Is REx worth it?

[Berlindriver]

Just one advice: No Rex! Buy a 120 Ah version!
Why?:
Less weight, more ev mileage.
Less maintenance, so much lower running costs.
Rex owners use the Rex almost never . Results of a bmw survey. That's why they ended selling them in Europe.
You can use the space of the Rex for extra storage (Wokeby trunk extension)
Why pay more for something you'll almost never use and can give you headaches.

Good luck with your next i3!

 

[user 7107]

I have had a REX car for 4 years now. And i have used it on some longer journeys. It has been an absolute blessing having the REX when either a) you don't have time to recharge (or cant be bothered waiting) or b) turn up at a charger and find its broken, wont work because of an account issue, or some one else is using it. I reckon on long journeys turning up at a charger which you cant use is about 1 in 5 so its not that rare an occurance. I do not know what I would have done without the REX. ~I will add however that if I had a longer range battery like Berlindriver suggests this might not have been a problem in the first place. Most of my driving in the i3 is in and out of town or to and from work and i charge at home and the REX is not needed.

 

[Minotaur]

I am rather interested in this aspect. Recently (1 month ago ) I purchased a BMW Rex with a 60ah range battery (2015 model) . Thanks to our London Mayor's fanaticism to get more money from people. Anyway. This Rex has a limited (upto 70mile) range. I have a high degree of trepidation regarding longer term reliability re engine. Charging on public networks costs from 50p upwards per kwh. Home charging is roughly 30p+ /kwh (I do understand there may be cheaper ways ). As I understand, petrol has theoretically 9kwh energy in 1 litre. Which on a pure cost works out to 17p/kwh (based on £1.50/ltr) . Appreciate petrol engines are not efficient hence possible loss of 50% in conversion (may be even less) ? which perhaps means it may cost 34p /kwh. Taking in to account grid cost of minimum 50p (as far as I have seen ), burning petrol (E5) to charge appears to me cheaper than charging at any grid stations. This also assumes there are chargers available, free to plug in. Not to mention the charging times which can be upwards of 1 hour to just get some charge in (7kw). Grid electricity is also inefficient. There will be transmission losses there too, not to mention generation losses ? Appreciate, electric is one of the most flexible energy sources to store and acquire. Unsure where BMW got there data from re Rex? As I do find the flexibility of a portable generator in the boot useful. Yes, of course, having a larger battery etc may go a way to alleviate problem I may face and does weigh less but the option to have a generator running and adding to the range is useful in the model I currently have. I would of course, love a 120ah or more battery. I do appreciate the lure of pure electric and the elimination of a petrol engine. Currently the cost of electricity on the charging network is too high ( UK ) Have seen charges of upto a minimum 75p /kwh and further a minimum connection charge at times of 30p just to connect? Yuk.

The figures re costings efficiency I obtained from a quick web scan are really a rough guess from the small research done.
I would love to read a better write up from more knowledgable persons here about costings and efficiencies.

 

[cpallenjr]

Making a truly accurate informed cost comparison is difficult for the average person because there are so many unknown variables. I do know impirically that charging at a public station ranges from 35 cents per kw on up to who knows how much at a highway rest stop. Ethanol-free premium gas costs about 3.00/gal in my market, and electricity at my home runs about 15 cents per kw.
The efficiency of my 2017 Rex averages 4.4 mi/kw electric and is supplemented by 36 mi/gal via the range extender.
Therefore:
Given 120 miles @ 27.2 kw = 4.4m/kw and 88 miles @ 2.4 gals = 36 mpg
4.4 miles @ 15 cents = 3.5 cents/mile
4.4 miles @ 35 cents = 8 cents/mile
36 miles @ $3.00 = 8 cents/mile

Clearly charging at home is the cheapest and most convenient method. Using the Rex to get home or to the next public charger is comparable to the cost of public charging but there are trade offs. I can fill my Rex tank in a minute at a self serve gas station, but then it takes a bit of annual maintenance and is another system that might fail and require repair. Purchasing a Rex equipped i3 is also arguably a little more expensive but there is a much broader market range 2014-2021.

I normally can go several days without charging but then maybe once a month or so I need to stretch my charge with the Rex. There is a grocery store on my route to the big city that has several free L2 chargers and I can pick up 20 miles or so while grocery shopping. I could do similar - charge enough to get home at a fast DC charger too kind of like getting $5 worth of gas instead of automatically filling the tank.

So my conclusion is whether you get an i3 with or without a Rex might be a wash depending on your budget, how much you use the car or how far you need to go every day, availability of public chargers in your area, and whether you can charge at home.

 

[ksnax]

This is one of those things where understanding the regional infrastructure and where the car will be driven really is critical to the decision. Again, a 60Ah BEV here (Eugene, OR) imposes impossibilities. A 120Ah imposes fewer of them, but they still exist. REx with the smaller pack overcomes all of them.

