REx is not a panacea!

[Surge]

I feel quite strongly about this, so felt it deserved a new thread.

If you opt for the REx, make sure you understand its limitations - and whether or not it fits your driving. The REx is not a solution for all range anxiety. It does not make the i3 a long-distance highway cruiser.

Let's review the relevant facts:

1) EPA rated the BEV (non-REx) i3 at 81 miles

2) The REx is about 20-28% LESS efficient (see my other post from today on this). It breaks down like this:
- REx is 10% heavier, so -10%
- REx drag coefficient is 0.30 instead of 0.29 -- this has the biggest impact on the highway, at 75 mpg you are down another -4.5% in range [recall from high-school physics that the force required to move a car requires speed^3, so with a higher drag coefficient, you get penalized exponentially the faster you travel]
- heat pump - lose about 7%, depending on how cold it is and how much you use the heater
- US/Canada, can't use 6% of battery with REx, so there's another 6% loss in range

Don't forget that this 20-28% loss of efficiency means 20-28% longer charger times!

3) Taking (1) and (2) into account, your EPA range on battery power for the REx is only about 58-65 miles

4) Ok, but I bet many of you are thinking, 'so what', the REx will double my range, so I get over 110 miles in total… right?
Well… not quite!
Not if you need that range for highway driving. And I bet most people don't drive more than ~60 miles per day in the city, so they will count on the REx when on longer trips involving the highway.
So here's where you need to know that at 70-75 mph, the REx won't give you more than around 60-65 miles. I am basing this on owner reviews (some, such as the UK Telegraph review, rate the REx at only 40-50 miles on the highway, even in moderate temperatures).
Even this sounds good - +60 miles on top of the 60 or so you get on battery, what's wrong with 120 miles?!

Well, nothing, if you don't drive in winter!

5) This is the death-blow of the REx, for me… because in below freezing, knowledgable people say that you could lose 40 over 50% of the range. This will affect the REx range as well, and it may affect it even more, because the little gas engine will have to run at the highest rpm possible to keep the battery charged, where it is less efficient than at lower rpms.
So your range is going to be more like 60 miles total - 30 + 30.

But with the BEV, you would get about 40-50 miles (based off the EPA estimate, adjusted for lower outside temp.).

To gain +20 miles and have to carry around a gas engine, and be penalized 20-30% in 100% of your driving.
Is having a safety net of 20 miles worth the extra cost, refueling hassles, and performance reduction worth it to you?

So be aware of what the REx will and won't do. When would the REx make tons of sense?
- if you don't drive on the highway
- if you don't drive in cold (below freezing) temperatures
- if your daily commute/travels are more than 80 miles but less than 120 miles

Finally, at least one member here advocates choosing the option that will work in the worst possible scenario. I respectfully disagree. If we chose all of our possessions for the 0.0001% outcome, we would be living in concrete bunkers.
More realistically, apply the 80-20 rule -- figure out how you do 80% of your driving and plan for that. Don't let the 20% dictate the 80%. (Or choose 90-10, if you feel more comfortable).

 

[mindmachine]

Man you are way out there, to the point of being OCD on this range and the degree of pessimism.

I am not going to argue with you but you are way overstating how terrible the i3 Rex is going to perform. And if you have been reading the discussions on this site you appear to be ignoring the equal number of positive stories about both range and flexibility of the Rex, and yes the high MPG's that the Rex gets when running strictly on it.

I don't think you should buy the BEV either, as you are not comparing apples to apples in your degree of pessimism on it as compared to the Rex. If you believe these numbers then the BEV is only going to have 40 mile safe range in the winter.

Come on now with the Rex really, 20 to 28% less efficient then BEV, I seriously question your estimates as being way off the mark.

Rex range estimate is way too low, the gas engine will not suffer anywhere near the efficiency losses of the batteries in cold weather. You are saying the Rex will only get 15 MPG in the winter??? You even really believe that number yourself. Not just because you have been down on the Rex yourself since the very beginning.

10% heavier can't be directly calculated/transferred as a 10% loss in range or efficiency.

Rex range no more than 30 +30 mile total range in the winter, wow you are way out in left field.

I sure don't know how Leaf owners survive or why anyone would buy one by now if performance of EV's were really this bad.

 

[MikeS]

I feel quite strongly about this, so felt it deserved a new thread.

If you opt for the REx, make sure you understand its limitations - and whether or not it fits your driving. The REx is not a solution for all range anxiety. It does not make the i3 a long-distance highway cruiser.

