Question on coding Rex to turn on below 75% battery level.

[Kash Bagal]

Although I have owned and driven i3’s for seven years, I have never understood why the need to activate the REX before the battery is low. I thought the point of having Rex was to ease range anxiety

Interesting point. Let me see if I can provide an explanation - though some members with more knowledge can perhaps provide additional information.
When this car was developed from 2007 - 2013 in Germany, there were very few charging stations. This car worked well within the city if it was charged each evening and would only be used in the city as a commuter car. However as soon as you would want to travel from city to city or for overnight trips, they saw the need for a Range Extender. In Europe, the Range Extender was able to be started at any percentage below 75%. And it came with a 9L (1.84 gal) gas tank.
As I understand it, when BMW decided to sell it in the US (I am in Canada so this info is based on my readings) the US government had promotions with grants for electric cars at the time. In order to qualify as a pure EV, they said that they would not approve it if the gas tank was over a certain size and if the gas tank could be used regularly or on demand. Thus in order to sell the car in the US and qualify as an EV along with the corresponding grants for buyers, BMW was forced to limit the size of the gas tank to 7L (1.84 gal) and the Rex could only come on at 6% of battery life. So the first thing that US and Canadian buyers would do was override the software limited settings to increase the gas tank to 9L and be able to turn on the Range Extender at any point below 75%.
Why is this important? Because I can tell you from experience, if you are not close to a charger, it is very unnerving if you only have 6% battery life left. If you happen to be on a highway and are going 120km/hr (75 miles/hr), the Range Extender cannot keep the battery charged and you start losing some of the capacity and soon you will be at 5%, then 4%, then 3% and since the car is an EV, even if you have gas, the car will stop at 0% battery life.
We have been driving the i3 for 6 years and we have been educating people on EVs since we got our 2 i3s. When asked about range anxiety on long trips we usually ask them how many long trips they take a year. When they say 3 or 4, we tell them that they will save so much money with EVs in a year that for those long trips, if they are still concerned, they can rent a gas car for those trips, and they will still come out ahead at the end of the year.
So for the 3 or 4 long trips we do each year, I use that same thinking with our i3 Rex, and instead of renting gas cars, we simply use it a gas car. I turn on the Rex at about 74%, and just use gas from that point onwards. It saves time since it only takes 2 mins to fill up the 9L gas tank every 100km (60 miles) or so and you are back on the road.
On a recent trip from Ottawa to Hamilton (500km - 300 miles), I turned on the Rex at about 74% and filled up every 100km or so, and since I was going about 120km/hr (75 miles/hr) by the time I got to Hamilton, I was down to about 60% charge left. I cannot imaging waiting to go down to 6% before the Rex came on - I would have to reduce my speed to about 90km/hr (55 miles/hr) in order for the Rex to maintain the charge. I am sure I would be getting looks from other drivers and it would have taken much longer to get to my destination. And if we had decided to use the car only as an EV, which we have done many times in the past, we would have had to plan the trip to visit multiple charging stations since the i3 battery is now getting lower, and at each stop we would have to hope that the charger is working and/or there is not a long line up making up for a long stop - both conditions which we have experienced in the past making for an even longer trip.
I am not saying that this is the best or only method but it is how we use the Rex, and there are many others who also use this method. We do not use the Rex in the city at all.
Hope that provides some explanation.

 

[GregM]

Excellent explanation! Now it all makes sense to me. Thank you so much for setting me straight. Now I just need to get my car coded. Thank you Kash!

 

[Kash Bagal]

Excellent explanation! Now it all makes sense to me. Thank you so much for setting me straight. Now I just need to get my car coded. Thank you Kash!

