Monday, December 23, 2013

BMW i3 Range Extender: How Robust Will It Be?

There are still many unanswered questions about the i3. What will the EPA range rating be? How will is fare in NHTSA and IICA crash tests, will it cost more to repair, and so on. However there is one particular feature of the i3 that continues to dominate the discussion boards and forums: What can and can't you do with the REx?

Back in June before the i3 was officially introduced, I did a blog post on this very topic which turned out to be very popular. In fact, it's one of my top viewed posts and has been viewed over 5,000 times so  far. We now know a few more details since then so I thought it was a good time to give an update here. While I have driven in REx i3's, unfortunately I haven't had the chance to thoroughly test on while it was in range extender mode. That wasn't a coincidence. BMW has still be fine tuning the software for the REx and hasn't allowed the press access to them for complete testing. However, somehow over in the UK The Telegraph got hold of one and was able to take to for a long drive and use the range extender. Overall the review was pretty favorable and the author gave the i3 four out of five stars. However that's not the whole story. The real "story" within this story is that the author reported this about the range extender and it's caused a but of a stir:

"I thrummed along at 70mph, but it soon became clear that at this kind of speed our comfortable range between fill-ups was more like 40-50 miles. Still, it was impressive how, even when it says it’s flat, the car maintains enough battery power to give an instant shove of torque. Only if you really run it down, which you’ll have to try pretty hard to do (or so I’d been told), would you compromise the performance. Which is what happened next. 

I’d just come through a heavy but localised rain storm on the M20 when the i3 started to slow. It was a gradual process, from motorway cruising speed all the way down to 44mph. By this time I was travelling up a slight incline and had effectively become a slow-moving obstacle. Lorries were catching me with quite frankly terrifying closing speeds. It was three or four minutes - which was long enough to make me consider pulling over - before the i3 recovered; just as slowly as it had lost speed, so it crept up. “It’s not a limp-home mode as such,” a BMW spokesman later told me, “but once the charge runs down to five or six per cent and the range extender cuts in, if you keep driving at 75-80mph it can’t maintain the charge.” Rather than damage the battery by running it completely flat, the i3 had restricted our performance."

This Telegraph video above doesn't mention the difficulty they experienced with the car slowing down at all even though they show it driving along on the M20 while it was raining like they printed version said. That's strange to me since the article seemed to make a big deal out of it. I would have like to have seen video of the car during the explained "slow down" event.

The i3 REx Engine
I've had quite a few conversations with the engineers and product managers at BMW about the range extender and while they are cautious no to over sell it's capabilities, everyone I have talked to promises it's not anything like a "limp mode", and in fact you can do just about anything with it as long as you understand how it works and drive accordingly - I take that as monitoring your speed and if you know you'll be driving up a long steep incline and take it easy for a few minutes before you begin your ascent so the REx can build up a little extra reserve power for the climb. There has been a lot of talk around the fact that in the US, you can't manually engage the range extender once the state of charge is below 75% as you can in Europe. The concern is with the range extender coming on at such a low state of charge (at ~5% SOC) that there may not be a large enough buffer for those instances when the car needs continuous supply of a lot of energy for high speed driving or to climb long, uphill grades at highway speeds.

I don't have the exact answers as to what exactly is possible and what isn't. In fact, I'm not even sure it's possible to offer such a definitive explanation. Sure, the engineers can offer a formula based on total passenger and cargo weight, vehicle speed, head or tail winds, percent of grade you are climbing, etc but who will even understand that let alone be able to transfer that to an actual real world driving situation? There are so many different situations and roads it's just not possible to give a clear black and white definition of what it can and what it can't do and I believe BMW will likely struggle with how to explain this to their customers - maybe that's why they haven't even tried to explain it yet! I have made a request to the program managers to let me drive an i3 REx for a day once one is available. I'll test it in every possible situation I can come up with in the time frame I have including a 220 mile trip to Vermont. Hopefully my request will be accommodated because I believe I can clear the air on this as much as possible, however as I mentioned above there are endless specific driving circumstances so no test will completely satisfy everyone. My advice: Drive one in REx mode to satisfy your concerns before you plunk down your money. I'm sure you can leave a deposit on one and have it refunded if you are not satisfied with the performance once you get the test drive opportunity.

i3 Product manager Oliver Walter
I can say I have spoken with program managers that have driven the i3 REx extensively, and they have assured me that on flat ground, you really have to purposely try to defeat it in order to use more energy than it produces and that it can easily drive along at 70 mph for as long as you need to and still have enough energy for short bursts of power to climb hills along the way. BMW i3 product manager Oliver Walter in particular has assured me the range extender is robust and will be able to power the car in just about any circumstance without the driver even noticing any difference than when it's in pure EV mode. The question becomes how fast can you drive and for how long, up how steep a grade, and right now, we just don't have definitive answers.

