rapid charge retrofit?

[elptex]

IMO no one should order this vehicle without it. Do yourself a favor and tick this box.

Unless you have a REX and then there is no need!

Disagree. Even with REX, more miles on electricity is cheaper to run.

 

[jadnashuanh]

None of the hills or ascents in the UK approach those in the USA in length where you may be going up for 20-miles or so and rise 5-6Kft or more. Obviously, they are not everywhere - it's a big country. Yes, there are parts of Europe where they do. The suitability of the i3 for your particular needs obviously will vary, and while I love mine so far, it certainly wouldn't suite everyone. FWIW, if the REx really is 34Hp, with no conversion losses, that equals 25.35Kwhr. So, to get 28Kwhr out of it usable, it has to be making a fair amount more than 34Hp when you consider it's probably around 80-90% efficient in the conversion. 25.35Kwhr is a long way from what you need to make 170Hp, and then, consider the electric motor is not the only load on the batteries: fan, lights, a/c (or heat), radio, power steering, power brake assist, windshield wipers, and something has to keep all of those computers running! Again, depending on your expectations and use pattern, the REx might keep the batteries from further discharging, but there are a lot of situations where it will not be able to.

 

[Zzzoom3]

...your speed will be limited to 40 mph maximum because that is all the power that the motor can deliver.

A common misconception. As long as there is useable charge left in the battery, it can deliver a full 170 hp. Luckily, sustained highway speed driving only requires around 30 hp, allowing excess power from the gen-set to raise the battery SOC to the level selected either manually or automatically.

Several folks on this forum have proven this to be the case, maintaining between 68 and 75 mph, discharging the battery on climbs and headwinds, recharging it on descents and tailwinds, with SOC varying only slightly. It is designed to maintain a minimum useable SOC of 6%, so it would take a very long climb, or many minutes in excess of 68 to 75 mph to eliminate that reserve of useable charge.

That's good to know. I was referring to the scenario when you have used the 18.8 kW. With REx motor only, you've got a max power of 37.5 hp (28 kW) available. You won't be able to drive above 40 mph which is not to practical. Which leads back to my point why you want DC Fast Charging. I'm not trying to argue for or against REx. I'm only saying that a wise purchase option is DC Fast no matter what version you buy.

 

[ultraturtle]

...25.35Kwhr is a long way from what you need to make 170Hp

Let's back up for a moment and discuss units, as they are often confused. Watts, Kilowats, and Horsepower are units of power, and are used in reference to power generation devices such as electric motors, and gasoline engines. Kilowatt hour (kWh) are units of energy and are useful when referring to energy storage media such as batteries and gallons of gasoline. Not only can 25.53 kWh of energy easily produce 170 hp of power, so can 1 kWh - just for a lot less time.

Next, there is no practical way for an i3 to sustain 170 hp for any reasonable length of time. 170 hp is roughly 6 times the power necessary to sustain highway speeds. Even if one were to theoretically sustain 170 hp by massively overloading the vehicle and driving it at ridiculous speed up an unrealistically steep grade against gale force winds, assuming it does not overheat it could only sustain that load for 9 minutes before depleting all 18.8 kWh of useable charge.

Finally, the REx gen-set is currently specified as 25 kW / 34 hp on the BMW USA website: http://www.bmwusa.com/Standard/Cont...d_Specs/BMWi3RangeExtenderSpecifications.aspx
The only reference to 28 kW came from delivery documents of European owners in this thread: http://www.mybmwi3.com/forum/viewtopic.php?f=13&t=1024
To date, no USA owner has verified 28 kW on that thread. Could be a simple difference in how various countries' regulations allow manufacturers to measure power.

 

[Zzzoom3]

...25.35Kwhr is a long way from what you need to make 170Hp

Let's back up for a moment and discuss units, as they are often confused. Watts, Kilowats, and Horsepower are units of power, and are used in reference to power generation devices such as electric motors, and gasoline engines. Kilowatt hour (kWh) are units of energy and are useful when referring to energy storage media such as batteries and gallons of gasoline. Not only can 25.53 kWh of energy easily produce 170 hp of power, so can 1 kWh - just for a lot less time.

Next, there is no practical way for an i3 to sustain 170 hp for any reasonable length of time. 170 hp is roughly 6 times the power necessary to sustain highway speeds. Even if one were to theoretically sustain 170 hp by massively overloading the vehicle and driving it at ridiculous speed up an unrealistically steep grade against gale force winds, assuming it does not overheat it could only sustain that load for 9 minutes before depleting all 18.8 kWh of useable charge.