You're most welcome. Coding is easy. If I may suggest . . . You will need an OBD reader (very cheap). I bought this one in 2018 from Amazon and it still works well: "VEEPEAK OBDCheck BLE OBD2 Bluetooth Scanner Car OBD II Diagnostic Scan Tool for iOS and Android Check Engine Light Code Reader Supports Torque, OBD Fusion App". There are many others.
And to communicate with you via your Smartphone you will need an app. The one most recommended is BimmerCode.
With these two items you can code not just the increase in Gas Tank, and the Hold State of Charge (which you cannot see currently in the My Vehicle area) but many other settings that i3 users change.
It is very interesting and fun to experiment with the many customizable items. If you want to see what others do, there are many YouTube videos on items that others have changed to suit their needs.
Good Luck.

 

[Whereitis]

Interesting point. Let me see if I can provide an explanation - though some members with more knowledge can perhaps provide additional information.
When this car was developed from 2007 - 2013 in Germany, there were very few charging stations. This car worked well within the city if it was charged each evening and would only be used in the city as a commuter car. However as soon as you would want to travel from city to city or for overnight trips, they saw the need for a Range Extender. In Europe, the Range Extender was able to be started at any percentage below 75%. And it came with a 9L (1.84 gal) gas tank.
As I understand it, when BMW decided to sell it in the US (I am in Canada so this info is based on my readings) the US government had promotions with grants for electric cars at the time. In order to qualify as a pure EV, they said that they would not approve it if the gas tank was over a certain size and if the gas tank could be used regularly or on demand. Thus in order to sell the car in the US and qualify as an EV along with the corresponding grants for buyers, BMW was forced to limit the size of the gas tank to 7L (1.84 gal) and the Rex could only come on at 6% of battery life. So the first thing that US and Canadian buyers would do was override the software limited settings to increase the gas tank to 9L and be able to turn on the Range Extender at any point below 75%.
Why is this important? Because I can tell you from experience, if you are not close to a charger, it is very unnerving if you only have 6% battery life left. If you happen to be on a highway and are going 120km/hr (75 miles/hr), the Range Extender cannot keep the battery charged and you start losing some of the capacity and soon you will be at 5%, then 4%, then 3% and since the car is an EV, even if you have gas, the car will stop at 0% battery life.
We have been driving the i3 for 6 years and we have been educating people on EVs since we got our 2 i3s. When asked about range anxiety on long trips we usually ask them how many long trips they take a year. When they say 3 or 4, we tell them that they will save so much money with EVs in a year that for those long trips, if they are still concerned, they can rent a gas car for those trips, and they will still come out ahead at the end of the year.
So for the 3 or 4 long trips we do each year, I use that same thinking with our i3 Rex, and instead of renting gas cars, we simply use it a gas car. I turn on the Rex at about 74%, and just use gas from that point onwards. It saves time since it only takes 2 mins to fill up the 9L gas tank every 100km (60 miles) or so and you are back on the road.
On a recent trip from Ottawa to Hamilton (500km - 300 miles), I turned on the Rex at about 74% and filled up every 100km or so, and since I was going about 120km/hr (75 miles/hr) by the time I got to Hamilton, I was down to about 60% charge left. I cannot imaging waiting to go down to 6% before the Rex came on - I would have to reduce my speed to about 90km/hr (55 miles/hr) in order for the Rex to maintain the charge. I am sure I would be getting looks from other drivers and it would have taken much longer to get to my destination. And if we had decided to use the car only as an EV, which we have done many times in the past, we would have had to plan the trip to visit multiple charging stations since the i3 battery is now getting lower, and at each stop we would have to hope that the charger is working and/or there is not a long line up making up for a long stop - both conditions which we have experienced in the past making for an even longer trip.
I am not saying that this is the best or only method but it is how we use the Rex, and there are many others who also use this method. We do not use the Rex in the city at all.
Hope that provides some explanation.

Your explanation makes a lot of sense, but I have a question. When I first got my i3 (before I found out a previous owner had already set up the HSOC function), I drove it far enough several times to run the battery down to zero, and the Rex seemed to work fine with zero charge showing even on the highway (more than once going through multiple tanks of gas with the battery at zero the entire rest of the trip). Now that I know about the HSOC, I activate it at 75% every time I think I'll be driving further than the battery alone will take me, using the Rex as much as I can and keeping as much battery power as possible in reserve. But you said the car won't run on the Rex if the battery has gone to zero. How can that be true when I found it still runs fine?

