Trying to understand the BMW i3 REX

I think that my last post about the BMW i3 REX and the possibility of having limited performance in some circumstances may have caused more confusion. My apologies if it did 🙂

I am going to try to explain the way I understand the REX and how it would be used to best effect on the BMW i3. Please everyone… I am not an expert on the BMW i3. All I am doing is taking the PR blurb, combining it with what I have heard from various media reports and also applying what knowledge I have from being a Nissan Leaf and a Vauxhall Ampera owner. Really all I am doing is what anyone else could do to come to some kind of assumption as to what life with a BMW i3 REX might be like. Once people start taking delivery and we see some real-world reports this may all be proven to be poppycock!

So, in my last post I showed a video of me driving my Leaf limiting my power use to 25kw to show in an approximate way what the BMW i3 REX might perform like with a completely flat battery. Of course this is not a comparison at all. The BMW is lighter, has skinnier tyres, has different aerodynamics etc and above all, when it has a REX it can recharge the battery. This is absolutely key to understanding how the REX works and how to use it to maintain full performance once the battery runs down.

I have an analogy that I would like to share that might make it a little clearer. It is quite simplistic but that is the beauty of it… hopefully it makes it easier to understand.

Imagine a car. A petrol car, not an EV. It needs energy to power it and it gets all of its energy it needs from petrol. So you can imagine the petrol tank as just an energy store. We fill up that store with petrol at the pump.

In a 100% EV (no REX) there is a battery and that is the energy store. It is filled up by plugging in and charging.

In the BMW i3 REX you can think of the battery and petrol tank both as simple energy stores and the REX can simply transfer energy from the petrol store to the battery store and top it up.

Now, the motor in the BMW i3 REX is 130kw. That gives it quite stunning performance and when you use that performance it uses energy from the battery. The battery can supply that energy at full speed… i.e. 130kw… and so will drain the battery energy store at that speed when being driven flat out.

Eventually of course the battery will run out of energy and run flat. In a normal 100% battery EV that would be the end of it and the car would stop. But with the BMW i3 REX you have an extra energy store in the form of petrol and the REX drives a generator which basically transfers the energy in the petrol to the battery ready to use and that is the basic of the REX.

The bit that is not so obvious though is that that transfer of energy from the petrol to the battery can only happen at 25kw but the motor is capable of using energy from the battery at 130kw. This has unexpected consequences.

Referring back to my video I showed the approximate performance we might expect from a BMW i3 REX when the battery is totally flat. The REX can deliver 25kw to the battery but the battery is capable of delivering 130kw to the motor. So if the car needs more than 25kw and the battery is flat then the REX can’t replensih the battery as fast as it is being used. The car will then have limited performance.

But what if the car only needs say, 10kw to drive? Well the REX can deliver 25kw to the battery but the battery is only now being drained at 10kw so there is a surplus of energy being transferred from petrol to battery and so the battery is charged. This has enormous implications because this now means that the battery is gaining energy as you drive and providing you don’t drive in a way that drains the battery down to zero you will always have full performance available and remember… the battery is being constantly topped up at 25kw so use less than an average of 25kw and you will not drain the battery.

So, in normal driving say, around town, where power requirements are low, the REX can deliver more than the car needs most of the time, the battery will be recharging all the time and so you will never see a drop in performance and should you need more than 25kw, a fast getaway at the lights for example, then you will have the extra energy you need already in the battery.

The complications come when on a motorway or when driving up a hill. Driving fast requires more energy as does driving up a hill. Often more than 25kw. So all the time you have enough energy in the battery as a buffer it can deliver whatever power the car needs to maintain speed. Let that buffer drop to zero though and you will then only then have the 25kw from the REX and that is when performance may suffer.

I most circumstances I would expect that if you drive the BMW i3 at sensible speeds, say below 70mph (i.e. speed limit!!!) , then the car will nearly always need less than 25kw and the battery can charge giving it that buffer should you need to go up a hill or drive a bit faster to overtake later in the journey. However, drive fast for too long or drive up a very long incline at normal motorway speed and you may find the buffer gets used up and then you are back to having just the 25kw from the REX again and the car will slow down.

The key to use the REX and maintaining performance then is to start running the REX as soon as you know you might need it. That way the battery is kept topped up throughout the full length of your journey and you should maintain full performance. But if like me, you want to use the car as much as possibly in battery only mode & charge up on the way great… until a charger fails. Now you are going to be low on battery and you will need to use the REX to continue your journey and that is the time you might encounter reduced performance.

 I am sure that after owning a BMW i3 REX for a while all this will just become second nature and it is more complicated trying to explain it all in words.

A long explanation (sorry!) but I hope by taking the time to explain all the different steps it makes things a little easier. If not – let me know… please 🙂 

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