Finally, the REx gen-set is currently specified as 25 kW / 34 hp on the BMW USA website: http://www.bmwusa.com/Standard/Cont...d_Specs/BMWi3RangeExtenderSpecifications.aspx
The only reference to 28 kW came from delivery documents of European owners in this thread: http://www.mybmwi3.com/forum/viewtopic.php?f=13&t=1024
To date, no USA owner has verified 28 kW on that thread. Could be a simple difference in how various countries' regulations allow manufacturers to measure power.

Turtle there is further confusion because the motor is 34 hp but it is used to turn an AC generator capable of producing 28 kW of power. That's all you get when the REx is running at full output.

 

[jadnashuanh]

Power is still power...you can't magically get more power out of a system than what you put in. The batteries act like a big storage container and can momentarily easily output much more than what you put in before they overheat, and thus get limited , or, worst case, depeleted. IF BMW's rating of the REx is truly 34Hp, it cannot make 28Kw. And, since you are turning rotational power into electricity, there is some penalty in that conversion so whatever you put in, you'll get less out as some of it is lost to friction, heat, etc. in the generator set. And, to accelerate the i3 briskly, you'll draw more than the 25Kw going in, so you'll be tapping the batteries so the energy flow will be out of them verses in.

I do not have a measure of how many watts it takes to run the i3 when it is just sitting still - I'm sure they've tried to make it as efficient as possible. All of that energy must come from the batteries, and when you're moving, that as well. The drag is a function of the square of the speed, then you have the potential load of trying to climb a hill. A headwind is just like trying to go at the sum of your road speed and the wind speed. Sometimes it helps if the vector is going the right way, sometimes it hurts.

 

[ultraturtle]

Turtle there is further confusion because the motor is 34 hp but it is used to turn an AC generator capable of producing 28 kW of power. That's all you get when the REx is running at full output.

Again, back to source documents. There is no mention of 28 kW of power outside of delivery documents to owners in Germany and the Netherlands. 25 kW, which equals 34 hp is all that is currently specified on the USA web site, with no power output specified on the international site: http://www.bmwusa.com/Standard/Cont...d_Specs/BMWi3RangeExtenderSpecifications.aspx

Just as there is no German or Dutch reference to their measurement of 28 kW equalling 37.5 hp, there is not yet a posting of any North American REx output that exceeds 25 kW, which equals 34 hp. My guess is that we are comparing apples to oranges in the manner that individual countries allow power output to be expressed.

 

[ultraturtle]

IF BMW's rating of the REx is truly 34Hp, it cannot make 28Kw.

I agree that it cannot, but we should note that BMW never rated the REx at 28 kW. It rated it at 25 kW, which is equivalent to 34 hp within 1%. The reference to 28 kW was only posted here by European owners who noted the number in their delivery documents, and did not post an equivalent horsepower value.

 

[Zzzoom3]

IF BMW's rating of the REx is truly 34Hp, it cannot make 28Kw.

I agree that it cannot, but we should note that BMW never rated the REx at 28 kW. It rated it at 25 kW, which is equivalent to 34 hp within 1%. The reference to 28 kW was only posted here by European owners who noted the number in their delivery documents, and did not post an equivalent horsepower value.

No argument 34 hp is 25 kW. It's just that we have folks who think they can run indefinitely on REx which isn't the case. Some think they have full power running on REx power only which is also not true. Furthermore, if running on REx only, you WILL be speed limited (max of 40 mph).
WRT the thread topic, my point is that DC Fast Charging is applicable, useful, and should be part of your purchase option choices in the i3 (either version). I think it stinks that we should have to pay for it in a $41K electric vehicle! We wouldn't be debating this if it was just included

 

[ultraturtle]

...Some think they have full power running on REx power only which is also not true. Furthermore, if running on REx only, you WILL be speed limited (max of 40 mph).

Again, not the case. Please carefully read the previous posts. There is no 40 mph limit. Once the REx system depletes battery charge to 6.0 to 6.5% SOC, it reverts to a series hybrid system much like the parallel hybrid systems of the Chevy Volt and Toyota Prius Plug-In. That 6.0 to 6.5% charge buffer supplies a full 170 hp on demand for as many seconds as it takes to accelerate from a stop or climb a reasonable hill. Upon decelerating to a stop, or coasting down a hill, the excess of 25 kW (34 hp) power produced by the REx gen-set brings the battery charge state back towards its preselected level prior to shutting off.

