Understanding Battery Capacity Loss From A Four Year BMW Electric Trial Veteran


(Editor’s Note:  This story is featured at BMWBlog, and our thanks to both Tom and the folks at BMWBlog for allowing us to run it here concurrently)

BMW is on the precipice of stepping into the future of personal mobility. The electrification of the automobile is inevitable; however the question facing the industry is when should they jump in and bring an all electric vehicle to market. BMW has decided the time is now.

Nissan LEAF Lithium-ion Battery Pack is Air-Cooled...Is That a No-No

Nissan LEAF Lithium-ion Battery Pack is Air-Cooled…Is That a No-No

Nissan was the first to bring a purpose built, all electric car to their showrooms a few years ago and have sold over 50,000 LEAF’s worldwide so far. While I applauded them for taking the lead and acting at a time when others were still talking about EV’s, I was also critical of their decision to exclude a sophisticated thermal battery management system which would help maintain a consistent range throughout varying ambient temperatures as well as help extend the battery’s life. This omission has proven costly to them as some LEAF customers that live in hot weather climates like Arizona have experienced unacceptable battery capacity loss; prompting buybacks, battery replacements and has even forced Nissan to change their battery warranty to now include capacity loss.

Battery Capacity Chart BMW ActiveE---Note: Each Data Point is the Average of 40 Measurements Taken at Every Recharge.  This Graph Represents 1,000 Recharges Over 15 Months and 44,000 Miles.

Battery Capacity Chart BMW ActiveE—Note: Each Data Point is the Average of 40 Measurements Taken at Every Recharge. This Graph Represents 1,000 Recharges Over 15 Months and 44,000 Miles.

I’ve been in BMW’s electric vehicle Trial Lease Program for nearly 4 years and have driven 120,000 all electric miles. I have carefully recorded data from every trip I have taken and have over 2,400 log entries. I have been monitoring how the battery reacts to factors like speed, ambient temperature and the topographic conditions of my journey, but I have been paying the most attention to how the battery pack has degraded over time.

BMW ActiveE

BMW ActiveE…This Ain’t Your Ordinary BMW

The battery in any electric vehicle is the most important and expensive component of the car. Electric vehicle battery packs are susceptible to the same capacity loss as any other battery, whether they are used for your laptop or a flashlight. Once you’ve used them for a while, they are never as good as they were when they were new. When people ask me about my ActiveE’s range, rarely do they ask what will the range be in three years – but they should. I don’t think most people that are considering an electric vehicle fully understand this.

Cutaway of BMW ActiveE

Cutaway of BMW ActiveE

The official EPA range rating was 94 miles per charge, which is about what the 2014 i3 is expected to deliver. When I first got it I was able to average about 96 miles per charge in moderate temperatures. Fifteen months and 1,000 recharges later, I can expect about 90-91 miles under the same conditions which translate to about an 8% reduction in range. My results are a bit extreme because I drive much more than the average person. In fact, I have 45,000 miles on the car after only 15 months and have charged it over 1,000 times. That kind of mileage would probably be typical after about 2 ½ to 3 years of driving for the average person so I’m sure I have brought on the battery capacity loss earlier than what should be expected under normal circumstances, but it does offer insight into what perspective i3 customers can expect over time.

The ActiveE has a 32 kWh battery pack but BMW claims only about 28kWh’s are usable. The remaining 4 kWh’s are kept as a buffer because it’s not good for lithium ion battery’s to fully charge to 100% or to allow them to be completely drained. When the car was new, I was measuring on average about 27.4 kWh’s available to me and now that number has shrunk to about 25.25 kWh which is about an 8% capacity loss. Battery capacity loss isn’t linear so it’s not possible to accurately predict future loss. There are also many factors that will affect the degradation that I can control, which further complicates the process of estimating future results. Since the ActiveE is purely a test car, and will be decommissioned after my two years with it, I don’t have to be really concerned with protecting the battery to help it last longer, but if I did there are some things that I could do to help fend off early capacity loss.

