Charging 80 or 100%

[JohnKelly]

I have always charged my 60 ah i3 to 100% whenever possible, and it often sits with a full battery for 3 or 4 days to a week at a time. Seven years I've been doing this... also plenty of DC fast charging. Not even a hint of range loss after 102,000 miles. It should be obvious that when you have a small battery, you need to make the most of it. Crippling it by reducing the charge percentage is just silly... you are effectively making the battery even smaller... which is especially silly when the BMS on these cars is so well thought out with already conservative charge profiles built in.

 

[alohart]

Calendar degradation occurs irrespective of charge level. Thinking that a low level of charge which save on degradation is naive at best and misleading at worst.

Cell degradation occurs for a variety of well-researched reasons that apply to i3 battery cells whose chemistries are the same as the cells in many other EV's (i.e., there's nothing magic about i3 battery cells or its 3rd-party BMS):

• Aging as you wrote. Nothing can be done about that for current i3 battery cells.

* Average charge level. The higher the average charge level, the higher the cell degradation rate. 50% is ideal but impractical for small capacity battery packs or for those who frequently drive a significant portion of the full range.

• Average depth of discharge. The higher the average depth of discharge, the higher the cell degradation rate. To minimize cell degradation due to depth of discharge, don't wait until the charge level is very low before charging, if possible. Instead, partially, not fully, charge whenever possible.

• Average cell temperature. The higher the average cell temperature, the higher the cell degradation rate. When an i3 is running, charging, or its battery pack is being preconditioned, the battery pack cooling system manages cell temperature automatically. However, when an i3 is parked with its HV system off, the cells will approach ambient temperature although quite slowly because of their high mass. I probably wouldn't buy a used i3 that spent its life in the desert southwest of the U.S. where it might have been parked frequently over hot asphalt in sunlight during hot weather. Some i3 drivers in this region have reported reduced battery output power after being parked in hot weather due to the battery cells being too hot.

• Average charging/discharging power. The higher the average charging/discharging power, the higher the cell degradation rate. Routine DC fast charging would statistically result in a higher cell degradation rate compared with lower-power AC charging. Routine rapid acceleration might also increase the cell degradation rate.

On our isolated island, the farthest round-trip that I might occasionally drive would be ~100 miles. Our usual round-trip driving distance is 20 miles. Our 2019 BEV has a actual range of just over 200 miles in our ideal EV driving environment (low speeds, no extreme temperatures), so I could maintain the charge level between 50% and 60% for our normal trips and still have enough range for an unplanned 100 mile round-trip. I've been maintaining a 50% to 80% charge level, but I could reduce that which would reduce the average charge level and depth of discharge. Doing so would statistically decrease the cell degradation rate. I would continue monitoring with the mi3 app the maximum and minimum cell voltages at 0% and 100% charge levels to check on cell balance. If I start to notice an increase in the difference between maximum and minimum cell voltages, I would charge to 85% before parking for several days which would allow cell balancing to at least begin and to maybe complete.

I plan to keep our i3 for the foreseeable future because there is no other EV available in the U.S. that meets our needs and preferences better than an i3. Because of this, I want to do everything possible to decrease the cell degradation rate. I understand that many i3 owners don't plan to keep their i3's for as long or they don't want to bother with managing their battery packs.

 

[alohart]

I have always charged my 60 ah i3 to 100% whenever possible, and it often sits with a full battery for 3 or 4 days to a week at a time. Seven years I've been doing this... also plenty of DC fast charging. Not even a hint of range loss after 102,000 miles.

There seems to be a wide range of experience with 60 Ah battery packs. I know of 4 local 60 Ah i3 owners whose battery packs were replaced by BMW under the capacity loss warranty. The Idaho National Laboratory bought 4 new 2014 i3's and conducted testing on their battery packs with instruments that few people have. After having been driven only 4k miles over 3 months, all 4 battery packs had lost 3.1±0.3% of their new usable capacities. The battery packs on the 3 i3's that were driven 11k miles over 15 months lost 7.5% of their new capacities.

Others like you have reported little cell degradation. This wide divergent of experiences doesn't seem to be happening with 94 Ah or 120 Ah battery packs which makes me think that poor quality control in 60 Ah battery cells might explain the different experiences.

It should be obvious that when you have a small battery, you need to make the most of it. Crippling it by reducing the charge percentage is just silly... you are effectively making the battery even smaller... which is especially silly when the BMS on these cars is so well thought out with already conservative charge profiles built in.

