Octopus Intelligient & Location

[colin123]

Battery is happier if you charge to full. Cell balancing. The i3 uses ‘passive top end cell balancing’ to burn down the highest charged cell to the next highest cell but only on completion of charging. Stopping at 80% means the cells don’t ever get balanced and capacity is reduced as charging stops as the first cell hits the high voltage limit. Leaving lower voltage cells missing out of additional capacity. As the highest cell charges faster and faster to the top voltage the trailing cells get less and less charge. Not just theoretical but anecdotal. I only partial charged on ‘free’ type 2 when shopping and never topped up at home. After a year the capacity was down to 33miles at 100% (not in winter). Picture attached. After religiously charging to 100% for 3 or 4 months it recovered to 75miles.

Hi, isn't the above the 'holy grail' of all dissagreements over what you should do with an EV battery ? I'm sure earlier models are different but my understanding is that in modern EV's there is a lower and upper buffer we can't view or use, so when we charge to 100% it may in effect only be actual 80% ? I guess a couple of points, where does it catagorically state this for BMW I3's ? Why is it so much slower charging in the 80-100% bracket and why are we always being told to obide the 20-80% rule, is this just older EV's? Be great if we could hear it from the 'horse's mouth' ?

 

[Mondomensch]

Hi, isn't the above the 'holy grail' of all dissagreements over what you should do with an EV battery ? I'm sure earlier models are different but my understanding is that in modern EV's there is a lower and upper buffer we can't view or use, so when we charge to 100% it may in effect only be actual 80% ? I guess a couple of points, where does it catagorically state this for BMW I3's ? Why is it so much slower charging in the 80-100% bracket and why are we always being told to obide the 20-80% rule, is this just older EV's? Be great if we could hear it from the 'horse's mouth' ?

Aren't the above two separate issues?

The slower charging after 80% is just to accommodate battery chemistry - as with filling a box with footballs, it's fast and easy to toss in the first 80% but after that it takes time and effort to fit each additional football in with the others, especially the last few.

The cell balancing issue is that BMW has apparently designed the battery management system to only do cell to cell balancing once the battery overall is charged to 100%. That's a design/engineering choice by BMW, but it's in conflict with the 80% rule of thumb.

My takeaway is that it makes sense to periodically (every month or two??) charge the battery to 100% to let the cell balancing happen.

 

[mojo]

The cell balancing issue is that BMW has apparently designed the battery management system to only do cell to cell balancing once the battery overall is charged to 100%.

There certainly isn't any agreement about that. Some sources (e.g. David Bricknell, author of a book on the subject) apparently gives the point at which cell balancing can happen as somewhere between 80 and 90%.

The real problem, and one that BMW is guilty of for many aspects of the car, is that BMW does not document this for their customers. In many cases we are either having to guess or infer information from observations.

 

[colin123]

There certainly isn't any agreement about that. Some sources (e.g. David Bricknell, author of a book on the subject) apparently gives the point at which cell balancing can happen as somewhere between 80 and 90%.

The real problem, and one that BMW is guilty of for many aspects of the car, is that BMW does not document this for their customers. In many cases we are either having to guess or infer information from observations.

some great replies, but are we not back at square one, if no BMW documentation are we all guessing ??

 

[colin123]

Aren't the above two separate issues?

The slower charging after 80% is just to accommodate battery chemistry - as with filling a box with footballs, it's fast and easy to toss in the first 80% but after that it takes time and effort to fit each additional football in with the others, especially the last few.

The cell balancing issue is that BMW has apparently designed the battery management system to only do cell to cell balancing once the battery overall is charged to 100%. That's a design/engineering choice by BMW, but it's in conflict with the 80% rule of thumb.

My takeaway is that it makes sense to periodically (every month or two??) charge the battery to 100% to let the cell balancing happen.

and thank you for explaining the slower charge between 80-100%, that does sound logical

 

[mojo]

if no BMW documentation are we all guessing ??

Regrettably, yes! Cell balancing is a good example. As far as I'm aware, BMW simply does not refer to it in any of their customer documentation. David Bricknell apparently had access to technical resources within BMW, and he (for example) says that cell balancing may happen at either the module or the pack level - but he doesn't say which. That's significant, because the former is 8 times more effective than the latter. Which is it? All we can do is guess.

Another example: the charging rules that the car uses whenever it is plugged in. We know (from observation) that there are at least two circumstances in which the car will ignore any timed charging schedule that the owner sets, and will simply start charging immediately. But BMW doesn't tell the car's owner anything about that. All we can do is observe what the car actually does, and try to figure it out from there.