BMW i3 Concept Coupe

BMW i3 Concept Coupe…Don’t Deep Discharge Me

How to guard against early capacity loss

  1. Avoid deep discharges. As mentioned above lithium ion batteries do not like to be frequently fully drained. Once in a while won’t hurt, but you don’t want to be rolling into your garage every night with the state of charge under 5%.
  2. Don’t leave a fully charged EV sitting unused for long periods of time. While charging to 100% daily isn’t really a problem, if you are not going to be using the car for a while, like days at a time then it’s best to leave it at about 80% charged. A typical example would be if you were going away on vacation for a while. In that case, don’t fully charge the car before you leave. It would be ideal to leave it between 70% and 80% charged until you get back.

    Like All Electric Vehicles With 20" Wheels, The Charge Port Has To Be Inconveniently Put At The Back Of The Vehicle

    Like All Electric Vehicles With 20″ Wheels, The Charge Port Has To Be Inconveniently Put At The Back Of The Vehicle…But I’m Not Using a Quick Charger

  3. Avoid excessive fast charging. The BMW i3 will have the capability of charging ona DC quick charger which will charge the battery to 80% in about a half hour. While the batteries are not damaged by quick charging process, they can be damaged by the heat created by fast charging. Unlike the Nissan LEAF, the i3 will have a complex thermal management system that is liquid based and its sole purpose is to keep the battery at safe operating temperatures to prolong the battery life and extend the cars range. This system will definitely allow you to fast charge more often without damage then if the car didn’t have it, but most industry experts still warn against consistent use of fast chargers. The science hasn’t really proven this one way or the other just yet, as DC quick charge is just beginning to be available to EV’s, but I would prefer to err on the side of caution and only use DC quick charge when I really needed to. I’m sure a few times a month won’t have any adverse effects.

    Sunlight is Evil

    Sunlight is Evil

  4. Don’t leave the car parked in a hot parking lot in direct sunlight if possible. I’m not suggesting you constantly hunt for a shaded paring spot when you run to the shopping mall, but if it is an extremely hot day(90+ degrees) and you’ll be leaving the car parked for many hours, it would be wise to find a spot where the car isn’t baking in direct sunlight. One of the biggest enemies to the li-ion battery cells is heat. The ideal temperature for the battery is 68 degrees Fahrenheit and as the battery temperature rises to about 90 degrees the cells begin to degrade. Once the battery temperature exceeds 105 degrees there is definite cell damage and capacity loss. I have only witnessed such a high battery temperature twice in my ActiveE since the thermal management system is constantly working to cool off the batteries when it’s hot out. I suspect the i3’s thermal management system will work even better since it’s been engineered and refined for about four years now, and the ActiveE’s system was only designed to be used on a short-term test car. In fact, if you look at the above graph you can see a period where the capacity dropped rapidly. That period was immediately following the summer of 2012, when I experienced my highest battery temperatures. I can’t say for sure whether or not that is directly related to the rapid capacity loss, but I do suspect it played a role.

    Maybe I Don't Need All That Range

    Maybe I Don’t Need All That Range…But the Hat…That’s a Necessity

  5. If you don’t need all the range the car can offer on a daily basis, then don’t fully charge it every night. I know above I said it’s not a problem for daily use, however if you don’t really need to then it’s better not to always fully charge to 100%. I may be nitpicking a bit here and others may say it’s not a problem, but if you know you only drive 30 or 40 miles a day commuting, then there is no need to fully charge your EV if it has an 80 -100 mile range. You can set it on a timer to stop charging before it’s fully charged or use the feature that many EV’s have which allows you to set the amount of charge the car accepts. You can charge to 80% daily and then set it to fully charge on the occasional days you need more range. I wouldn’t really worry too much about doing this, but if you are a low mileage driver, then it certainly won’t hurt.

What BMW needs to do

Three Camoflauged i3s With Swirls Were at BMW's Facility in New Jersey Too.  This Isn't One of Them.

Camouflaged i3s Recently Took Journalists On Some Test Rides

Capacity loss is a fact of life when you have an electric car, I’ve witnessed it first hand and have the data to back it up. However I wonder if the average prospective BMW i customer understands all this? Probably not. How BMW educates the customer will play a crucial role in their long product satisfaction. The customer must know what to expect before they buy the car or they are surely to be disappointed a couple years down the road when the destinations that they used to travel to are suddenly out of range. I know capacity loss is a moving target and there isn’t any way to offer exact predictions, but it is possible to produce charts and graphs that will offer estimates for the owner so they are at least prepared for what is to come. I’m sure BMW has much more sophisticated capacity loss data than I do and they can certainly prepare a “Battery 101” brochure for prospective i3 customers so they can learn about this before they buy the car and will be prepared to take better long term care of their battery.