 

[mojo]

Grid electricity is also inefficient. There will be transmission losses there too, not to mention generation losses ?

There will, but why do you need to care about these? There will be numerous energy losses in refining crude oil into petrol and distributing it to your local forecourt, but do you have to account for those when working out if a petrol car makes financial sense? Those losses are already factored in to the price that you pay at the pump, and the same applies to losses in the electricity network.

 

[user 7107]

You are correct that the cost of road side chargers has increased a lot and financially its not so attractive. The electric charging makes more sense when you charge at home. As i said in my first response to this question the REX makes sense from a practicality point of view, and i have never tried to make cost comparisons, since for me that's not the point.

 

[eNate]

I'm not the only person with an employer who allows EVs to be plugged in at work. So my charging expenses are largely paid for. I occasionally plug in at home, and even more rarely on the road. If DCFC charging rates were 4x what they are today, I'd still be saving a ton of money vs. buying gas.

I'm mindful of the fact that DC fast charging is a service, not a commodity. I'm paying a premium for a fast charge. The comparison of the DCFC kWh cost vs. what I pay at home is almost irrelevant.

 

[Guv]

I have a i3s ReX.. love it

Pros:
- the S is better looking than the older models
- got the safety of the range extender
- very good and smooth ride
- never had any problems with mine, would got for 2018 model or higher
- the ReX charges the battery and doesnt run the wheels, its a small motorcycle engine

Cons:
- tax is a lot for the first 5yrs
- ReX does make a fan noise, after parked up.. i suppose its cooling down
- pot holes can be dangerous
- the doors at the back don't open until you get out which can be annoying

 

[TornadoRedV]

Depends on your situation but having Rex is a god send for the odd longer journey for us, plus we live in the middle of nowhere, lengthy power cuts recently in the storms, couldnt charge the polestar at home. But little i3 was perfect, straight on the pertrol to a local type 2
I rate hybrid power trains of any kind, having had a few of them now, pure EV has a lot of limitations

 

[kweiske]

Driving 110-120 miles round trip in my '18 REX, knowing I've got 50 miles in reserve is nice. If I had an EV with 200 mile range, I wouldn't want a range extender.

 

[johnheddy]

I would not have bought my 2017 i3 without the REX. There are no charging options near me above level 2 until I exhaust more than half my 120 mile battery range (summer). My REX gives me 90 to 100 actual highway miles per tank if full. I regularly travel 320 miles (twice a month) and use the REX and carry 2 extra gas cans in the frunk. I can just pull off at the closest rest stop or fast food place and fill up, even if there is not a gas station where I run out of gas. When on either end, I use electric only, but it is not worth making 3 half hour charging stops (only to 80%) on the trip, and DC fast charging is bad for the battery anyway.

I have been looking at the articles about adding an 8 gallon aux tank (racing fuel cell) in the frunk area to allow refueling without stopping. It seems safer than carrying two regular plastic gas cans in the frunk.

Each time I stop to refill the tank, I lose some of the state of charge I told it to hold. The marker for hold SOC starts over when you power up after re-fueling, so if you didn't make it back to 75% SOC, you can't set the marker at 75%. Yes my car is coded for hold state of charge. Losing some SOC each stop is a problem if the hotel has no charging options nearby, and i end up burning some SOC to find a good place to refill.

Sometimes I can use the 110V 10A trickle charger out my hotel room window if I can park near the room. I carry a 100ft heavy duty extension chord. But usually I try to stay as close to 70% SOC during the trip as possible by refilling with gas and holding SOC so I can go to the office without charging the next morning.

Refilling in route without shutting off the car would keep the marker at the original 75% with no loss of setpoint during the trip. If you fall behind, you will catch back up when you run into slower traffic, or go downhill for a while. You fall behind if driving over 70 MPH on level, faster at 80, faster still at 80 uphill. but it will catch up if you just slow down for a while.

 

[alohart]

DC fast charging is bad for the battery anyway.

That's not true for i3's due to their paltry 50 kW maximum DC charging power. 50 kW / 33 kWh = 1.5C max which is quite gentle. 50 kW / 22 kWh = 2.3C max which is more stressful for a 60 Ah battery pack, but it's actually less than 2.3C because the charging power never quite reaches 50 kW and remains lower during most of the charging session. There are many EV's with much higher maximum DC charging power that, despite larger battery packs, are reaching C levels that might result in increased cell degradation. Inside EV's recently posted an article about the effects of DC fast charging.

 

[johnheddy]

Ok, confused by your math. Why are you calculating 1/hours and calling it C? Why not divide 33kWh by 50 kW and get 0.66 hours charge time? Isn’t that more useful?