 

[colin123]

Regrettably, yes! Cell balancing is a good example. As far as I'm aware, BMW simply does not refer to it in any of their customer documentation. David Bricknell apparently had access to technical resources within BMW, and he (for example) says that cell balancing may happen at either the module or the pack level - but he doesn't say which. That's significant, because the former is 8 times more effective than the latter. Which is it? All we can do is guess.

Another example: the charging rules that the car uses whenever it is plugged in. We know (from observation) that there are at least two circumstances in which the car will ignore any timed charging schedule that the owner sets, and will simply start charging immediately. But BMW doesn't tell the car's owner anything about that. All we can do is observe what the car actually does, and try to figure it out from there.

well that's helpful unhelpfulness mojo, but what can we do ?? I'm going to chase Octopus again, they are very slow to come back to me, I wonder if they withdraw BMW owners from IO if this can't be resolved and put u sback on the more expensive Octopus Go Tariff?

 

[Gonville]

Hi, isn't the above the 'holy grail' of all dissagreements over what you should do with an EV battery ? I'm sure earlier models are different but my understanding is that in modern EV's there is a lower and upper buffer we can't view or use, so when we charge to 100% it may in effect only be actual 80% ? I guess a couple of points, where does it catagorically state this for BMW I3's ? Why is it so much slower charging in the 80-100% bracket and why are we always being told to obide the 20-80% rule, is this just older EV's? Be great if we could hear it from the 'horse's mouth' ?

Have a read of David Bricknell’s authoritative book. Yes the i3 has buffers. Yes the i3 ‘as an older car’ has an innovative thermal management system using refrigerant. Highly efficient at removing heat from the battery pack which is heating due to a full charge. LEAF never had this. LEAF didn’t have buffers. LEAF had a physical stop at 80% at least in earlier models. This isn’t just ny view/opinion there is plenty to show that the i3 has outstanding battery management which other early models skimped on. Also some later models didn’t have top end buffers - as per Model S - so encouraged only for longer journeys and only if you were departing after charging so that the pack didn’t heat soak at the top end. The Tesla does active cell balancing during charging due to the many more cells. However the LFP version due to the different chemistry does recommend going to 100%. So the blanket 20-80% rule is totally misleading unless you know the engineering, the thermal management and also the chemistry. It isn’t a battery university type of answer it’s an engineering question. Attached buffer sizes. Not that the Jaguar iPace has had significant issues. Many being bought back so it’s not later models being perfect.

Slower charging at the top end is due to the electrons being packed in and also heat being generated. That’s physics (and chemistry) but the engineering slows the current to avoid problems. It’s just sensible not to wait around on a rapid to get to 100%. Of course a laptop with a single cell has no need of cell balancing. Even Elon recommends deep discharge of a Tesla to ‘level the pack’ once in a while. (Is in Google somewhere when I searched)

 

[Gonville]

well that's helpful unhelpfulness mojo, but what can we do ?? I'm going to chase Octopus again, they are very slow to come back to me, I wonder if they withdraw BMW owners from IO if this can't be resolved and put u sback on the more expensive Octopus Go Tariff?

This isn’t an octopus problem. It’s BMW server.

 

[Gonville]

Aren't the above two separate issues?

The slower charging after 80% is just to accommodate battery chemistry - as with filling a box with footballs, it's fast and easy to toss in the first 80% but after that it takes time and effort to fit each additional football in with the others, especially the last few.

The cell balancing issue is that BMW has apparently designed the battery management system to only do cell to cell balancing once the battery overall is charged to 100%. That's a design/engineering choice by BMW, but it's in conflict with the 80% rule of thumb.

My takeaway is that it makes sense to periodically (every month or two??) charge the battery to 100% to let the cell balancing happen.

The ‘rule of thumb’ is based on naked cells - single ones at that. (Only useful for thumbs not engineers). The i3 has a top end buffer already. So if you only charge to 80% you are in fact only charging to much less a value of the physical battery. 80% of usable in the 60Ah would be only 14.4kWh out of a total capacity of 21.6. So only charging to 66%? Why would you do that?

It makes sense to always charge it to 100% indicated - I.e. 18.8kWh capacity to allow cell balancing. I have posted what happens if you only charge to 70% you end up with a GOM of 35miles.

 

[Gonville]

Regrettably, yes! Cell balancing is a good example. David Bricknell apparently had access to technical resources within BMW, and he (for example) says that cell balancing may happen at either the module or the pack level - but he doesn't say which. That's significant, because the former is 8 times more effective than the latter. Which is it? All we can do is guess.

