2015 i3 Battery Degradation (20% after 30k miles)

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sogorman

Member
Joined
Jul 7, 2018
Messages
9
Long time lurker, first time poster.

Last week, I picked up a 2015 i3 with 33k miles on the ODO here in Phoenix. I have a 2015 Model S (which is soon to be sold and a 3 in it's place) that my wife and I share and I wanted an EV for my daily commuter into work. I love the interior of the i3 and while I have never owned a BMW I am a fan of their engineering. My comute is 49 miles round trip and I figured that even with a used i3 with 10% battery degradation (side note my tesla has 4% degradation after 68,000 miles) and with the over estimated EPA range I would have more then enough range to get me to work and back home every day.

Friday was the first attempt at the 49 mile round trip commute and i made it home with only about 5 miles left on the ODO.... not what I was hoping for. Today I decided to do a range test going from 100% SOC down to near 0%, recording the consumption and range and then calculate the used kWh and determine how much degradation this i3 with 33,000 miles had on it from the original usable 18.8 kWh. Using the tools at my disposal, it looks like my i3's battery has degrated about 21% and is now sitting at or near 14.84 usable kWh.

Here is a spreadsheet with my calculations.. I plan on attempting this experiment every 45 days or so and see how the battery is doing.

https://docs.google.com/spreadsheets/d/1K3tfxXNBYDHnIroQKyG0R_G3-q9Es_fJ3cbE8AB8eZI/edit?usp=sharing

I love the car but 20% battery degradation seems a bit extreme, part of me hopes that it gets up to 30% so I can deal with BMW but the other part of me wishes I could have a car with more range than ~54 miles.

Any thoughts?
 
You can check the battery capacity in the hidden menu. Search the internet using your favorite search engine for batt.kap.max. Here's one link from this forum
http://www.mybmwi3.com/forum/viewtopic.php?t=2793

I recall one post, maybe on this forum or perhaps the Facebook group, where the capacity dropped below 70% and BMW replaced the battery. I assume your car is still under warranty....you might want to get the dealer to check the battery...perhaps they can tell if it has a bad cell or something.
Others can comment if ~54 miles is normal for a 2015 BEV, but of course it will depend on driving conditions and driving style.
 
Thanks @theothertom. The Max. Kappa. kWh secret menu is an estimation from the BMS. My reads 14.8 which is inline with my actual measurement. You are correct about the 100k 70% warranty, I guess I am just a bit disappointed at almost 20% capacity / range loss at only 33k miles.
 
If your i3 has spent its entire life in Phoenix, its battery pack could have been hot long enough to increase the battery cell degradation rate. Unlike a Tesla Model S and probably Model 3, the i3's battery pack cooling system does not automatically operate when an i3 is parked, so an i3 parked in the sun on hot pavement or in a hot garage won't automatically keeps its battery pack cool. If battery pack preconditioning is on, cooling would operate only long enough to cool the battery pack immediately before a set departure time which would not keep the battery pack cool at all times. This might require an EVSE to be plugged in as well, but those more familiar with battery pack preconditioning will correct what I've written if it is incorrect.

So it's possible that significant irreversible battery cell degradation has occurred, especially if the previous owner always charged the battery pack to an indicated 100% (an actual 90-93%) and allowed it to remain full for extended periods of time during hot conditions.

If you're lucky, the previous owner may have rarely charged to full such that the accelerated cell self-discharge rate at higher temperatures has resulted in the charge levels of the cells varying considerably (a.k.a., cell imbalance). Unfortunately, cell balancing occurs most rapidly when the battery pack is left at full charge for an extended period, the very conditions that result in an increased permanent cell degradation rate.

If our battery pack seemed to have a low capacity and I knew that it had not had much opportunity for cell balancing (i.e., it had not remained at full charge for an extended period of time), I would try charging it to full and leaving the EVSE plugged in to keep the charge level full as cell balancing decreases the pack's charge level, all under cool conditions, if possible.
 
FWIW, once the battery is fully charged, other than some cell balancing that might occur after that, it could be a number of days before the SOC dropped enough to cause the i3 to turn the EVSE back on. After 4-years, mine seems to provide about the same max range, and I almost always just plug in when I get home, and unplug only just before I leave. I try to set a departure time when it's not a spur of the moment event, whether it's summer or winter. Mine sits outside where the temperature can vary between about -15 to over 100F, depending on the season. WHen it's hot, I do try to put the car cover on it that tends to limit the heat buildup, at least to ambient or so.
 
@alohart, yes I know that the Tesla's (S,X,3) not only use active cooling in the case of the i3 but also passive cooling (i.e. the car is 'turned off' and sitting in the parking lot the car will keep the pack temp within specs). I doubt that the pack is just out of balance... I don't see how 20% range loss can be accounted to cell imbalance. Unfortunately I think my best best is to keep her parked in the sun until it hits 30% loss at which time talk to BMW

@jadnashuanh you are correct, once the SOC reaches 100% and even when balancing needs to occur the car will signal to the EVSE to open the contractors which disconnects the car from the charger. The BMS units will then balance the cells with the car disconnected and then once the overall pack SOC reaches a lower threshold will signal the EVSE to close the contractors to begin charging again.
 