It's not silly to manage the battery pack for those of us who don't typically need the full range of even a 60 Ah battery pack. We bought a 2014 BEV new and drove it for over 7 years before selling it because its battery pack capacity and range had decreased by ~25% over only 12k miles making some trips we wanted to take impossible without charging en route which could be time consuming or impossible. Unfortunately, that wasn't enough to qualify for a new battery pack under warranty. The BMS hadn't prevented significant cell degradation. When it became obvious that the battery cells were degrading rapidly, I began managing the charge level to try to reduce the cell degradation rate. Unfortunately, this might have reduced the degradation rate enough so that our battery pack did not qualify for a warranty replacement.

 

[eNate]

when you have a small battery, you need to make the most of it. Crippling it by reducing the charge percentage is just silly...

Because I choose to limit my charge to 80%, my battery is not "crippled." When I need 100%, I charge to 100% -- and it's all there for me to use. That's not most days. Most days I use around 15% of my capacity.

To each their own of course. That should go without saying. If somebody needs 100% most days, they shouldn't hold back on topping off the charge.

 

[tjsean0308]

Because I choose to limit my charge to 80%, my battery is not "crippled." When I need 100%, I charge to 100% -- and it's all there for me to use. That's not most days. Most days I use around 15% of my capacity.

To each their own of course. That should go without saying. If somebody needs 100% most days, they shouldn't hold back on topping off the charge.

It is highly likely that your pack is not completing the balancing discharge step. These cars only balance the pack at a high state of charge due to the way they balance. They discharge high-voltage cells to match lower ones. This requires a high state of charge so the range of the car isn't negatively impacted by the balancing discharge. Some users report gains in overall range once they change charging habits to allow the car to charge to 100% between drives. The Reddit community has a decent sample size showing this data. I imagine you would see a higher range with more frequent top charging and therefore balancing.

We charge the car to 100% when it gets down to 30-40% for us which ends up being once a week or so, sometimes every other. That is what we feel is a good balance between allowing the car to top charge and balance, and also reducing the amount of time the car sits above 80%. We have seen no reduction in range or Batt Kappa Max (I check it regularly and average the findings)

The 60Ah cars have the smallest buffer with the 120Ah cars coming in second. The larger packs show very little degradation in all the testing done by Bjorn Nyland and others. The changes in the buffer and pack size, as well as small chemistry changes, are major contributors to the early car battery warranty claims.

 

[alohart]

It is highly likely that your pack is not completing the balancing discharge step. These cars only balance the pack at a high state of charge due to the way they balance. They discharge high-voltage cells to match lower ones. This requires a high state of charge so the range of the car isn't negatively impacted by the balancing discharge.

It's easy enough to check how well the cell charge levels are balanced using either the free mi3 iOS app or the free electrified Android app. I've not routinely charged to 100% or even 90% in either of our i3's over 9+ years and never saw any evidence of cell charge level imbalance. The i3's passive cell charge level balancing works very well, apparently even at charge levels less than 90%.

We charge the car to 100% when it gets down to 30-40% for us which ends up being once a week or so, sometimes every other. That is what we feel is a good balance between allowing the car to top charge and balance, and also reducing the amount of time the car sits above 80%.

The degradation rate of lithium ion battery cells is lower when the average depth of discharge is lower. So if you routinely charged from 40% to 60% rather than from 30% to 100%, your battery cells would statistically retain their capacities longer. If you noticed the minimum and maximum cell voltages beginning to increase due to an increasing cell charge level imbalance, you could charge to 100% and park your i3 for a few hours to allow cell charge level balancing to proceed.

 

[duanew]

This ignores the fact that there is a top end ‘reserve’ not available which prevents charging above 85% actual? So unless one qualifies the remarks about ‘100%’ that advice in itself is worthless. The manual used to state leave it plugged in and there was no system iDrive mechanism to ‘stop’ at say 80% as there now is in the newer i models.

Many have said that the top buffer is only about 5%. That is still to high a charge level to stay out of the accelerated aging range around full charge. My point of saying that is the ABC (Always Be Charging) recommendation is bad advice.

 

[duanew]

Calendar degradation occurs irrespective of charge level. Thinking that a low level of charge which save on degradation is naive at best and misleading at worst. Calendar degradation is what it says. 1-2% per annum irrespective of charge level.