As I understand it, from a physical examination of the pack, whilst at the engineering centre in Farnborough the BMW Head of engineering showed us the module control units in the battery. When asked the value of the resistor in each module used for cell balancing he gave an answer - I didn’t record that - but it was very small. The presence of myself and David at that meeting (reference the pack photographs attributed to myself in the book) leads us to strongly infer that the cell balancing is at the module level. I hope so anyway. That meeting was before David wrote the book but you are correct that he hasn’t been more definitive on the whole 85% thing except that we know categorically it occurs after charging is completed.

 

[Gonville]

Hi, isn't the above the 'holy grail' of all dissagreements over what you should do with an EV battery ? I'm sure earlier models are different but my understanding is that in modern EV's there is a lower and upper buffer we can't view or use, so when we charge to 100% it may in effect only be actual 80% ? I guess a couple of points, where does it catagorically state this for BMW I3's ? Be great if we could hear it from the 'horse's mouth' ?

We do have a buffer I think with the BimmerFlow (now eFlow?) gives us these values. Let me see if I have some pictures. I have posted what happens if you don’t charge to full but apparently that is disputed in this forum as being anecdotal. I’ll post it again 35 miles for a 60Ah and not in winter.

The buffers is 11% and 94% so charging, and discharging, from 0-100% should be no worries for anyone.

 

[mojo]

leads us to strongly infer that the cell balancing is at the module level. I hope so anyway.

So do I! It would be a crazy decision (IMO) to try to do it at the pack level. And it's not as if small resistors are so expensive that they could only afford one per 96 cells...

 

[Gonville]

So do I! It would be a crazy decision (IMO) to try to do it at the pack level. And it's not as if small resistors are so expensive that they could only afford one per 96 cells...

It’s the supervisor circuit which dictates where and how many. There are 8 supervisor circuits managing 12 cells each? Did I get the ManMaths ™ right? So one resistor for each which I guess it runs round it’s 12 cells so that the top one gets burned down each charge cycle.

 

[Gonville]

Hi all, Octopus Intelligent has been running great for many months, recently it just has a mind of its own no matter what Octopus or the BMW has set (my charger is just set to dumb mode usually so has no say). So, it will just start charging randomly and not stop until 100% charge. My first email to Octopus they say they have been receiving errors about my car’s location? My app always shows my i3's correct location when parked - any ideas anyone ?

Check you remote history. This is also what Octopus say :-

 

[Gonville]

…but unfortunately BMW made it complicated by including no API command to stop charging.

It’s because it’s so old. Ten years ago no one could think why any sane person when attached to a charge source would ever prematurely stop charging? I mean why? Except if you needed to…..? getting more than 50 miles was the challenge so to deliberately stop charging was….inexplicable. Yes we had a hidden top buffer but we didn’t know that really. That’s why ABC was the mantra.

I note that the newer cars with much bigger batteries and less thermal management such as the iX3, i4 all have that stop functionality and a % limit stop as well. Probably they don’t have hidden top buffers?

See latest from Octopus which matches our understanding I think.

 

[Parky14]

Count me in with the Remote Services issue disabling Time Slot Charging.
BMW said it was an issue affecting many UK BMW customers, but the Connected Drive people simply have no idea how to fix it.
We can only hope there is some work being done in the background to resolve the issue which appears to be BMW server based.

I suggest all those affect write to BWM customer services and raise a complaint with them, maybe copy in some of the EV Bloggers, i3 evangelists. We need to put some pressure on!

 

[mojo]

It’s because it’s so old. Ten years ago no one could think why any sane person when attached to a charge source would ever prematurely stop charging? I mean why?

Even ten years ago, you would think that they would have been able to conceive of the idea of someone charging at home, using a time of use special tariff? Possibly a tariff that allows the energy supplier to dynamically move charge slots around? Even 10 years ago, I don't think that idea would have been radical. But BMW were not forward-thinking enough to include the commands necessary in the command set of their API? To me it seems hard to fathom - perhaps to others it seems obvious that they wouldn't have thought about it.

 

[mojo]

I suggest all those affect write to BWM customer services and raise a complaint with them, maybe copy in some of the EV Bloggers, i3 evangelists. We need to put some pressure on!

Agreed - I haven't yet done so, but I will try to find the time tomorrow...

 

[colin123]

so this is the email address for The Managing Director of BMW UK, I suggest the more emails the merrier: [email protected] with a cc to [email protected]