Custom Rex Retrofit Project

[ProtoThad]

I recently bought an older i3 BEV as my around-town errand car. Being a 2014 with a well used 60Ah battery, its range isn't great, but well within my needs. Nevertheless, as an embedded electronics nerd, I thought it might be fun to try and boost the range by hacking up a custom range extender... basically a secondary battery pack that plugs in and pretends to be a real rex.

Yes, I realize there are better / easier ways to get more range (including just trading up to a newer model)... but where is the fun or challenge in that?

I'll probably start with a simple proof of concept providing a relatively limited range boost, but with the potential for improvement as better battery options become available. I just need to know two things to get started:

1) What resources are available regarding the technical details of the i3, particularly regarding the ECUs, charge controllers, etc.

2) Do the BEV vs Rex versions use the same primary ECU hardware and software, just with the Rex features enabled / disabled as appropriate? Or will I need to swap out some components before adding my fake Rex?

My current idea is to harvest the electrical components from a dead Rex, combining them with a battery array that duplicates the DC output of the original Rex generator/transformer with some added electronics to spoof the fuel level signal (thus ensuring proper range calculation).

Any thoughts?

 

[mojo]

To start with, if you just need an overview of how the many control modules in an i3 are connected together, BMW USA published a document which describes this in a good level of detail. Nowhere near enough detail to build a "REx simulator" but enough to see what you're dealing with.

Essentially there are the following main control modules that you would need to deal with - for the electric motor (EME, EDME), for the range extender (REME, RDME, TFE), for battery management (SME) and for charge management (KLE) - plus, of course, the main Body Domain Controller. You can assume that these control modules interact in a way that is proprietary to BMW, and the interactions probably aren't published anywhere outside BMW (because they don't need to be).

The point of the REx is that petrol is a very dense power source - you can pack a lot of energy in a small space, and you can replace it easily.

The problem with trying to replace the REx with a second battery is that you would need a physically big battery to make the effort worthwhile - but you don't have loads of space in which to put a big battery - unless of course you are willing to sacrifice loadspace and passenger space.

Also... don't forget that the REx incorporates no concept of "recharging", so any mechanism to recharge the batteries in the "fuel tank" would need a complete, separate BMS and physical charging connection.

TLR: Way, way too complex for the benefit you would gain. Sorry!

 

[BobDowser]

All of the above plus you're going to sacrifice acceleration, have top-heavy handling with the extra battery weight higher than the current ones, and the front-rear balance would suffer. You might also have more of a "boat" feel to the steering with more weight stuck at the front or rear instead of centrally.

 

[alohart]

All of the above plus you're going to sacrifice acceleration, have top-heavy handling with the extra battery weight higher than the current ones, and the front-rear balance would suffer. You might also have more of a "boat" feel to the steering with more weight stuck at the front or rear instead of centrally.

This would depend on the weight and location of the auxiliary battery pack. As long as it weighs less than the REx system weight, ~300 lb., acceleration would be no worse than an i3 REx which is slightly less than an i3 BEV due this additional weight.

It would be interesting to mount an auxiliary battery pack below the cargo area floor where the REx system is mounted. That would keep the center of gravity as low as that of the i3 REx with the additional bonus of no reduction in cargo volume and a front-rear balance similar to the i3 REx.

Integrating an auxiliary battery pack into an i3 BEV would be a significant challenge, however.

 

[alohart]

Please check your private messages.

 

[ProtoThad]

It would be interesting to mount an auxiliary battery pack below the cargo area floor where the REx system is mounted. That would keep the center of gravity as low as that of the i3 REx with the additional bonus of no reduction in cargo volume and a front-rear balance similar to the i3 REx.

This is what I was thinking... reuse the existing REX location, as I understand that is basically unused space in a BEV. I was initially thinking of limiting my battery pack to the weight difference between the 60Ah and 120Ah battery pack, which is 43kg. That would be a rather limited range boost using older reclaimed LiOn batteries (which I probably will use for the prototype)... but solid state batteries with up to 400wh/kg sweetens the deal. That get's us to 17kWh of extra juice.. more than doubling the capacity of my existing battery.

But If we can fit 100kg of solid state batteries in that space, that would give us a 40kWh of extra juice, which is more than a stock 120Ah battery. That, plus the original 60Ah battery... and our weight is still less than the 120Ah+Rex. And all this without even considering what we can get if we completely swap out the original battery with a custom build using solid state. My back of the envelope calculations says you could make an i3 with better than 400 mile range.

And before anyone jumps in with additional reminders of how complicated and impractical this would all be... I will remind you that I'm a nerd who is doing this for the fun of it. Practicality has no place in this conversation.

Thanks for all the great input!

Thad

 

[agzand]

It makes more sense to retrofit a larger battery, a used 94ah or 120 ah pack should be pretty easy to source.

 

[ProtoThad]

It makes more sense to retrofit a larger battery, a used 94ah or 120 ah pack should be pretty easy to source.

The challenge in that is you need the equipment to lift the chassis and drop the battery, not to mentioned what is needed to drain and refill the thermal management system (expensive). And again... if I just wanted a bigger battery, I could have bought one of the other reasonably priced i3's that I looked at. I instead picked this one up cheap specifically because I wanted a project car. I probably will do a battery swap out at some point, but it will be a custom battery pack when I do it.

Thad

 

[mojo]

You might already be aware of the product, but if not, the German company Wokeby makes a storage unit that fits in the space where the REx would otherwise be in a battery-only car. A look at that will give you an idea of the area that you have to work in. It's not a particularly convenient shape for a battery array, because it's probably going to force you more into a vertical stack than you would like.

https://wokeby.de/en/product/bmw-i3-trunk-extension/

And yes, the Wokeby product is a good idea - shame about the price!!