Numerous owners have posted sustained REx operation with no loss of SOC at between 68 and 75 mph of continuous operation, which is absolutely in line with the physics of the situation. I simply cannot understand why you claim this 40 mph limit.

 

[Zzzoom3]

...Some think they have full power running on REx power only which is also not true. Furthermore, if running on REx only, you WILL be speed limited (max of 40 mph).

Again, not the case. Please carefully read the previous posts. There is no 40 mph limit. Once the REx system depletes battery charge to 6.0 to 6.5% SOC, it reverts to a series hybrid system much like the parallel hybrid systems of the Chevy Volt and Toyota Prius Plug-In. That 6.0 to 6.5% charge buffer supplies a full 170 hp on demand for as many seconds as it takes to accelerate from a stop or climb a reasonable hill. Upon decelerating to a stop, or coasting down a hill, the excess of 25 kW (34 hp) power produced by the REx gen-set brings the battery charge state back towards its preselected level prior to shutting off.

Numerous owners have posted sustained REx operation with no loss of SOC at between 68 and 75 mph of continuous operation, which is absolutely in line with the physics of the situation. I simply cannot understand why you claim this 40 mph limit.

I am referring to the scenario where a 0% state of charge exists. Are you saying you can't get to 0% SOC with a REx? If you can, can you still drive the car the same way? Where is this extra power coming from? Have you seen "8 Key Questions about the BMW i3 Electric Car": http://www.plugincars.com/8-key-questions-about-bmw-i3-electric-car-129816.html

 

[ultraturtle]

Are you saying you can't get to 0% SOC with a REx?

No. I'm just saying that it is difficult to do so under normal driving conditions. You would somehow have to deplete 1.1 kWh of energy by exceeding an average of 68 to 75 mph, climbing an extended hill, or driving against an insane headwind for a longer amount of time than most of us would consider doing knowing the limitations of the most efficient range extended battery electric vehicle on the planet.

1.1 kWh (6% of 18.8 kWh) is a lot of energy to eat up. On level ground, it can power the i3 for about 5 miles of EPA range.

 

[Zzzoom3]

Are you saying you can't get to 0% SOC with a REx?

No. I'm just saying that it is difficult to do so under normal driving conditions. You would somehow have to deplete it by exceeding and average of 68 to 75 mph, climbing an extended hill, or driving against an insane headwind for a longer amount of time than most of us would consider doing knowing the limitations of the most efficient range extended battery electric vehicle on the planet.

Ok agreed. Then you get into the scenario I was discussing. In any case, when you get there or close to it, wouldn't a 30 minute charge to 80% be highly desirable?

 

[ultraturtle]

Ok agreed. Then you get into the scenario I was discussing. In any case, when you get there or close to it, wouldn't a 30 minute charge to 80% be highly desirable?

While I do not agree that most drivers would be likely to get into the scenario you were discussing (1.1 kWh (6% of 18.8 kWh) is a lot of energy to eat up - on level ground, it can power the i3 for about 5 miles of EPA range), I do a agree that an occasional high power DC charge to 80% might be somewhat desirable as long as you understand that frequently doing so will likely wear out your battery prematurely.

This is not a big deal for the folks that will be leasing the i3 for a few years and moving on, but it is a very big deal for the few of us who see it as the most sustainable personal transportation solution currently available to cover our needs for the next few decades.

 

[KurtEndress]

You might have to change that 1.1 kWh buffer to a 0.86 kWh buffer between 6.5% and 1.9%. According to the BMW University High-Voltage Components training document from http://www.bimmerfest.com/forums/showthread.php?t=787263 The i3 REx Will drive without restrictions down to 1.9% relative battery SOC. Under 1.9% is the "Range in which driving with restricted drive power takes place".

I suppose it's too much to expect full performance down to 0%. It's nice to know where the restriction kicks in.

The document also contains some other interesting figures. The motor is rated to 125 kW for up to 30 seconds and 75 kW continuous. The battery is rated at 147 kW for the short term and "at least 40 kW" continuous. So the REx running isn't a difference between 25 and 125 kW, it is a difference between 25 kW continuous average and 40 to 75 kW continuous.