Let's review the relevant facts:

1) EPA rated the BEV (non-REx) i3 at 81 miles

2) The REx is about 20-28% LESS efficient (see my other post from today on this). It breaks down like this:
- REx is 10% heavier, so -10%
- REx drag coefficient is 0.30 instead of 0.29 -- this has the biggest impact on the highway, at 75 mpg you are down another -4.5% in range [recall from high-school physics that the force required to move a car requires speed^3, so with a higher drag coefficient, you get penalized exponentially the faster you travel]
- heat pump - lose about 7%, depending on how cold it is and how much you use the heater
- US/Canada, can't use 6% of battery with REx, so there's another 6% loss in range

Don't forget that this 20-28% loss of efficiency means 20-28% longer charger times!

3) Taking (1) and (2) into account, your EPA range on battery power for the REx is only about 58-65 miles

4) Ok, but I bet many of you are thinking, 'so what', the REx will double my range, so I get over 110 miles in total… right?
Well… not quite!
Not if you need that range for highway driving. And I bet most people don't drive more than ~60 miles per day in the city, so they will count on the REx when on longer trips involving the highway.
So here's where you need to know that at 70-75 mph, the REx won't give you more than around 60-65 miles. I am basing this on owner reviews (some, such as the UK Telegraph review, rate the REx at only 40-50 miles on the highway, even in moderate temperatures).
Even this sounds good - +60 miles on top of the 60 or so you get on battery, what's wrong with 120 miles?!

Well, nothing, if you don't drive in winter!

5) This is the death-blow of the REx, for me… because in below freezing, knowledgable people say that you could lose 40 over 50% of the range. This will affect the REx range as well, and it may affect it even more, because the little gas engine will have to run at the highest rpm possible to keep the battery charged, where it is less efficient than at lower rpms.
So your range is going to be more like 60 miles total - 30 + 30.

But with the BEV, you would get about 40-50 miles (based off the EPA estimate, adjusted for lower outside temp.).

To gain +20 miles and have to carry around a gas engine, and be penalized 20-30% in 100% of your driving.
Is having a safety net of 20 miles worth the extra cost, refueling hassles, and performance reduction worth it to you?

So be aware of what the REx will and won't do. When would the REx make tons of sense?
- if you don't drive on the highway
- if you don't drive in cold (below freezing) temperatures
- if your daily commute/travels are more than 80 miles but less than 120 miles

Finally, at least one member here advocates choosing the option that will work in the worst possible scenario. I respectfully disagree. If we chose all of our possessions for the 0.0001% outcome, we would be living in concrete bunkers.
More realistically, apply the 80-20 rule -- figure out how you do 80% of your driving and plan for that. Don't let the 20% dictate the 80%. (Or choose 90-10, if you feel more comfortable).

I guess you better buy a different car then :twisted:

 

[Surge]

I'm using facts and reports from actual owners. This is not my bias, these are the facts and owners' experiences.

- Read the UK Telegraph review of the REx, note what they say about the highway range of the REx.

- 10% increase in weight = 10% more energy required to push it around! Especially at low speeds, at higher speeds, wind resistance takes over as the dominant force to be overcome (see next point)

- on top of that, the coefficient of drag is worse for the REx (not sure why, but it was reported here:
http://insideevs.com/bmw-i3-range-extender-to-offer-up-to-87-more-miles-decreases-performance/
So the force necessary to push the i3 increases with the speed cubed, really hitting you at highway speeds
I have analyzed using this calculator: http://ecomodder.com/forum/tool-aero-rolling-resistance.php
that it will require 10% more power to move the REx i3 at 75 mph than the BEV.

- on top of that, if you drive in winter, the cabin heat is 30% less effective. The 7% hit came from the bmwi3owner blog; but it will vary based on how much you use the heat, the temp outside, etc.

- and finally, you can't use the last 6% in the US/Canada

No need to get defensive. The REx will make sense for some, but I think in far fewer cases than most want to believe.

Unless you really need an additional 20 miles in winter, or 60 miles in summer (on the highway),
or you drive in the city more than 80 miles/day, I don't see what you're benefitting with the REx, other than psychologically (hopefully -- for me, I would always regret not being in the more efficient model).

 

[MikeS]

'Read the UK Telegraph review of the REx, note what they say about the highway range of the REx.’ - You believe what journalists write? :evil:

- 10% increase in weight = 10% more energy required to push it around! But only significant if you ignore the other weight factors such as the driver and passenger! :evil: .

 

[Surge]

'Read the UK Telegraph review of the REx, note what they say about the highway range of the REx.’ - You believe what journalists write? :evil:

I tend to, especially when they are from respected publications and they are reporting observed facts.