So why can't the i3's in the US have the same ability the European i3's have to manually turn on the REx ahead of time, when you know you'll need a lot of energy for demanding driving conditions like climbing a mountain later in your trip? The answer is in the CARB certification of the i3. BMW needs the i3 to be certified as a zero emission vehicle in order to claim the most ZEV credits they can for each i3 sold. However the benefits aren't only for BMW. BMW has been working behind the scenes to make sure that i3 buyers get the same tax credits, tax exemptions and benefits like HOV access that zero emission vehicles get, which in many cases is better than what is offered to PHEV buyers. For example here in NJ, if I buy a zero emission car like a Tesla Model S, I am not required to pay sales tax, but if I buy a Chevy Volt which has a range extender, it's classified as a PHEV and I have to pay sales tax. So if BMW by getting the i3 REx certification approved, manages to get New Jersey to classify the i3 REx as a zero emission vehicle, then I will save about $3,500 in sales tax. To me, that's worth losing the ability to manually turn on the REx early because there will be so few instances that I would ever need to; it's just simply not worth it. This isn't just for NJ though, there are quite a few other States that offer different incentives for ZEV's as compared to PHEV's. However as far as I know, BMW is still working out these details State by State and they haven't announced exactly what the certification will mean in each particular State just yet. As soon as there is more to report on the certification and State benefits I'll have it up here, and hopefully I'll get to report first hand on how the REx performs sometime soon. Happy Holidays everyone!

Of course a robust network of DC quick chargers like the one pictured here would render the range extender irrelevant. But are they coming?


  1. Thanks again Tom. This blog is the best place to go for i3 details and the latest news. I hope you get to conduct complete tests soon!

  2. It is strange that bloggers/forum discussions are doing a better job of explaining this subject rather than journalists/BMW.

    Quite a few journalists in the UK have had the chance to drive the REx long distance. There was a press event in the first week in November where they took REx cars from central London to Brands Hatch, did a few laps of the circuit and drove back again, the idea being that the total mileage was large enough to ensure that the REx would be required (this was the day that spawned all of the videos of journo's drag racing the E92 M3).

    There were some specialist EV blogs who took part in that as well so it's a bit disappointing that it's only the Telegraph that has gone out of it's way so far to test the limits of the REx. I don't particularly like the way he reported it (I'm not sure he fully understood that his manual intervention to turn the REx off created limitations that wouldn't have existed if he'd just left it on auto) but at least he had a go and it certainly stirred up a lot of discussion on the topic which is always good.

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  4. I think the poignant lesson here is that the US version of the i3 should not be driven above 70 mph in range-extended mode for extended periods of time. This should allow the REx to keep the small buffer available in the battery topped off. This buffer should facilitate about 500 feet of elevation gain without any performance penalty. Additionally, this buffer should allow for normal acceleration and it should support the maximum speed of 93 mph for several minutes. All of these scenarios are important, since drivers would like to be have the confidence that the i3 will behave normally in range-extended mode.

  5. The tax exemption would be a great benefit Tom, I hope you are right about that. Here in Washington we also have the same tax exemption on zero emission vehicles. Can I assume BMW is working on this for WA residents also? They didn't forget about us up here right? ;)

  6. There will probably be software mods that will bypass this limitation and allow the range extender to come on early in the USA. It is because of the CARB Bevx rating, and the sales tax break thing that BMW put in this limitation where the engine can only come on when the battery is very low on charge.

    1. Yes, I'm sure you're right about that. It won't take long for somebody to introduce a hack for this.

    2. If you could turn the REx on manually, couldn't you then fill the tank while you still have battery power and extend the range to an unlimited amount?

    3. Steve, yes, you certainly can do that. It should work even without the manual override, which will not be available in the US version. The only difference between the two scenarios is that without the override, the driver will have to be a bit more mindful about the sustained top speed, long inclines and steep hills. The REx will provide enough power to replace the average energy consumed by the vehicle. The buffer in the battery is there to bridge the times when you need more than the average power, such as jackrabbit starts, going more than 75 mph on the freeway or climbing long hills. The larger the buffer, the longer the car can sustain those situations. With about 60% buffer in the battery, and assuming a manual override at this state of charge, you could climb the tallest freeway pass in the US at sustained 75 mph. While that won't be possible with the US version of the i3, the question is who really needs this type of performance?

    4. You can enable hold mode on the USA i3 Rex now via the OBD2 port - canbus hacking, as well as using the full 2.3 / 2.4 gallon tank rather than being limited to 1.9 gallons. - danwat1234 - also as 'Unknown' above

  7. Well sure, and carry a leak proof 5 gallon gas can in the back for trips with gas stations not close by.

  8. Should the nav system work out if you are about to come to a high speed grade, and put the rex on early?

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