 

[tob1as]

Excellent explanation! Now it all makes sense to me. Thank you so much for setting me straight. Now I just need to get my car coded. Thank you Kash!

It’s really easy to code yourself with an OBDII dongle and the BimmerCode app. There are also a number of other customization options which are fun. You can customize heated seat temperature settings, set to always start up in Eco Mode, chose from mini cooper and rolls Royce warning sounds, chose a different startup screen (Alpina in my case) …
https://bimmercode.app/cars/bmw/i3/i01/
I have no affiliation, other than having used it for the entire 4 years I have had my i3.

 

[Kash Bagal]

Your explanation makes a lot of sense, but I have a question. When I first got my i3 (before I found out a previous owner had already set up the HSOC function), I drove it far enough several times to run the battery down to zero, and the Rex seemed to work fine with zero charge showing even on the highway (more than once going through multiple tanks of gas with the battery at zero the entire rest of the trip). Now that I know about the HSOC, I activate it at 75% every time I think I'll be driving further than the battery alone will take me, using the Rex as much as I can and keeping as much battery power as possible in reserve. But you said the car won't run on the Rex if the battery has gone to zero. How can that be true when I found it still runs fine?

Wow, You are brave running the battery to zero. I am no expert at all, but the only thing that I think of is that zero is not zero. Just as in an ICE car, when the gas tank registers empty, there is a small reserve; I believe that BMW has left a small reserve charge in the battery. And if you use the Rex and keep your speed down so that the Rex can sustain the charge you will probably be OK. Have you kept up your speed to say 75 miles/hr with the battery registering zero for any length of time? I know that the Rex cannot maintain a certain charge at that speed. I think I read about the battery reserve somewhere but can't remember where.
The reason I said that the car will not run when the battery is zero is because the i3 is truly an EV (and not a Hybrid). The 650cc motorcycle engine cannot possibly run the car at any substantial speed and in fact the car does not have a drivetrain that connects the Rex engine to the wheels.
So I wouldn't rely on the fact that you can run the car at zero battery life for any substantial length of time.
Now on the other hand, I have run the gas tank dry and the car simply keeps running. The only thing with this is that the charge now gets used up instead of being maintained when the Rex was running.
I hope that makes sense, but please someone correct me if I am wrong.
Thank you.

 

[Whereitis]

Wow, I you are brave running the battery to zero. I am no expert at all, but the only thing that I can say is that zero is not zero. Just as in an ICE car, when the gas tank registers empty, there is a small reserve, I believe that BMW has left a small reserve charge in the battery. And if you use the Rex and keep your speed down so that the Rex can sustain the charge you will probably be OK. Have you kept up your speed to say 75 miles/hr with the battery registering zero for any length of time? I know that the Rex cannot maintain a certain charge at that speed. I think I read about the battery reserve somewhere but can't remember where.
The reason I said that the car will not run when the battery is zero is because the i3 is truly an EV (and not a Hybrid). The 650cc motorcycle engine cannot possibly run the car at any substantial speed and in fact the car does not have a drivetrain that connects the Rex engine to the wheels.
So I wouldn't rely on the fact that you can run the car at zero battery life.
Now on the other hand, I have run the gas tank dry and the car simply keeps running. The only thing with this is that the charge now gets used up instead of being maintained when the Rex was running.
I hope that makes sense, but please someone correct me if I am wrong.
Thank you.

Thanks for engaging. To clarify, I did not choose to run the Rex. I just thought that's how this car works. I was not aware of the HSOC feature when I bought the i3 (I learned about it later) and on a day I needed to go 65 miles to get where I was going, I simply drove my car until the battery ran out, then continued to drive it until I got there. When I got back in the car to drive home, it wasn't charging (since my charger was at home), so I simply put it in gear (with battery showing zero) and drove home, stopping for gas when the remaining Rex range dropped under 10. The battery charge remained at zero the whole time. I never slowed down on the highway, maintaining 75 to 80 mph all the time. So clearly, the Rex Is capable of powering the car at highway speeds. My question is still about your stating the Rex won't run the car when the battery is at zero. That's not my experience, so why would it be yours?