- 10% increase in weight = 10% more energy required to push it around! But only significant if you ignore the other weight factors such as the driver and passenger! :evil: .

Well unless you plan to make your passengers lose 120KG of their body weight before getting into the REx, then this is a constant -- whatever you plan to carry in the i3 will be the same whether its a REx or not. So it won't matter. As a % it would be a little less than 10% if you really load up the i3, but I'm not sure that's what you meant.

 

[MikeS]

I don’t think that in th eUK we consider any papers to be ‘respected publications' :evil: In fact as I remember it the telegraph article tried to claim they could hear the Rex when driving at 70 mph!

 

[fdl1409]

Dear surge,

with all due respect, but what you write is completely off the mark.
You multiply penalty for weight and drag coefficient and cold and...
This is just not true. The i3 BEV is rated with 12.9 kWh/100km, the i3 REx with 13.5 kWh/100km. This is a difference <5%, not 20-28%.
Yes, the last 6.5% the REx will start. But still, those 6.5% are available und you can and will use them if the fuel runs dry.
You may lose 40-50% in very cold weather, but only if you feel you have to heat full power AND drive with high speed. This is something which is not specific for the REx, but for any electric vehicle.
As I have stated in another thread, heating will not reduce your range by such a high percentage if you drive 75 mph.
With my i3 REx, which I is in my possession for 50 hours now, I have driven 95 miles yesterday with 12 electric miles remaining. That means I could drive 140 miles without the REx in your opinion? You are way off the mark here again.
Weight penalty for example. Net weight is 1.315 compared to 1.195 kg of the BEV. With only a 70kg driver, that would be 8.6% extra weight. That does certainly not increase your power consumption by 10%. You have some more losses on the tires, but weight does not change the air drag at all, so you have to take this out of your calculation. If running with 75mph, the extra weight alone won`t cost you more than 2%. That is why you cannot multiply all the factors. Same is true for air drag. Yes, that is higher with a REx, but again you have to take out all other factors first. In the end, you have to sum it up, not multiply.

Frank
Germany

 

[Surge]

Actually, with all due respect, there is 10% more power required to push the REx.
Look at the link to the air and rolling resistance formula I sent and try it out.

At 75 mph you need 22.16 hp to keep the i3 REx moving,
vs. 21.16 hp to keep the BEV moving. That's a difference of 10%.

True, I didn't include the weight of the driver, cargo, etc.; but it shouldn't matter, since the weight is constant in both cases.

My other percentages are just approx., it's not a science, there are too many variables. But readers should get a sense that, yeah, we're talking about 20% worse efficiency here (in cold, on the highway)… it's not trivial!

Even if you don't drive in below freezing at 75 mph, 10% is not a small difference.

I suppose you are right, you could let the REx run out of fuel on US/Can cars and then have the last 6% of battery…

But still, you are looking at 10% less due to weight and aero drag,
and around 7% due to the lack of cabin heat pump. So that's 17%.

Not including the likely higher fuel consumption in -15+C weather…

And the other point is that because range suffers so badly in cold and at normal (70-75 mph) highway speeds, the fact that the REx doubles ranges doesn't mean that much, when range is 30 miles!

That was the 'aha' moment for me… doubling a short range is not that significant (30->60 miles).

If I routinely need the REx (i.e., to go 30-60 miles), then there's another drawback because it's nowhere near as efficient as a diesel or a Prius, for example. If my winter commute exceeded 30, I would have to look at a diesel or hybrid, if I wanted to maximize my mpg.

So what are you paying for with the REx, aside from psychological benefit, about +30 miles in the conditions described above.

Again, if you don't drive at 75 mph in below freezing temps, the REx makes more sense, since you'll get more like +80 miles.

And we haven't even begun to talk about the free car sharing that BMW is offering, or the opportunity cost for the cost of the REx.

 

[amateurish]

My Rex is showing an estimated range of 98 miles in eco pro plus today...

 

[fdl1409]

Sorry, but you are wrong with your mathematics all the way in your calculations.

So the force necessary to push the i3 increases with the speed cubed, really hitting you at highway speeds

Yes, but your conclusion that higher air drag increases power demand for overcoming air resistance cubic is wrong. It goes up square, not cubic. Why is that? If you take a given car and double speed then air drag will rise square, 2x2=4. The third dimension is speed, which is double. 4x2=8.
Now if you look at another car with higher air drag, the difference will increase resistance square, but the third dimension speed remains the same in comparison. So difference in air drag will raise power demand square, nor cubic.