Close-Up of BMW i3 Battery System

Close-Up of BMW i3 Battery System

Secondly BMW needs to show confidence in their product and offer a robust warranty that not only covers defects, but also guarantees battery capacity. GM and Nissan both came out with strong 8yr / 100,000 mile warranties for their EV batteries, and Nissan recently added a capacity loss warranty after their recent problems. Nissan now guaranties the battery will be greater than 66.25% of its original capacity for 5 years or 60,000 miles. I see this as a step in the right direction for Nissan, but I am hoping BMW shows even more confidence in the i3’s battery. I would like to see them guarantee 70% capacity for 5 years or 75,000.

I think this is a reasonable offer considering BMW will be utilizing a state-of-the-art battery thermal management system to help maintain proper temperature. Plus being a premium manufacturer, I believe their customers expect a premium product to have a warranty that instills confidence, especially since this will most likely be the first electric vehicle that virtually all of them purchase. A strong warranty may be the deciding factor in whether or not they are willing to take that leap into e-mobility.

I maintain a blog about my experiences living with an electric BMW and it can be found at: http://activeemobility.blogspot.com/

Source: Credit to Tom and the guys at BMWBlog

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38 Comments on "Understanding Battery Capacity Loss From A Four Year BMW Electric Trial Veteran"

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GM has an 8 year/ 100,000 miles/30% max capacity loss.
Am i to understand that GM has more confidence in its cars than BMW ?……
If so, that says a lot in my eyes.

I’m curious if charging to 100% and coming home with 20% remaining every day is better or worse than charging to 80% and coming home near 0%. After all, he said depleting the battery is bad for it.

Regardless of the effect on the battery, I would definitely chose charging to 100%, just so that I have a buffer.

What Tom alluded to (but didn’t outright say) is that the time spent at 100% is an important factor. If you set a timer such that you charge to 100% and then immediately leave for work, you aren’t stressing the battery anywhere near as much as charging as soon as you get home, then letting the car sit at 100% for 6 hours overnight.

Outstanding report Tom. What GM did with the Volt need not be PHEV vs BEV, as you said it is just good battery management. What is your opinion about the report posted by the American Chemical Society suggesting that a “well managed” battery may last 20 years?

In the future EVs will take over the world AND we will have an edit function here…

I see those two things as happening concurrently, (=

We encourage all commenters to flawlessly execute their spelling/grammar on their original posts…just like the writers here at InsideEVs. ***cough***

I would if I could. Tom writes a novel without flaw and I can’t write two sentences. Some engineers are flawed that way. The technical info here is fantastic. If you come for the grammar you are in for a surprise.

Another option is battery leasing. Leasing removes any concerns about longevity, but if you’re like me, you don’t like leasing things. 🙂

For low-mileage drivers, it was suggested not to fully charge the battery. I’m a low-mileage driver (my daily use is ~20 miles, or ~20% of the battery on my Leaf), so I can drive for a few days without charging. Does anyone know if it is better to let it go from 80->20% over three days, then recharge back up to 80%, or better to do 80-60% and recharge it back to 80% every night?

…and thanks, Tom, for the helpful data and conclusions!

Great article Tom. You must have a driving cycle that exceeds the “typical” less than 40 miles/day one that GM designed the Volt battery around. I suspect your miles to work are high enough that you are also charging at work (Restaurant) before going home. This is also my case at least in the winter where my trip is around 68 miles w/ no charging station at the destination (in Podunk, Arizona….oops I mean Payson). Thus the reason I have a Volt.

I am however very interested in the i3 (with RE) and eagerly await more info on it.