 

[Kash Bagal]

Thanks for engaging. To clarify, I did not choose to run the Rex. I just thought that's how this car works. I was not aware of the HSOC feature when I bought the i3 (I learned about it later) and on a day I needed to go 65 miles to get where I was going, I simply drove my car until the battery ran out, then continued to drive it until I got there. When I got back in the car to drive home, it wasn't charging (since my charger was at home), so I simply put it in gear (with battery showing zero) and drove home, stopping for gas when the remaining Rex range dropped under 10. The battery charge remained at zero the whole time. I never slowed down on the highway, maintaining 75 to 80 mph all the time. So clearly, the Rex Is capable of powering the car at highway speeds. My question is still about your stating the Rex won't run the car when the battery is at zero. That's not my experience, so why would it be yours?

This is really interesting. It is not my experience at all - meaning I have never let the charge go to zero. I am just stating that an EV cannot run when the battery is at zero just as an ICE car cannot run without gas. This runs counter to any logic. I cannot provide a rational answer to you. Perhaps someone with much more knowledge can provide an explanation.

 

[tob1as]

This is really interesting. It is not my experience at all - meaning I have never let the charge go to zero. I am just stating that an EV cannot run when the battery is at zero just as an ICE car cannot run without gas. This runs counter to any logic. I cannot provide a rational answer to you. Perhaps someone with much more knowledge can provide an explanation.

I concur. I don’t have a Rex, but here in Colorado people have proven that when you head uphill, or drive in inclement weather, the battery will continue to lose charge even with the Rex running at full tilt. So @Whereitis may have been driving with a tail wind and/or on a slight downhill and it might have worked in that context? Environmental conditions will have a huge impact.

 

[alohart]

Your explanation makes a lot of sense, but I have a question. When I first got my i3 (before I found out a previous owner had already set up the HSOC function), I drove it far enough several times to run the battery down to zero, and the Rex seemed to work fine with zero charge showing even on the highway (more than once going through multiple tanks of gas with the battery at zero the entire rest of the trip). Now that I know about the HSOC, I activate it at 75% every time I think I'll be driving further than the battery alone will take me, using the Rex as much as I can and keeping as much battery power as possible in reserve. But you said the car won't run on the Rex if the battery has gone to zero. How can that be true when I found it still runs fine?

The displayed charge level is the BMS' best estimate; it can't be measured directly. Your BMS must have been underestimating the charge level if you were able to drive at highway speeds with a 0% displayed charge level.

When the BMS detects the voltage of a cell approaching its lower voltage limit, it begins decreasing the maximum output power and climate control power consumption to increase the remaining range. An i3 driver can see when this is happening by noticing the gray power segments on the right half of the power gauge begin to disappear starting from the top, so it's important to pay attention to the power gauge when the charge level is very low. At that point, if a REx driver doesn't slow down, more power is being consumed than the REx generator is producing which would result in the maximum output power continuing to be reduced. When this happens, REx drivers have reported being unable to keep up with traffic which could be dangerous. The REx generator output power would allow a REx driver to continue moving unlike a BEV driver whose speed would continue to decrease until the car stops moving.

The 6.5% charge level when the REx engine automatically starts is twice the remaining energy for a 120 Ah i3 compared with a 60 Ah model, so 120 Ah i3 drivers wouldn't experience power reduction when driving at higher speeds as often as 60 Ah i3 drivers. The attached chart shows what happens to a BEV (left) and REx (right) as the charge level decreases. Notice that the absolute charge level is shown along the static capacity test lines whereas the displayed charge level is shown along the usable capacity line.

 

[Kash Bagal]

I concur. I don’t have a Rex, but here in Colorado people have proven that when you head uphill, or drive in inclement weather, the battery will continue to lose charge even with the Rex running at full tilt. So @Whereitis may have been driving with a tail wind and/or on a slight downhill and it might have worked in that context? Environmental conditions will have a huge impact.

