on this route. I found that at times the car slowed to about 33mph. Obviously this is no scientific test and to be fair to BMW, this is not the way the REX would operate. The REX can charge the battery when the full 25kw is not needed so it would be a lot better than just limiting power to 25kw as I did. Nonetheless, power requirements are a basic fact of physics and so I think this shows what kind of performance we can expect from the BMW i3 REX when the battery is at 0% charge.

Here is the test drive I did. Again, remember that this is not how the REX would work but when the battery is flat and you need power then this is representative in my opinion.

BMW are making it clear that the REX is not intended for regular use or for long trips. In other words they intend it to be used to help with that “range anxiety”… for occasional use if you get stuck without somewhere to charge. They even say that they don’t expect that many owners will ever use the REX! I think that is a rather misleading statement however. Having owned a Nissan Leaf for 2.5 years and a Vauxhall Ampera for 1.5 years and having done over 20,000 miles in each, I can assure you that if there is an option to extend the range then people will use it. With the Ampera or Volt this is normal. The generator is 1400cc and easily capable of maintaining full car performance even with the battery flat and it is designed to do just that. However, the size and low power of the BMW i3 REX makes it unsuitable in my opinion, and in the opinion of BMW, for anything other than occasional “get you out of trouble” use.

I am awaiting with interest some real-world reports from real owners. Owners that are not in the media and that have real-world experience. However, until then we have only what BMW say and what is reported in the press to go on and from that I conclude that the BMW i3 looks like a great 100% battery EV with the option for a REX for the odd occasion when you cannot charge or the public charger breaks down. Obviously, if it is there then there is nothing stopping you using it all the time to double the range of the battery but I can’t see people wanting to do that very often. It is reported as being like having a lawnmower in the boot and this, coupled with what is likely to be very poor performance when on REX, will not make running on the REX very appealing.

There is a way though to use the REX and still maintain the performance of the full 125kw electric motor. If you run the REX before the battery runs flat then you still have the full battery performance whilst topping up the battery as you drive. This would work but I can’t see this as being an attractive option. You would need to run the REX whilst driving with battery remaining. Now I would hazard a guess that most people that buy a battery EV want to run it on battery… not petrol. You would normally only want to run the REX if it were necessary, as BMW suggest it should be used, but to maintain performance over the full 190km claimed range you have to run the REX. I wouldn’t find that an acceptable option. Also, one of the benefits of an EV is the quiet, smooth driving… this would be shattered by running what seems to be a noisy petrol generator in the boot!

No, I like the concept of the REX. It will reduce “range anxiety” for some and it will make it less likely that you will get stuck without anywhere to charge. But I don’t believe that it will be an attractive alternative to an ER-EV (extended range electric vehicle) such as the Volt or Ampera. If you regularly want to drive beyond the range of the battery (beyond about 80 miles) then you might want to consider the Ampera/Volt as the better choice.

I see the BMW i3 REX as a direct alternative to the Nissan Leaf, not the Ampera or Volt, and should be seen as a 100% battery EV with a “get out of jail free” card – the REX.

2 thoughts on “BMW i3 REX – Some Initial Thoughts on the REX”

  1. Paul, this is a good idea, but the test does not consider the fact that the REx is supplemented by battery power. Additionally, this report does not consider the fact that the i3 is about 17% lighter, which would translate to proportionally higher speeds on inclines. About 20% improvement on hills is not a huge difference, but a significant improvement nonetheless.

    According to an interview with Andreas Feist, head of i3 vehicle development, and this test drive report, the US version will not allow the REx to engage before the SOC reaches 6.5%. Once the SOC has declined to 3%, a critically low level, the REx will recharge the battery. Otherwise, it won't. This will give the US version about a 0.65 kWh buffer to supplement the REx. While that's plenty for accelerations, it's not enough to traverse more than 500 feet of altitude difference.

    Yes, this is a very slim operating margin, and while the speed and performance won't be as bad as you have demonstrated in your video, courtesy of the significantly lower weight, there is no denying that the US version of the i3 will not be a practical replacement for a Volt. The REx in conjunction with 6% battery SOC will really be just enough to avoid the tow truck or make it to the next charging station. It would appear that the driving experience, especially in hilly areas, won't be very pleasant in range-extended mode.

    This, of course, won't be the case in Europe, where the REx can be switched on at or below 75% SOC according to the most recent reports. That should allow the i3 to go on a cross-country trip, if one is willing to refuel frequently. The US version will be much more limited, courtesy of the low reserve SOC. This is a very unfortunate limitation, which wastes the potential of this feature.

    With a slightly larger tank, say 5 gallons instead of 2.37, and a slightly higher maximum REx output power, say 30 kW, and a minimum battery SOC of 20%, the i3 would be a potent and more efficient Volt competitor. The projected performance limitations in REx mode really killed my enthusiasm for what I thought is going to be one of the defining features of the i3.

    That said, I estimated nearly a year ago that the power supplied by the REx will propel the i3 at 75 mph on flat ground. Andeas Feist confirmed that assumption in his interview by stating that the speed in REx mode will be limited to 120 km/h. Furthermore, based on the weight of the i3 and the assumed aerodynamic drag, I estimated the speed going up a 6% grade to be 45 mph.

  2. Thanks for your comment.

    You are of course correct, and that is why I said it was not a direct comparison for those very reasons 🙂 but well worth pointing that out again.

    I did hear that the US version had a very small buffer but I didn't realise it was that small!

    Like you, in my opinion, a 30kw generator and larger gas tank would have put it right up there as a Volt/Ampera competitor. As it is it is close but I wonder just how much of a problem it will be in real-world use? perhaps not as much as I am suggesting but we shall see.

    Thanks again for your comments.

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