Also BMW engineered a refrigerating cooling system not present in the newer models and heat due to charging is indeed a factor.

The "1-2% per annum irrespective of charge level." statement goes against expected norm of all Lithium batteries I know of. Charge level matters quite a bit. Find some tests or evidence that proves me wrong.

 

[duanew]

Cell balancing also is passive top end. That only happens on completion of charging when the level is above 85%. Indicated. If you only charge to 80% indicated the cells will become unbalanced and a reduction in charge capacity and therefore range will occur.

While there are many devices that work exactly as you describe, I have not seen any evidence that the i3 BMS works in that manner. It is not difficult to install a Smart BMS that is capable of controlling the charge level at different states of charge. Would BMW install a cheap BMS in a 50k car. I hope not.

 

[Gonville]

That can't be correct! A 60 Ah battery pack has a 21.6 kWh gross capacity and 18.8 kWh usable capacity which is 87% of the gross capacity. That means that only 13% of the gross capacity is divided between low and high charge level buffers, so your 15% high charge level buffer size is impossible, especially because ~10% of the gross capacity is reserved for the low charge level buffer. This means that the size of the high charge level buffer is very small, less than 5%, which means that when fully charged, the actual charge level is >95%.

Attached is a screenshot of the mi3 app when connected to our 2019 BEV at a 100% charge level. The estimated battery pack state of health is 99%, so very little of its new usable capacity has been lost. In this case, the maximum actual charge level is 96.1% while the minimum actual charge level is 12.0%.

No one, including BMW these days, recommends leaving a lithium ion battery cell at a 96% charge level if one wants to minimize the cell degradation rate. However, if full range might be needed soon, charging to 100% is fine.

First - I like the logic but the 10% buffer was at the top in the early models. So that negates most of what is calculated from there on. Current models are different I agree.

Second - if you don’t charge higher there will not be cell balancing. I can’t see how that fits with the argument except the newer models (non i3 I mean) don’t have the top end buffer.

Third - calendar degradation isn’t impacted by how much you charge it or to what level. It will happen anyway each year by 1–2%. Can’t be avoided by any 20-50% scheme made up by battery university.

Unless these points on calendar degrdation and cell balancing are addressed the percentages being discussed are moot. We know that BMW released more top end % in early 2016 because of the relatively good performance and at least in the UK with ‘mild’ weather we have not had more than 1 or 2 failed cell replacements and no call on the 70% capacity warranty. So something is up with the above assertions.

 

[Gonville]

Cell degradation occurs for a variety of well-researched reasons that apply to i3 battery cells whose chemistries are the same as the cells in many other EV's (i.e., there's nothing magic about i3 battery cells or its 3rd-party BMS):

• Aging as you wrote. Nothing can be done about that for current i3 battery cells.

* Average charge level. The higher the average charge level, the higher the cell degradation rate. 50% is ideal but impractical for small capacity battery packs or for those who frequently drive a significant portion of the full range.

• Average depth of discharge. The higher the average depth of discharge, the higher the cell degradation rate. To minimize cell degradation due to depth of discharge, don't wait until the charge level is very low before charging, if possible. Instead, partially, not fully, charge whenever possible.

• Average cell temperature. The higher the average cell temperature, the higher the cell degradation rate. When an i3 is running, charging, or its battery pack is being preconditioned, the battery pack cooling system manages cell temperature automatically. However, when an i3 is parked with its HV system off, the cells will approach ambient temperature although quite slowly because of their high mass. I probably wouldn't buy a used i3 that spent its life in the desert southwest of the U.S. where it might have been parked frequently over hot asphalt in sunlight during hot weather. Some i3 drivers in this region have reported reduced battery output power after being parked in hot weather due to the battery cells being too hot.

• Average charging/discharging power. The higher the average charging/discharging power, the higher the cell degradation rate. Routine DC fast charging would statistically result in a higher cell degradation rate compared with lower-power AC charging. Routine rapid acceleration might also increase the cell degradation rate.

On our isolated island, the farthest round-trip that I might occasionally drive would be ~100 miles. Our usual round-trip driving distance is 20 miles. Our 2019 BEV has a actual range of just over 200 miles in our ideal EV driving environment (low speeds, no extreme temperatures), so I could maintain the charge level between 50% and 60% for our normal trips and still have enough range for an unplanned 100 mile round-trip. I've been maintaining a 50% to 80% charge level, but I could reduce that which would reduce the average charge level and depth of discharge. Doing so would statistically decrease the cell degradation rate. I would continue monitoring with the mi3 app the maximum and minimum cell voltages at 0% and 100% charge levels to check on cell balance. If I start to notice an increase in the difference between maximum and minimum cell voltages, I would charge to 85% before parking for several days which would allow cell balancing to at least begin and to maybe complete.