Yes I agree with you. The unique condition with @Whereitis is that the car went to zero on the outbound leg and it was still zero when he used the car on his return home so the slight downhill cannot be applied as it cannot be such on both legs. I really don't understand it as even with a small reserve, it would have eventually been expended as @Whereitis states he was going 70 to 80 mph on the highway which in my mind is too fast for the Rex to maintain the State of Charge.
I haven't read the comment by @alohart yet but it may provide the necessary explanation.

 

[Whereitis]

This is really interesting. It is not my experience at all - meaning I have never let the charge go to zero. I am just stating that an EV cannot run when the battery is at zero just as an ICE car cannot run without gas. This runs counter to any logic. I cannot provide a rational answer to you. Perhaps someone with much more knowledge can provide an explanation.

I appreciate your recounting your experience. I may never understand it though, since I don't see how starting a 60-mile drive with the battery already at zero can be anything but a Rex-only drive. Since I've been using HSOC, I've noticed when running on Rex with HSOC engaged at high speeds with some battery juice left, the battery will lose a little power when really hammering it so it obviously is affected, but the car still maintained highway speeds with no discernable degradation for over an hour even when starting out with zero charge. Got no clue why if you're correct about the Rex not being able to power the car with no battery assistance, just that it's never left me stranded (yet).
.

 

[Kash Bagal]

The displayed charge level is the BMS' best estimate; it can't be measured directly. Your BMS must have been underestimating the charge level if you were able to drive at highway speeds with a 0% displayed charge level.

When the BMS detects the voltage of a cell approaching its lower voltage limit, it begins decreasing the maximum output power and climate control power consumption to increase the remaining range. An i3 driver can see when this is happening by noticing the gray power segments on the right half of the power gauge begin to disappear starting from the top, so it's important to pay attention to the power gauge when the charge level is very low. At that point, if a REx driver doesn't slow down, more power is being consumed than the REx generator is producing which would result in the maximum output power continuing to be reduced. When this happens, REx drivers have reported being unable to keep up with traffic which could be dangerous. The REx generator output power would allow a REx driver to continue moving unlike a BEV driver whose speed would continue to decrease until the car stops moving.

The 6.5% charge level when the REx engine automatically starts is twice the remaining energy for a 120 Ah i3 compared with a 60 Ah model, so 120 Ah i3 drivers wouldn't experience power reduction when driving at higher speeds as often as 60 Ah i3 drivers. The attached chart shows what happens to a BEV (left) and REx (right) as the charge level decreases. Notice that the absolute charge level is shown along the static capacity test lines whereas the displayed charge level is shown along the usable capacity line.

View attachment 1394

Thank you for this explanation!

 

[Whereitis]

The displayed charge level is the BMS' best estimate; it can't be measured directly. Your BMS must have been underestimating the charge level if you were able to drive at highway speeds with a 0% displayed charge level.

When the BMS detects the voltage of a cell approaching its lower voltage limit, it begins decreasing the maximum output power and climate control power consumption to increase the remaining range. An i3 driver can see when this is happening by noticing the gray power segments on the right half of the power gauge begin to disappear starting from the top, so it's important to pay attention to the power gauge when the charge level is very low. At that point, if a REx driver doesn't slow down, more power is being consumed than the REx generator is producing which would result in the maximum output power continuing to be reduced. When this happens, REx drivers have reported being unable to keep up with traffic which could be dangerous. The REx generator output power would allow a REx driver to continue moving unlike a BEV driver whose speed would continue to decrease until the car stops moving.

The 6.5% charge level when the REx engine automatically starts is twice the remaining energy for a 120 Ah i3 compared with a 60 Ah model, so 120 Ah i3 drivers wouldn't experience power reduction when driving at higher speeds as often as 60 Ah i3 drivers. The attached chart shows what happens to a BEV (left) and REx (right) as the charge level decreases. Notice that the absolute charge level is shown along the static capacity test lines whereas the displayed charge level is shown along the usable capacity line.