Outstanding article Tom! After nearly two years with my LEAF, I would notionally agree with your recommendations, but without the data to back it up. What did you use to measure battery capacity? I , however, have basically not followed any of them. I charge to 100% every night, live in a climate that is regularly over 90 deg (Houston), use DCQC 2-3 times per week, and return home with < 10% charge every commuting day. Unfortunately my use case requires this, and that is why I leased. Out of all of these factors, the only one that seemed to show a noticeable drop-off in range was extreme summer heat, for which I can do little about (I already garage park at home and work). Interestingly, I noticed it more after the second summer (received my LEAF in June 2011). I have not done any formal measurements on my battery, but I can say the Level II charge time has dropped from roughly 7.5 hours to 5.5 – 6 hours. I hope data points like mine will help Nissan engineer more heat tolerant battery packs in the future. In regards to other aspects of the battery pack, it performs outstanding.… Read more »

Great article Tom and I love that you respond to everybody’s questions and comments. You’ve been a great BMW EV advocate for several years now and quite visible in the online EV world. Other automakers should be encouraging others to do the same (I have no idea if BMW has or will do anything nice for you or not – but that’s a testament to your unbiased accounts).

I assume you will get an i3 when it’s available, how many more months must you wait? Will you get the REx?


I’m a little surprised that your capacity so abruptly drops off. It looks like this happens right as the cold weather starts. Are you sure that some of this capacity loss isn’t due to cold temperature and therefore may increase as things warm up? If not, why would the loss be so abrupt and not consistent, assuming your usage is consistent?

I was thinking about upgrading from my Volt to a full BEV at the end of the year, but after seeing this, I’ll probably upgrade to another Volt. At about 2.5 years and 24k miles, I haven’t noticed any capacity loss.

You may not need to worry about draining to zero or charging to full 100%. If BMW designed it properly, the “zero” in the display should not be an actual zero on the battery charge and 100% on the display should not be 100% in actual capacity. There should be a buffer on both ends. This will extend the life of the battery and in fact with LiFePO4 you can reach more than 5000 cycles depending on these buffers.
Certain Li-ion chemistries dont have problem in fast charges and in higher operational temperature(>90 degF). your “ideal temperature of 68 degF ” is pretty low, in this temperature the batteries are useless in the tropics and even in summer here in the US.

Can anyone point to any studies about whether Level 2 charging is any worse for the battery than Level 1? (I’ve heard it said numerously that Level 3 charging should not be used often.)

Generally, the difference between 120V and 240V charging will be negligible in terms of battery longevity. I’ve heard before that C/2 was the sweet spot for lithium-ion batteries, and this study seems to corroborate it. C/2 would be about 12 kW for the LEAF and 16 kW for the ActiveE. Much more than what is currently used to charge the car. When you look at the 1C rate, you get an idea about the impact quick charging has on battery life. Although we see charge rates of up to 2C when quick charging, the average is about 1C. Please keep in mind that the capacity loss indicated in the 1C column would correspond to a cell, which was charged with 1C rate for 800 cycles. In other words, that’s like quick charging an EV exclusively to travel about 60,000 miles. It’s worth pointing out that a single cell, like tested during the study, does not a pack make. When stacked tightly like in the LEAF, heat developed during the charging process might not dissipate as quickly as it would be the case with a single cell. This report demonstrates how valuable test data is. It would be very helpful if… Read more »

Sorry, the image of the study wouldn’t show in an tag. Here is the raw link: http://bit.ly/chargeratesvsgegradation

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Tom, great article. You could also add additional rules pertaining to preferred time of charging for minimal impact on the grid (night time vs. day time). Another rule I have implemented is to time the end of charging just prior the vehicle use, which helps the battery to warm up on those cold days. This is particularly useful for batteries without thermal management. The problem I see is the customer education. It is now getting too complicated. We, the EV owner species, enjoy thinking about our cars and planning our charging & trips. However, when you look around, those drivers with gas cars, they take the freedom of NOT thinking. Why should they? The big oil companies spoiled them by building rich network of gas stations. There is no problem with topping off, no tank shrinking and refill takes 10 minutes or so. When people ask me about charging my EV, I could engage in a long conversation explaining the best practice of EV charging but I figured that this could have detrimental effect on EV promotion. Why should I buy EV if it is so complicated to charge it? The last think I want to say to my friends/colleagues… Read more »