And the car can’t charge 33kWh. There is reserve at top and bottom that can’t be used. That is why it is ok to charge to100% at home, as it is really less than 90% of the 33kW. Lithium batteries don’t like sitting at 100% for long periods of time, or sitting at 0% for somewhat shorter periods of time, so BMW protects the battery by not letting users reach those levels.

 

[Map40]

For us, the Rex makes the i3 the best electric vehicle ever. Before, with my tesla or my leaf, in an emergency if I had to go and visit my son, you were always tied to the battery. With the Rex I simply don't care. The i3 stopped being a "special use car" and it became "a car"

 

[Devlin13]

We love our Rex. Gives us the ultimate flexibility. Short drives, no problem. Long drives, also no problem. That said, to give you full control, turn on the feature that allows you to hold charge at 75%, as opposed to the US default of no manual control over the Rex and it will only turn on when you hit 5%.

If I’m on a long drive, I turn on the Rex after it hits 75%, or time it so that I get to the next charging location with 10-20% battery left. That way, I maximize charging speeds when I get to a super charger.

 

[ksnax]

Every 3-6 months, I travel to a doctor appointment out of town. It's 65 miles, with only one DC charger about 40 miles into the trip. With less than ideal weather, I would not make it to the dr. office without a charge mid-way. In that case, it really means TWO charges for my 2015 to make a single 130 mile trip - but only IF that DC charger is working correctly. The last 3 trips I have made, it has not been, shutting down early or just not charging at all. Without the REx, I'd be SOL without toiling to find a level 2 charger somewhere nearby, and yesterday I did, both occupied by cars plugged in and not actually charging. Fortunately, there are more than 5x as many gas stations on the route.

So while I would love to not use gas at all, the truth is that I only use the REx about 10x a year. It fills the void in range that I don't actually need the other 95+% of the time. Even a 120Ah i3 would still be problematic for this trip. I would not own an i3 at all with the current charging infrastructure if it were not for the REx.

 

[alohart]

Ok, confused by your math. Why are you calculating 1/hours and calling it C? Why not divide 33kWh by 50 kW and get 0.66 hours charge time? Isn’t that more useful?

Depends on what you're considering. The C rate is related to the charging or discharging stress on a battery cell. The charging time doesn't indicate stress directly.

Because of the non-linear DC charging curve (see attached charging curves for all 3 i3 battery packs), the charging time isn't necessarily easy to calculate. The charging time depends on the number of kWh added to the battery pack, the charging losses due to some of the energy converted to heat, and the ending charge level. 33 kWh wouldn't be added to a 33 kWh battery pack with a nominal new usable capacity of 27 kWh unless the charging losses are higher than they actually are. 50 kW wouldn't be the average charging power.

And the car can’t charge 33kWh. There is reserve at top and bottom that can’t be used. That is why it is ok to charge to100% at home, as it is really less than 90% of the 33kW. Lithium batteries don’t like sitting at 100% for long periods of time, or sitting at 0% for somewhat shorter periods of time, so BMW protects the battery by not letting users reach those levels.

Most lithium battery chemistries, lithium ion phosphate excluded, degrade more rapidly when left at an actual 100% charge level for a significant period of time. They degrade less slowly when left at an actual 95% or 90%, but they still degrade more rapidly than if left at a lower charge level. The BMS in all i3's reserve ~8-12% of the gross capacity for a low charge level buffer (displayed as a 0% charge level). My second attachment shows the usable percentage of each i3 battery pack type. The unusable portion is split between low and high charge level buffers. The 94 Ah battery pack has the highest unusable portion, 18%, so its actual charge level at a displayed 100% charge level is lower than either the 60 Ah or 120 Ah battery packs. However, it's almost certainly greater than an actual 90% charge level, depending on the size of the low charge level buffer.

Those many 60 Ah i3 owners whose battery packs lost capacity quite rapidly, some so rapidly that their battery packs were replaced under warranty (I know of 4 where I live), would probably have preferred a larger high charge level buffer to reduce the cell degradation rate, but that would have decreased the already low range even further.

 

[agzand]

I am probably biased (I drive my second i3 REX), but here is what I think. As much as I like the i3, as a pure BEV there are better options out there these days. Just buy a Model 3 SR and you don't have to deal with peculiarities of the i3, better range, practicality warranty, easy tires, etc.

What make the i3 a very appealing car for me is that even after 10 years, the REX model is still the best PHEV on the market by a wide wide wide wide margin. You get 99% of the benefits of an EV, in combination with 99% of the benefits of a PHEV.

 

[Candece]

I consistently get 4.5kw (not sure if I am using it correctly) for my range efficiency. I definitely think it's the most efficient