Cell imbalance may be the culprit as well. I’m not sure how the i3 BMS works, but in Leafs the range estimate and other metrics are said to be calculated by the lowest voltage cell and reported on this basis. Once this cell (or range of cells) is balanced with the others, the reported capacity my increase dramatically in case if a significant difference before balancing. Try leaving the car plugged in after reaching 100% for some days to check this assumption.
 
My 2015 i3 REX does run its compressor when parked (presumably to cool the battery), but only when plugged in. During summers in Austin, I hear the compressor running quite a bit, even when the car is parked under a carport. Also, after a drive on a hot day, my car often runs the compressor for a while after I park it and plug it in. I have not noticed much degradation, but I could see that maybe if the previous owner left the car unplugged in the heat a lot, it could have damaged the battery.

It would be nice if BMW would let us set different battery schemes to optimize for reduced battery degradation, or for increased range. I could also see having the app alert drivers when the car is parked and the battery is heating up too much, so that the driver could make the call about turning on active cooling at the expense of some range.
 
arodi3 said:
My 2015 i3 REX does run its compressor when parked (presumably to cool the battery), but only when plugged in.
Have you noticed whether the A/C compressor continues running after the battery pack is fully charged? My sense is that the A/C compressor, DC-DC converter charging the 12 V battery, and the high-voltage system in general shut off after the battery pack is fully charged even though an EVSE is still plugged in. However, I might certainly be incorrect.
 
@Oleksiy I am sure the cells are not imbalanced, I have had the car at fully charged connected to a L2 for two days before running the first test in my google doc.

https://docs.google.com/spreadsheets/d/1K3tfxXNBYDHnIroQKyG0R_G3-q9Es_fJ3cbE8AB8eZI/edit?usp=sharing


@alohart and @arodi3 Yes the car will attempt to cool and regulate the HV pack temperature when plugged into L2 even at 100% SOC. Sometimes I can catch both my model S and now the i3 with the cooling fans on even at 6:00am when they have been in a shaded garage overnight. Note that the temperature in the garage is around 100F when this happens. This indicates that the i3 would like to have it's cells under 100F. Now, if we park the i3 in the phoenix sun where today it's 110F out and a car parked in the sun the interior will easily reach 140F (and eventually the HV pack as well) I hate to think what is happening to the cells. My Tesla will run the fan and even the compressor to attempt to keep the cells at a healthy temp.

I really can't decide whether to further push the cells into an unhealthy state and trigger the 30% warranty or try and baby it.
 
sogorman said:
I am sure the cells are not imbalanced, I have had the car at fully charged connected to a L2 for two days before running the first test in my google doc.
According to British i3 owners who have spoken to BMW engineers, cell balancing occurs extremely slowly due to the high resistance resistors used to discharge high charge level cells. They have written that a severely imbalanced battery pack could take weeks to rebalance completely. If true, and if the charge levels of the cells in your battery pack vary significantly, 2 days of cell balancing might be inconsequential.

I wish we had a way to measure charge level imbalance so that we could determine whether cell balancing were required.

sogorman said:
Yes the car will attempt to cool and regulate the HV pack temperature when plugged into L2 even at 100% SOC./quote]
Was this with battery pack preconditioning enabled? I know that this would result in active cooling occurring if the battery pack temperature were sufficiently high. If cooling could occur whenever an EVSE is plugged in but not actively charging, why would the preconditioning functionality be needed?
 
sogorman said:
I really can't decide whether to further push the cells into an unhealthy state and trigger the 30% warranty or try and baby it.
I think you would do the former inadvertently whatever strategy you choose to employ. Charging up to 100% and trying to balance cells would lead to accelerated degradation. And it would be difficult to employ a 80%-20% use approach anyway given the already degraded battery with the correspondingly decreased range. You can't control the weather either, temperatures over 100F will just add to this process.
 
@Oleksiy you are probably correct. with the low available total capacity (~15.0kWh) and my daily commute (~49Mi round trip) I will be forced to cycle the pack from about 100% - 5% every day in the phoenix heat. To your point whether I want to or not this will accelerate the degradation and the further the pack degrades the further I must discharge it every day.

I am used to babying my Tesla's pack the best I can (charge only to ~80% SOC every day, pre condition for about 10 minutes before every trip, etc...) with the i3 I had hoped to do the same but BMW cripples us in some regards (not being able to set charge target SOC) and my i3 with the battery in the shape it is will require near 100% DOD every day just for getting me to and from work.
 
@sogorman I don't think that it has to be a level 2 charger for the i3 to cool the cells while at 100% SoC; I just have a level 1 at work and I observe the compressor running in the middle of the day on hot days.