Now let us look at rolling resistance. Here a picture from Tesla:

Power consumption to overcome rolling resistance here is 60Wh/miles, rather independent of speed. This is for a much heavier car, for an i3 it is more likely around 40 Wh/mile. At 75 mph you will need something like 3kW for rolling resistance. An extra 8.6% weight for an i3 REx with a 70kg driver will increase that by 258W, which is definitely less than 2% of power needed to maintain an average speed of 75 mph.

If power to overcome air drag at 75 mph is for example 15 kW, the difference of 0.30 vs. 0.29 in cw will raise that by 30/29 square, which is 7%, making 1.05 kW here.

With heating being one third less efficient, this may be another kW if heating demand is as high as 3 kW.

Sums up to possible 10% all together in unfavourable conditions ( mainly high speed ), but certainly not 20-28%. Well, you can compensate this by reducing your speed a little bit, because that will influence power demand for the dominant factor air drag cubic. 2.2% less speed will equal air resistance power demand between cw 0.30 and 0.29.

Frank
Germany

 

[ultraturtle]

2) The REx is about 20-28% LESS efficient (see my other post from today on this). It breaks down like this:
- REx is 10% heavier, so -10%
- REx drag coefficient is 0.30 instead of 0.29 -- this has the biggest impact on the highway, at 75 mpg you are down another -4.5% in range [recall from high-school physics that the force required to move a car requires speed^3, so with a higher drag coefficient, you get penalized exponentially the faster you travel]
- heat pump - lose about 7%, depending on how cold it is and how much you use the heater
- US/Canada, can't use 6% of battery with REx, so there's another 6% loss in range

Don't forget that this 20-28% loss of efficiency means 20-28% longer charger times!

Rather than relying on assumptions, you can simply go to BMW's website, where they have measured the efficiency of the two vehicles, taking all of the above into account: http://www.bmw.com/com/en/newvehicles/i/i3/2013/showroom/technical_data.html. The BEV consumes 12.9 kWh/100km, and the REx consumes 13.5 kWh/100km, or 4.65% more. That makes the REx 4.4% less efficient than the BEV. That means 4.4% less range, and 4.6% more charging time.

3) Taking (1) and (2) into account, your EPA range on battery power for the REx is only about 58-65 miles

EPA range for the REx should come out to be about 77 miles based on the above measured efficiency difference.

... in below freezing, knowledgable people say that you could lose 40 over 50% of the range. This will affect the REx range as well, and it may affect it even more, because the little gas engine will have to run at the highest rpm possible to keep the battery charged, where it is less efficient than at lower rpms.

Incorrect. Gasoline engines operate most efficiently (greatest energy output per gallon burned) at near full load. Exploiting this feature is another reason I tip my hat to those brilliant engineers at BMW - it could turn out to be the most efficient gasoline engine to ever power an automobile.

 

[Surge]

Sums up to possible 10% all together in unfavourable conditions ( mainly high speed ), but certainly not 20-28%.

So we agree on the 10% due to weight and drag coefficient at highway speeds (75 mph).

The 20-28% comes from:
- 10% due to weight (as discussed -- again, the online equation link takes into account everything you described, and more)
- 7% for less efficient cabin heat (this came from BMW i3 Owner's blog post); this is obviously an estimate - it depends how warm you want to be and how cold it is outside!

So it should be 17% rather than 20-28% (my original assumption was not being able to use the last 6% of charge in the battery in the US/Canada).

Still, 17% is not trivial. Let's put it into context:
- 17% longer charge times (to have the same range)
- EPA range of 98 miles instead of 81, or 17 miles extra per charge

I would much rather have a range of 98 miles instead of 81, 100% of the time; and save $4000, and not need gas.
The only time I imagine needing a REx, will be on the highway on a longer trip. At precisely those times, the REx won't add more than 30-60 miles (depending on outside temp).

 

[fdl1409]

No, I agreed on 10% including heating, drag and weight much less, maybe 5-7%. By the way, in very cold weather, efficiency of the heat pump deteriorates. Without the heat pump and at lower speed, the difference is smaller.

Frank
Germany

 

[I33t]

At 75 mph you need 22.16 hp to keep the i3 REx moving,
vs. 21.16 hp to keep the BEV moving. That's a difference of 10%.

21.16 + 10% = 23.276

I don't know where you got your hp figures from but your math is way off! The difference is a bit less than 5%. In real life, it would probably be less than that (if the hp figures are accurate)

Perhaps consider a hybrid rather than a Rex?

 

[Stevei3]

Don't let the 20% dictate the 80%.

