Custom Rex Retrofit Project

BMW i3 Forum

Help Support BMW i3 Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
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?
I built a 12kw paralell battery for my first i3 BEV using all the modules from a 2019 volvo hybrid. It did extend range but experimental and you had to know what you are doing to connect and disconnect it. the car couldn't estimate its new range, as the SME didn't see the power that was comming from the aux battery. I gave up on the idea after I learned to do the 60ah - 94Ah battery swap upgrade. If I were to try it again I would connect the aux battery inside the main battery so that the SME and safey box sees and controlles all the battery power. I still have the low miles volvo batteries in Baltimore, if anyone is interested in experimenting
I drive an electric Renault Kangoo minivan(factory-prototype build in 500 quantity) from 2002, which I bought in 2011. It had the famous Saft NiCad batteries, which provided about 55 miles of practical range. I changed the old pack for a Lithium pack in 2014, for more than double the range. Of course I could not integrate the lithium pack into the original nicad control and charge electronics. Therefore the car electronics cannot determine state of charge etc. I use a separate BMS and charger so I check SoC with an app. As this car also came with a Rex version (yes, in the early 2000's!), I can see via software that the car is "thinking" that it runs on the Rex forever.
Do you have any guidance for this swap? What resources/procedure did you use?
I am working with as their installer in the USA. Part of our deal is that I don't provide info on how it is done, and of course that would not be in my interest anyway. I have 2 tested 94 Ah battery packs on hand, that I need to sell. I can install one in your car, coded and guaranteed for a year (it should last the life of the car) for $7k and your old battery. Can do it in 1 day. You can check for junk yards selling HV batteries. I can install a batterie you have, but fixing one that was shut down in an accident will take extra time and money.