 

[alohart]

You might already be aware of the product, but if not, the German company Wokeby makes a storage unit that fits in the space where the REx would otherwise be in a battery-only car. A look at that will give you an idea of the area that you have to work in. It's not a particularly convenient shape for a battery array, because it's probably going to force you more into a vertical stack than you would like.

https://wokeby.de/en/product/bmw-i3-trunk-extension/

And yes, the Wokeby product is a good idea - shame about the price!!

This is a great suggestion for estimating the shape and size of the space available where the REx system would be.

I bought this trunk extension and a frunk sealing from Wokeby. Yes, the price is premium, but so is the design, quality, fit and finish, etc. Shipping half way around the world to Honolulu was only ~60€ which is amazing considering the large volume of the shipping box.

 

[alohart]

But If we can fit 100kg of solid state batteries in that space, that would give us a 40kWh of extra juice, which is more than a stock 120Ah battery. That, plus the original 60Ah battery... and our weight is still less than the 120Ah+Rex. And all this without even considering what we can get if we completely swap out the original battery with a custom build using solid state. My back of the envelope calculations says you could make an i3 with better than 400 mile range.

Adding that much weight would probably require installing REx springs. This video includes a description of the suspension parts for all i3 models:

https://www.youtube.com/watch?v=MG6XmK95HxY

 

[ProtoThad]

Adding that much weight would probably require installing REx springs. This video includes a description of the suspension parts for all i3 models:

A good point. I suspected as much, which is why I intend to keep my initial proof of concept relative light / low range. I am still intrigued by the possibility of a custom solid state battery swap-out with zero or minimal weight increase... but that of course can't happen until solid state batteries start hitting the secondary market. They are only just making it into production cars at this point.

Thad

 

[Brooklyn tech geek]

My back of the envelope calculations says you could make an i3 with better than 400 mile range.

And before anyone jumps in with additional reminders of how complicated and impractical this would all be... I will remind you that I'm a nerd who is doing this for the fun of it. Practicality has no place in this conversation.

Thanks for all the great input!

Thad

Have you seen this? I Bought a Ten Year Old i3 REX for $3500 EV and I Increased its Range To Over 400 Miles!

Sounds like what your looking for. Rich Rebuilds shares your optimistic view on practicality by doing exactly what you hypothesized.

I want to do the same. While he shows how to, I have no idea on the extra cost to do so and who would be able to do it for me since I'm not a mechanic.

If you have an idea of cost of parts and more "how to" based on the vid, would appreciate your input.

1. Battery that he used cost
2. Other accessories cost + approx time to charge the new battery (was thinking about 25 -35hrs level 1 wall charger).




Hope it helps.

 

[Simoni3]

I3 with x amount of petrol in the frunk,yikes !!!!!

 

[REvans2001]

I3 with x amount of petrol in the frunk,yikes !!!!!

You saved me the time watching to the end! Thank you.

 

[Deepsouth]

More info about this:
https://www.theautopian.com/why-bmw...-dangerous-gas-tanks-into-their-front-trunks/

 

[JohnKelly]

In my opinion, the easiest way to do this is get one or two 5.12 kWh batteries weighing 100lbs each (strap them down) wired in parallel, and a simple 110v inverter. Then use your level 1 charger when you park somewhere... for lunch or whatever... cumbersome but doable and only weighs what an extra passenger might. No vehicle integration headaches... adding an extra 40 miles of range or so.... or get a level two 24 amp charger and a larger inverter for faster speeds. Total extra weight... well under 300 lbs.

 

[ksnax]

In my opinion, the easiest way to do this is get one or two 5.12 kWh batteries weighing 100lbs each (strap them down) wired in parallel, and a simple 110v inverter. Then use your level 1 charger when you park somewhere... for lunch or whatever... cumbersome but doable and only weighs what an extra passenger might. No vehicle integration headaches... adding an extra 40 miles of range or so.... or get a level two 24 amp charger and a larger inverter for faster speeds. Total extra weight... well under 300 lbs.

That would be a long wait though at 110V. Sadly, there is no way to carry extra fuel with a REx that is truly safe in any case. If it needs to be done at all, the frunk is still probably the best place for it, being outside of the passenger compartment.

The up side is that few people are unable to walk away from a crash in these cars. It's still a big deal if there are kids in car seats to extract, but most adults would be able to escape the flames if ignited.

 

[Rossifer]

A good point. I suspected as much, which is why I intend to keep my initial proof of concept relative light / low range. I am still intrigued by the possibility of a custom solid state battery swap-out with zero or minimal weight increase... but that of course can't happen until solid state batteries start hitting the secondary market. They are only just making it into production cars at this point.

Thad

Thad, which production cars have a solid state battery? I am unaware that these batteries are large enough for cars, and that they've figured out how to make them rechargeable.

 

[JohnKelly]

That would be a long wait though at 110V. Sadly, there is no way to carry extra fuel with a REx that is truly safe in any case. If it needs to be done at all, the frunk is still probably the best place for it, being outside of the passenger compartment.

The up side is that few people are unable to walk away from a crash in these cars. It's still a big deal if there are kids in car seats to extract, but most adults would be able to escape the flames if ignited.

Which is why I mentioned a portable 24 amp 240V charger.. They are smaller than the 32 amp versions by a lot and much faster than level 1. By the way, fuel cells are very safe when installed properly. But if my Rex ever fails and is too expensive to fix, I will go with 10+kWh of server rack batteries which can be used for something else long after the car is unrepairable from a financial point of view. Charging while stationary for any length of time is fine for some people. Will not work for everyone, but it will certainly work for me.