Range Vs mileage Vs charging speed?

[Steve999]

Here is the data that no one wants to see- I pulled data from car gurus dash pictures of mileage and remaining range (normalized to 100% and comfort settings) and various utube videos i could pull data from (bjorn nyland, ElectroBMW-russia) for the 20ah batteries. My own data for for my car plotted over the past several years has a similar loss rate. Here is the result:

https://photos.google.com/photo/AF1QipO0qr1odAI3vZ7js4237u6fCDyHvo4FB-WEKyoC

More disturbing ( but may not be true due to limited dataset ) is the degradation I'm getting with different charge rates over the past two years. I've gone back to 700watt charging again (and collecting data currently) , after using L2 charging at 7.6kw, then 3.7 kw on a home chargepoint unit, to see if reduced degradation rate holds.

https://photos.google.com/photo/AF1QipPB3zw8LHGcMZ4ZkU-aWu0aWUuDC08lt3MLSjGb

Has anyone else experienced this trend even at such low charge rates?

Here is the same data by mileage and temperature: (it's garaged so temps do not hit extremes, plus northern CA -bay area... we don't have weather)

https://photos.google.com/photo/AF1QipMUvN4yrBZ-bFuL5aDBtbJ8SAqN6erPqNsXRG-U

 

[alohart]

Please check your photo links. Accessing each link resulted in 404 Page Not Found errors.

Because I'm unable to view your photos, I don't know how you have decided that range has been lost. Remaining range is usually an estimate calculated with an undocumented algorithm making using remaining range unreliable, especially when comparing different i3's, drivers, driving conditions, etc.

Since we bought our 2014 BEV new in November, 2014, I've charged almost exclusively at 3.3 kW (16 A @ 208 V), typically to an indicated ~90%. Despite having driven less than 11k miles, actual range as well at Batt. Kapa. max have decreased ~20%, a very disappointing degradation rate that will probably not reach the 30% warranty limit by November, 2022, when our battery pack capacity warranty expires.

Some 60 Ah battery cells seem to be degrading much more rapidly than 94 Ah cells. Another explanation could be the battery management system gradually decreasing the usable capacity, maybe to reduce the actual degradation rate to minimize the number of warranty claims.

 

[SwissDI3]

Aloha Art,
by charging the car to only 90% you don't allow the Battery Management System to perform the cell balancing that normally occurs when the battery is full.
This is essential, because not all cells are absolutely the same.

This could explain your very high degradation rate at relatively low milage.

If you do not allow cell balancing, than the weakest cell will determine when the battery pack is indicated at 0%. The strongest cells could still have 10% in them, but it is not usable.
When you now charge to only 90%, than the strongest cell will determine that point and your weakest cell has only 80% in it. This effect happens during each charging/discharging cycle - and you can see where this is going.

When the battery pack gets recharged all the way to 100%, than the strongest cells will reach 100% first and at that time the balancing will start.
It will stop to charge the strongest cells (or even discharging them via a resistor), while the weakest cells get charged all the way to 100%.

Cell balancing takes a long time. So you need to leave your car plugged in after it indicates full (best overnight) for a few recharge sessions. I believe you will see an increase in max. capa if you do this.

Give it a try and please report back.

 

[Steve999]

Sorry about the photo links- here's a utube link with graphs and brief explanation:

https://www.youtube.com/watch?v=2H-nJzDjHKM

 

[alohart]

by charging the car to only 90% you don't allow the Battery Management System to perform the cell balancing that normally occurs when the battery is full.

The i3's passive cell balancing is implemented with resistors that slowly discharge the cells at the highest voltages until their voltages match those cells with the lowest voltages. This occurs whenever an i3 is off and not actively charging. This happens most rapidly at a full charge. However, it still occurs when the charge level is less than full. Because we drive so infrequently and not very far, our i3 sits off and not charging almost 100% of the time. That's plenty of time for cell balancing to occur.

This could explain your very high degradation rate at relatively low milage.

To ensure that cell imbalance isn't the cause of our range loss, I left an EVSE plugged into our i3 for 3 weeks. The Batt. Kapa. max value and the actual range did not increase at all and continued slowly decreasing. I'm pretty confident that battery cell charge imbalance isn't the cause of our decreasing usable capacity. It could be due to the battery management system (BMS) decreasing the usable capacity, maybe to minimize the number of battery pack capacity warranty claims.

Cell balancing takes a long time. So you need to leave your car plugged in after it indicates full (best overnight) for a few recharge sessions.

I haven't read documentation that describes in detail the i3's cell balancing system. I do know that BMW buys the i3's BMS from a 3rd-party vendor, so the BMS likely operates like typical passive cell balancing systems. If so, it's not necessary to keep an EVSE plugged in for cell balancing to complete. Leaving an EVSE plugged in would result in charging starting up again if cell balancing discharged the battery pack enough for charging to start again, but wouldn't affect cell balancing.

 

[alohart]

More disturbing ( but may not be true due to limited dataset ) is the degradation I'm getting with different charge rates over the past two years. I've gone back to 700watt charging again (and collecting data currently) , after using L2 charging at 7.6kw, then 3.7 kw on a home chargepoint unit, to see if reduced degradation rate holds.

In general, the capacity of a battery cell varies with the charge or discharge rate. So charging at a lower rate would result in more energy being added to the battery cell compared with higher charging rates. This isn't a result of degradation and thus isn't a permanent effect. If one wants maximum EV range, charging and driving very slowly would maximize range.

 

[jadnashuanh]

Recharge cycles tend to be the limiting factor, so with a smaller capacity battery, to go the same number of miles, you'll have more cycles, thus, more degradation than you would with a larger battery pack. A cycle is considered 100-0-100%. But 100-90-100% times 10 is the same degradation as one between full and empty.

 

[3pete]

If I'm reading your chart correctly, it looks like you have 4 data points at the higher charge speed. Based on what I've seen, there's some sizeable variability in tracking battery degradation and I'd be skeptical of basing anything off of that small of a sample. The data that I've been tracking shows that +/- 0.2 kWh is very possible between days/ charges and if you happened to get a "high" reading followed by a "low" one in one of your charge speed data sets but not the other it could be a significant factor.

Just something to consider.