View attachment 1394

I think I understand your chart and explanation, but my personal experience is quite different, as I never felt the car losing power and never had any problem keeping up with or passing slower traffic even after an hour on the road or longer on Rex with zero charge showing at the outset. Just my experience, please don't expect me to know why.

 

[sibumi]

Your explanation makes a lot of sense, but I have a question. When I first got my i3 (before I found out a previous owner had already set up the HSOC function), I drove it far enough several times to run the battery down to zero, and the Rex seemed to work fine with zero charge showing even on the highway (more than once going through multiple tanks of gas with the battery at zero the entire rest of the trip). Now that I know about the HSOC, I activate it at 75% every time I think I'll be driving further than the battery alone will take me, using the Rex as much as I can and keeping as much battery power as possible in reserve. But you said the car won't run on the Rex if the battery has gone to zero. How can that be true when I found it still runs fine?

Sorry, can you explain to me what is the HSOC function? Thank you

 

[mojo]

Sorry, can you explain to me what is the HSOC function? Thank you

If you read this thread, it explains what HSOC is:

https://www.mybmwi3.com/threads/question-on-coding-rex-to-turn-on-below-75-battery-level.18971/

 

[Whereitis]

Sorry, can you explain to me what is the HSOC function? Thank you

HSOC is "Hold State Of Charge" which is accessible by using your iDrive. Press the big Menu button, scroll down to Settings, press down on iDrive, scroll to Range Extender, press down on iDrive to see that submenu, and if it's enabled, you'll see a checkbox for HSOC. Even if enabled, it's grayed out unless your HV battery has less than 75% charge (current charge level will be displayed on that screen). Activate HSOC and your Rex will turn on immediately and from then on you'll be using gas instead of battery. It resets every time you park, so you have to re-enable it every time you start out.

 

[alohart]

Got no clue why if you're correct about the Rex not being able to power the car with no battery assistance, just that it's never left me stranded (yet).

Both the battery pack and the REx generator send power to the HV circuits. The propulsion motor doesn't know or care where its input power originated. When the BMS prevents the battery pack from providing power when its displayed charge level is 0%, the REx generator will continue providing power. However, the maximum output power of the original REx system was 23.3 kW at 4300 rpm and 330 VDC. This might have been increased a bit in later years. The continuous output power of the 120 Ah battery pack is 75 kW with a peak power of 135 kW for 30 seconds. The maximum REx power output is no more than ⅓ of the battery pack's output, so an i3 cannot drive as fast using electrical power produced only by the REx generator as it can using electrical power from battery pack.

My interpretation of the REx engine not being able to power an i3 without battery pack power is that it cannot propel the rear wheels mechanically.

 

[Whereitis]

Both the battery pack and the REx generator send power to the HV circuits. The propulsion motor doesn't know or care where its input power originated. When the BMS prevents the battery pack from providing power when its displayed charge level is 0%, the REx generator will continue providing power. However, the maximum output power of the original REx system was 23.3 kW at 4300 rpm and 330 VDC. This might have been increased a bit in later years. The continuous output power of the 120 Ah battery pack is 75 kW with a peak power of 135 kW for 30 seconds. The maximum REx power output is no more than ⅓ of the battery pack's output, so an i3 cannot drive as fast using electrical power produced only by the REx generator as it can using electrical power from battery pack.

My interpretation of the REx engine not being able to power an i3 without battery pack power is that it cannot propel the rear wheels mechanically.

That all makes sense, especially the part about the energy output of the battery vs. the Rex. I can only recount my experience when I say the car seems just as quick (or if not, still plenty quick) on the highway running on the Rex with the battery showing zero for many miles.

 

[Dutchi3]

Great and fascinating chart, thanks. If you consider that one liter (0.22 gallon) of gasoline contains 10kWh of energy (of which 70% is wasted as heat in an ICE) it is really fascinating what we are dealing with here.