i3 Battery charging - Degradation real life experience

[alohart]

If I right-click on the "image" and then select "Open image in new tab", then I get to see the image.
Does that work for you(s)?

No, because these images are in YOUR Google account so YOU can see them. They do not appear to be set up for sharing with anyone else. I cannot "open the image in a new tab", all I get is a 404 error.

When I right-click on the image, the response is a Google login page. For those without a Google account or who would prefer not to log in to Google, the photos would not be viewable. However, after I logged in, the response was the same 404 error which is probably due to no sharing as you suggest.

 

[user 10276]

Does anyone know what the i3 60ah kappa max reading would be from new? Is it 18.8 kWh or 22?

 

[EvanstonI3]

Does anyone know what the i3 60ah kappa max reading would be from new? Is it 18.8 kWh or 22?

2 separate owners who have had their battery packs replaced under warranty have reported that the reading was 19.6kWh after the warranty replacement of an entire pack.
The 18.8 number is a "de-rated" number provided by BMW for warranty purposes so the 70% warranty is based on that number, not 19.6 or 22

 

[user 10276]

2 separate owners who have had their battery packs replaced under warranty have reported that the reading was 19.6kWh after the warranty replacement of an entire pack.
The 18.8 number is a "de-rated" number provided by BMW for warranty purposes so the 70% warranty is based on that number, not 19.6 or 22

Ah I see. I’m on 17.6-17.8 I know it’s just a rough estimate but not bad going for 8 years old and 51,000 miles

 

[Gonville]

I'm an 80% kinda guy. BMWs BMS is a decade old compared to what's available on EVs being introduced today, and all of these newest cars – including the new BMW EVs – come with user-selectable charge management. The new BMWs even allow charge current selection (not just "reduced rate" like on the i3)

I believe that BMW was attempting to make the i3 ownership experience trouble-free when they decided to leave this out, and adding it in later would be an admission of their oversight, and potentially open them to liability – not to mention paying to update software on a lot of cars.

I hope my 120 Ah battery had better chemistry and lasts longer than the original 60 Ah models.

One complicating factor in assessing this is that 120 Ah batteries are used less (on average) than their 60 Ah counterparts. In other words, if the average daily distance driven is 40 miles, then the early batteries were using 50% of their capacity daily, where the 120 Ahs are only using a quarter of their capacity. That right there should account for the larger battery degrading at half the rate of original.

Of course the i3's generous buffer helps, but it's divided between the top and bottom of the pack, so 100% indicated is really 95% or so – nowhere near 80%.

Anyhow I'll continue to manage my 80-ish% charge manually, no biggie, I also don't sweat it if I occasionally charge to 100%. I hope this pays off as the car ages.

If you don’t go beyond 80% cells will come unbalanced. Doesn’t matter to degradation but will reduce capacity as ‘passive top end cell balancing’ only happens after a charge is complete one cell at a time per charge. 80% is old school single cell battery university with no thermal management.

 

[Gonville]

This is my original 60Ah after I charged it only occasionally on free vend public posts for few hours whilst shopping. So never beyond 80% just enough for 50 miles a week couple of shopping trips 5 miles max. So look what happened to the 100% GOM. Ok GOM is variable but 35 miles! Anyway took a month or 2 charging every day at home on a 7kW to 100% to get back to 70 odd miles. Cell balancing. Passive. Top end.

(Also that Elon noted that Tesla users should discharge the car to a low level and charge slowly to full to enable bottom end pack balancing. Tesla uses active cell balancing as it charges. )

 

[Gonville]

BMW Max Kappa



Max Kappa (short for “Kapazität”) is the BMS estimated snapshot capacity at that point in time, its based on external factors (temperature, current load), coulomb count, these factors are then fed into a statistical model to provide an educated guess of what the current capacity of the pack. This is what is shown in the service menu. It is not like a fuel tank you can't just measure it as there is chemistry involved.



Max Kappa as such is not a measure of anything, it is a guess of what the BMS thinks the pack is doing at that moment in time and therefore how many kWhs are available. It is not a true reflection of capacity.



Consider the following scenario, it has been cold overnight you haven't preconditioned so the battery is at 4c well below its preferred operating temperature of 24-28c, Max Kappa will show a corresponding low number as due to the cold the pack is less chemically active, as you drive the load starts to warm the pack up, so you then lose range but Max Kappa goes up due to battery reaching operating temperature.



The only way to test true capacity and health of a battery is to charge and discharge the pack to known points and record the input and output rates. It is perfectly possible for Max Kappa to read 19.1kWh (on a 60aH) on a full charge but the battery actual be degraded, or vice versa for it to read well under 15kWh but be perfectly fine.







Orange Machine. Battery safety tester (including pressure). Discharge charge full cycle.

 

[eNate]

I don't have official sources from BMW, but have seen enough discussion on this topic – here on this forum, in Reddit, and maybe FB – including user data, to indicate that i3 cell balancing occurs at all voltages.

My own dongle shows voltages roughly matched, though I've never bothered to track them.

100% is still the gold standard because it also allows the BMS to recalibrate indicated 100% and may result in more accurate miles remaining displayed.

But for my day-to-day use, that doesn't matter, and for occasional longer distance trips, when I charge to 100%, even if the estimated miles remaining starts out indicating low, I get the full 185-ish miles of real world range by the time my %SOC is down to the teens.