@alohart Yes, I have heard the compressor running while my car is parked and at 100% SoC. My car is often at 100% SoC since it is just a 10-mile drive to work for me. During the winter, I don't hear the compressor coming on randomly while the car is parked.
 
The Batt. Kapa Max. on my i3 service menu is 15.9 kWh, or 80% of new spec of 19.8, and even less of actual when new, which was just over 20.

My '14 i3 BEV is just shy of 30K miles. I live in the Phoenix desert, but the i3 is rarely parked outside, and the garage rarely gets much above 90 degrees. In the summer, I keep the i3 plugged in (L2) whenever home so that the cooling system may run as needed, but when cooler, I typically will not plug in unless the charge is below 75% (per the display) except when I know I will need a full charge the next day, which is typically 2 days/week.

I suspect I have about a year or so to hit the battery warranty replacement, as battery degradation tends to speed up rather than slow down (all else the same). Unfortunately, before I get there, there is one weekly trip I make that is already starting to push my round-trip remaining range.
 
@i3Alan Sounds like we are in the same boat. With the long commutes the further the pack degrades the further the daily DOD (depth of discharge) cycles get... it's a cycle that is self destructive at a certain point.

I think I am about 1 year away from a new pack as well which I guess is not all a bad thing. If you don't mind me asking what part of town are you in?
 
sogorman said:
I am used to babying my Tesla's pack the best I can (charge only to ~80% SOC every day, pre condition for about 10 minutes before every trip, etc...) with the i3 I had hoped to do the same but BMW cripples us in some regards (not being able to set charge target SOC) and my i3 with the battery in the shape it is will require near 100% DOD every day just for getting me to and from work.
I heard that when you charge a Tesla to the indicated 100% you will reach the actual 100% charge on the battery, while indicated 100% on i3 correspond to 80-90% of the charge.
There is also a question if the dashobard SOC meter is accurate and linear. I hasn't paid much attention to i3, but my Chrysler Pacifica can go about the same distance from 5% to1% as from 1% to 0.
 
@gt1 I agree that the Max Kappa. Kwh reading in the diagnostics is only an estimate made by the BMS which is why I did an actual road trip recording distance, consumption, etc... to calculate the actual usable energy details here: https://docs.google.com/spreadsheets/d/1K3tfxXNBYDHnIroQKyG0R_G3-q9Es_fJ3cbE8AB8eZI/edit?usp=sharing

As to
I heard that when you charge a Tesla to the indicated 100% you will reach the actual 100% charge on the battery, while indicated 100% on i3 correspond to 80-90% of the charge.
You are correct to a degree. The original usable capacity of the i3 battery as stated from BMW is 18.8 kWh of the actual 22 kWh. This indicates that there is 1.2kWh (5.45%) of SOC that is reserved by the BMS. Some of that is for anti bricking on the low end which would mean that "fully charged" is to your point not 100% SOC but closer to 95%.
 
arodi3 said:
@alohart Yes, I have heard the compressor running while my car is parked and at 100% SoC. My car is often at 100% SoC since it is just a 10-mile drive to work for me. During the winter, I don't hear the compressor coming on randomly while the car is parked.
The battery pack is warmed by resistance elements rather than the heat pump, so the heat pump compressor would not run to warm the battery pack.

BMW's top electric vehicle technical services manager in the US answered the following questions:

Q. Will the car ever turn a battery warming on by itself if the battery temperature gets critically low. For example, the car is parked outside and plugged in and the battery temperature drops below 30 degrees, will the preconditioning turn on and warm the battery up without owner intervention?
A. No. User intervention is required for battery preconditioning. If the battery temperature is very low, it will be outside its normal operating temperature. As a result, the power output and usable energy of the battery will be reduced.

Q. How about if it gets critically hot – over 105 degrees?
A. If the battery temperature is higher than the optimal operating range and preconditioning is activated, the battery can be cooled. This is not very common due to the fact that the battery is such a high thermal mass, is located close to the ground, and is not exposed to direct sunlight.

With the limited capacity of an i3's battery pack, it makes sense that battery pack preconditioning, either heating or cooling, would not occur unless the car is in drive readiness state, is being charged, or has had battery pack preconditioning enabled with the time being less than 3 hours prior to the set departure time.

When you heard the A/C compressor running, were any of the above conditions being met?
 
gt1 said:
I heard that when you charge a Tesla to the indicated 100% you will reach the actual 100% charge on the battery, while indicated 100% on i3 correspond to 80-90% of the charge.
For example, a 60 Ah i3's battery pack has a total nominal capacity of 21.6 kWh with a nominal 18.8 kWh usable. That means that 18.8/21.6 = 87% of the total capacity is usable. We know that a significant percentage of this cannot be accessed at the low charge level end which means that an indicated 100% charge level is likely 90-93% of the absolute full charge. In this charge level range, the capacity degradation rate, especially at high temperatures, is significantly faster than at a charge level 10% less (i.e., an indicated charge level of ~90%).
 
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