Hi Surge,

interesting discussion you've started. I can't seem to withold myself from contributing, knowing in advance that this discussion can be endless and/or fruitless

#1: see quote: I beg to differ. We've bought our REx exactly to accomodate for the dictation of the 10, or say 20%. The REx enables us to use the i3 most of the time, even when E range is barely enough or plain insufficient, or charging facilities at destination are few (like today, for there is but one (!) charging outlet in Bergen aan Zee). The result of this is we are driving ~2100 miles/month, of which 85% electric. Quite brilliant in my book. True: the REx will be a little less efficient, but do keep an open eye @ the big picture here, I would say.

#2: in our experience, > 6500 miles so far, REx in E mode isn't as inefficient as you might think, if we compare it to the i3/BEV of our friends. BMW's numbers are sort of truthful it seems. As are your own, because, unless you've made a typo, the difference between (and I quote) '22.16 hp to keep the i3 REx moving, vs. 21.16 hp to keep the BEV moving' isn't 10%, but an increase of 4.7%. Please do correct me if I'm wrong. (PS: ah, I see i33t beat me to this comment)

Regards, Steven

 

[Surge]

Frank, go to this calculator and see for yourself:
http://ecomodder.com/forum/tool-aero-ro … stance.php

The REx needs 10% more power to push it down the road at 75 mph, vs. the BEV.
This is due to weight and Cd.
It's just a fact.

Second, you have the loss due to the heat pump. Correct, if you don't use the heat, it won't make the REx any worse. But in the winter, you will lose range.
From BMWi3Owner's blog post:

A heat pump reduces energy about 30% and since the display says that the interior heating consumes about 12 km during a charge cycle the heat pump should be able to increase the range 5-10 km.

We don't know what his driving cycle was at that time, but that's between 5-10% of a winter range drive. So I chose 7% to be in the middle.

So 17% impact to range/efficiency/charge time/{however you want to think about it!}

 

[Surge]

At 75 mph you need 22.16 hp to keep the i3 REx moving,
vs. 21.16 hp to keep the BEV moving. That's a difference of 10%.

21.16 + 10% = 23.276

I don't know where you got your hp figures from but your math is way off! The difference is a bit less than 5%. In real life, it would probably be less than that (if the hp figures are accurate)

Perhaps consider a hybrid rather than a Rex?

You are correct, it is more like 5%. I stand corrected!

But why 'in real life it would be less than 5%'…? I don't see how it would be less.

 

[Surge]

Don't let the 20% dictate the 80%.

Hi Surge,

interesting discussion you've started. I can't seem to withold myself from contributing, knowing in advance that this discussion can be endless and/or fruitless

#1: see quote: I beg to differ. We've bought our REx exactly to accomodate for the dictation of the 10, or say 20%. The REx enables us to use the i3 most of the time, even when E range is barely enough or plain insufficient, or charging facilities at destination are few (like today, for there is but one (!) charging outlet in Bergen aan Zee). The result of this is we are driving ~2100 miles/month, of which 85% electric. Quite brilliant in my book. True: the REx will be a little less efficient, but do keep an open eye @ the big picture here, I would say.

#2: in our experience, > 6500 miles so far, REx in E mode isn't as inefficient as you might think, if we compare it to the i3/BEV of our friends. BMW's numbers are sort of truthful it seems. As are your own, because, unless you've made a typo, the difference between (and I quote) '22.16 hp to keep the i3 REx moving, vs. 21.16 hp to keep the BEV moving' isn't 10%, but an increase of 4.7%. Please do correct me if I'm wrong. (PS: ah, I see i33t beat me to this comment)

Regards, Steven

Steven, yes, great comments! There is a 10% weight difference, and I think the (my!) confusion arose from that… but yes, I stand corrected, it's more like 5% at 75 mph.

My motivation here was just to cause potential buyers to think twice (or more!) about their needs before deciding, and not assuming that the REx will just do it all for ~$4000.

In my specific case:highway driving at 120 kph/75 mph, often in below freezing temp., the REx won't really add that much. That's the point I'm trying to make. Is gaining ~ 40 miles in very cold weather worth it? Maybe you never have to deal with these temps., or you don't drive @ 75 mph, so you'll get more range.

For me, I'd rather have the most efficient vehicle possible, and enjoy the fastest acceleration possible (to get around congested traffic), and have 12% (5% + 7% more range per charge, then to have the 40-80 mile extension when/if I ever need it.

 

[Surge]

It's interesting to think about that 12% increase in efficiency with the BEV, in terms of charging time.

So to get the same range with the REx, you'd need to spend an extra 22 min. charging (on a level 2 charge). (but that's just theoretical because the battery is the same, of course)

Put another way, for every 8 full recharges of the BEV, you would need 9 for the REx.