I plan to keep our i3 for the foreseeable future because there is no other EV available in the U.S. that meets our needs and preferences better than an i3. Because of this, I want to do everything possible to decrease the cell degradation rate. I understand that many i3 owners don't plan to keep their i3's for as long or they don't want to bother with managing their battery packs.

I agree on a lot of this but the practicalities are not quite how asserted. Again it is battery university, single naked cell, no deep discharge protection, no overcharge management, no thermal management, 1C charge and discharge. We don’t have that.

- Calendar degradation - yes nothing special no one can beat that. Ours is 1-2%pa
- High charge level - we can’t ‘over charge’ - top buffer
- Deep discharge - we can’t do that either - bottom buffer
- Cell temperature - sophisticated thermal management - it will shut down or reduce power if it gets outside the 25-40c optimal range. Inverters and battery. Arizona yes in that case I agree.
- Charging ? Rapid charging is about too much heat. See the Benelux Tesla study on those that only ever supercharged.


None of the above mentions for a multi cell system - rather than battery university - the absolute importance of cell balancing. Read up on what Elon says about deep discharge to level the cells occasionally. Ours is top end passive cell balancing. If you don’t charge it up it will never do cell balancing and whilst that doesn’t cause degradation it kills capacity.

Someone please outline how they expect cell balancing to work with a 20-50% charge regime?

 

[Gonville]

The "1-2% per annum irrespective of charge level." statement goes against expected norm of all Lithium batteries I know of. Charge level matters quite a bit. Find some tests or evidence that proves me wrong.

I think like all these discussion disagreement requires proof. Battery university has all that and unfortunately it all related to single cell with no thermal management and no BMW safe guarding deep discharge etc etc etc. I leave the reader to Google.

 

[Gonville]

While there are many devices that work exactly as you describe, I have not seen any evidence that the i3 BMS works in that manner. It is not difficult to install a Smart BMS that is capable of controlling the charge level at different states of charge. Would BMW install a cheap BMS in a 50k car. I hope not.

Worth reading what aloha has said about the accuracy of the capacity reporting of the various apps.

 

[Gonville]

Many have said that the top buffer is only about 5%. That is still too high a charge level to stay out of the accelerated aging range around full charge. My point of saying that is the ABC (Always Be Charging) recommendation is bad advice.

I disagree. There is no BMW technical data that supports anything other than plug the car in. This is for an i3. i4 is different as are all the other i models none of whom use the refrigerant cooling. Even the Mini SE has same drive but different glycol based cooling. There is evidence to support the ‘top end cell balancing’. Only charging to less than 80% we know will lead to reduced charge capacity see my earlier picture. That isn’t just a myth or needs to be proven it is a non photoshopped picture of what happens if you don’t charge past 80% for a long time. Doing it once in a while is obviously better but as it only does one cell at a time per charge session. ABC for the win. At least in the UK. I think the USA Arizona is a whole different ball game and the lesson is don’t overheat the battery they hate that. Perhaps in mild continental climates in the USA ABC is OK. Remember the Electric Mini, Electric 1 series all these were trialled extensively in the USA to hone the BMS control mechanisms. From 2014 until 2020 ABC - Tom Moloughney on his recent Monroe video.

The biggest issue we have here in the UK is the 12v AGM failing and producing a brick. If the cars is used as a daily driver and charged daily the AGM seems to like that. Over 5 years - leave it in the cold for 2 weeks and it will die. Without notice. HV is fine.

 

[RicardoF1RST]

The info that applies to the BMW i3. If it was otherwise they would have provided an option to set the charge to other than 100%.
100% on this car is not 100% because there is a buffer at the top and bottom ends, so the car ends up charging to 80-85% anyways but shows as "100%".

Interestingly, my BMW i3s owners guide (Jan 2022) doesn't mention "ABC" anywhere. But it does say charge to 80% unless more range is required to benefit battery longevity...I only read it recently and was surprised. Personally, I have done 40,000m in 2yrs and I charge to 100% approx 2-3 times a week with either a home 32A or work 13A supply, maybe used a rapid twice. The rest of the time I charge to 80%.