 

[alohart]

This is my original 60Ah after I charged it only occasionally on free vend public posts for few hours whilst shopping. So never beyond 80% just enough for 50 miles a week couple of shopping trips 5 miles max. So look what happened to the 100% GOM. Ok GOM is variable but 35 miles! Anyway took a month or 2 charging every day at home on a 7kW to 100% to get back to 70 odd miles. Cell balancing. Passive. Top end.

It would have been interesting to use the mi3 (iOS) or electrified (Android) app to display the maximum and minimum cell voltages at a high charge level. That would have shown how much the voltages differed to provide you with an indication of how unbalanced the charge levels were. Imbalance must have been considerable for a 50% loss of predicted range.

My practice is to charge to ~80% routinely but to monitor the cells' voltages periodically to determine when cell charge level balancing is needed. I would then charge fully and allow our i3 to be parked for as long as possible to give passive cell balancing as much time as possible knowing how slow it is. However, I have never seen significant imbalance, so I must be charging to a high enough charge level.

 

[eNate]

I just reviewed my charging history in the app and plugged in my dongle to check voltage.

I'm at 72% today. I've put 1,300 kWh into the battery since I last charged to 100% in October (equivalent of ~30 battery cycles, mostly in the 50% to 80% range). My min/max voltage spread is 1.49%. Average pack voltage is 68.56.

 

[Gonville]

It would have been interesting to use the mi3 (iOS) or electrified (Android) app to display the maximum and minimum cell voltages at a high charge level. That would have shown how much the voltages differed to provide you with an indication of how unbalanced the charge levels were. Imbalance must have been considerable for a 50% loss of predicted range.

My practice is to charge to ~80% routinely but to monitor the cells' voltages periodically to determine when cell charge level balancing is needed. I would then charge fully and allow our i3 to be parked for as long as possible to give passive cell balancing as much time as possible knowing how slow it is. However, I have never seen significant imbalance, so I must be charging to a high enough charge level.

It only levels down the highest voltage cell to the next highest for each charge cycle. Leaving it a long time between charges makes no difference as it will only burn down one cell.

 

[dkis]

Hi,

I have a 2019 120ah i3, i has 100000km when I bought it with ~35.5kwh capacity in hidden menu. Now it shows 33.5-33.6kwh after 6-7000kms. When I go far I charge it to 100%, for everyday useage I charge it 40 -> 60% or 20-25->80-85% depens on the required range.

 

[dkis]

Hi,

I have a 2019 120ah i3, i has 100000km when I bought it with ~35.5kwh capacity in hidden menu. Now it shows 33.5-33.6kwh after 6-7000kms. When I go far I charge it to 100%, for everyday useage I charge it 40 -> 60% or 20-25->80-85% depens on the required range.

When I charge to 100% it shows me a little bit more than 300km range in eco pro + mode.

 

[Edwin Grenfeldt]

Does anyone know what the i3 60ah kappa max reading would be from new? Is it 18.8 kWh or 22?

Hello the total amount is 21,6 kWh and the you could use is 18,8 kWh, you see

 

[jfk68]

Hello! I just tried the Mi3 app with my 2019 i3s 120Ah with 178000 km. Last month I started to have power losses around 20% of the Soc. What is your opinion on this result?
I'm a little worried because of the difference between the cells. How to do a good cell balancing? Thanks.

 

[alohart]

Hello! I just tried the Mi3 app with my 2019 i3s 120Ah with 178000 km. Last month I started to have power losses around 20% of the Soc. What is your opinion on this result?
I'm a little worried because of the difference between the cells.

Yes, that could explain what you're experiencing. However, an i3's cell charge level balancing is top balancing meaning that cell charge levels are balanced at high charge levels (>80%). The charge level in your mi3 screen captures is only 34%. It would be interesting to see the BMS data when the charge level is 100%.

If you typically charge to 100%, how long is your car typically parked after charging but before being driven again. This period is when cell charge level balancing occurs most rapidly and should be maximized.

How to do a good cell balancing?

A battery pack whose cells have very divergent voltages (i.e., charge levels) can take many days for the voltages to equalize because the cells with the highest voltages are discharged through resistors to lower their voltages. To avoid generating too much heat, these resistors have a high resistance to limit the current flow. This is why voltage balancing is so slow.

About all that you could do is to charge to 100% when you park after work and leave your car for as long as possible without driving. Try to do this for many days using mi3 at a 100% charge level to display the balancing progress. Hopefully, this will restore some of the usable capacity. However, the highest capacity of all 96 cells is calculated to be 114.10 Ah compared with the nominal new capacity of 120 Ah, so some degradation has occurred that cell charge level balancing won't restore. This is normal for an i3 that has been driven as much as yours has.

 

[jfk68]

Thank you for this very detailed answer! I will do a reading when car is at 100%. Your answer fully explains the observed drop for the SOCmax. Since buying the car I was doing 150-180km/day and I plugged it in every evening (charging at night during off-peak hours).
Two years ago I changed job location and only do 40km/day. Since then the SOCmax has been dropping regularly and much faster than the first 4 years. The car is charged to 100% only twice a week and remains at this level for 2-3 hours max until I drive it... not enough for cell balancing as you said.