 

[fdl1409]

As regards to 25 vs. 28 kW, this information is from official BMW documents. Here a link to the thread:

http://www.mybmwi3.com/forum/viewtopic.php?f=13&t=1024

Until March 2014, REx in the price list is rated 25kW at 4.300 rpm.
From April, this has changed to 28 kW at 5.000 rpm.

The same engine in the BMW C 650 GT Scooter is rated 44 kW at 7.500 rpm.

This is not just another definition of power. This engine is able to deliver much more power. Initially, it was reduced to 25 kW. Seems like later they changed their mind and gave us a few more horsepower.

To verify this, anyone out there who can measure rpm?

Frank

 

[jadnashuanh]

There are lots of mountain passes and long grades, especially in the western USA where you may have a 10-mile or longer grade...great on an EV when going down, but hell on the way up. Since you cannot turn the REx on early in North America, you could easily get into a situation where you essentially run out of power on the way up, depending on where you were when you got to that grade, REx or not, regardless of however fast they run the thing.

 

[Zzzoom3]

You might have to change that 1.1 kWh buffer to a 0.86 kWh buffer between 6.5% and 1.9%. According to the BMW University High-Voltage Components training document from http://www.bimmerfest.com/forums/showthread.php?t=787263 The i3 REx Will drive without restrictions down to 1.9% relative battery SOC. Under 1.9% is the "Range in which driving with restricted drive power takes place".

I suppose it's too much to expect full performance down to 0%. It's nice to know where the restriction kicks in.

The document also contains some other interesting figures. The motor is rated to 125 kW for up to 30 seconds and 75 kW continuous. The battery is rated at 147 kW for the short term and "at least 40 kW" continuous. So the REx running isn't a difference between 25 and 125 kW, it is a difference between 25 kW continuous average and 40 to 75 kW continuous.

Really good post! .. thank you

 

[i3Alan]

There are lots of mountain passes and long grades, especially in the western USA where you may have a 10-mile or longer grade...great on an EV when going down, but hell on the way up.

The i3 is sold as a city car, not a cross country touring car or an off-road trekker or a drag strip champ or a mountain pass slayer. Sure sounds like a lot of criticism here for the i3 not being well suited for major mountain passes. It will do great in many situations it was not designed to handle, but no buyer should expect it to do all things automotive well.

 

[ultraturtle]

Until March 2014, REx in the price list is rated 25kW at 4.300 rpm.
From April, this has changed to 28 kW at 5.000 rpm.
...
This is not just another definition of power. This engine is able to deliver much more power. Initially, it was reduced to 25 kW. Seems like later they changed their mind and gave us a few more horsepower.

To verify this, anyone out there who can measure rpm?

Frank

My post was not so much to dispute the claim that BMW upped the horsepower as much as refute the claim that BMW is somehow lying about a 25 kW gen-set producing 28 kW worth of power. Just pointing out that there is a direct link between 25 kW and 34 hp on the BMW USA website (they are equivalent values), but as there is no current official document that specifies that the increased 28 kW is equivalent to 37.5 hp for European i3s, some members of this forum have concocted something of an illogical conspiracy theory attempting to link the originally posted 34 hp output to the current 28 kW output of European spec vehicles. What is missing is both a European document stating that the 28 kW output is in fact 37.5 hp, and any documentation that North America REx output has been increased to to 28 kW / 37.5 hp.

Thanks to KurtEndress giving us a link to some pretty sweet BMW documents, we can gather some insight into the two power levels:

Turns out that with nothing but a software change, BMW can up the RPM to 5000, and increase the power output to 28 kW, but that drops the torque to significantly less than that available at 4300 RPM. While it may look to morons like myself that values in the range of 4500 RPM (max torque) to 4800 (max power) might have been a wiser limiting value, we do not have access to efficiency curves, nor do we have access to the incredibly complex algorithms used to manage actual power production. What it can do, and what it does do may be quite different.

I might also guess that a 4300 RPM / 34 hp / 25 kW limit may well be imposed upon North American i3s in order to maintain its astonishing 39 mpg EPA rating.

I for one do not have any intention of overloading my i3 REx, driving it for 72 miles with no limitations, then flooring it uphill at ridiculous speed for as long as it takes to deplete it's useable SOC. I recognize its limitations, and am willing to deal with them in exchange for the privilege of driving the most well thought out, most earth-friendly and versatile vehicle on the planet.