Weighing Battery Preconditioning vs Daily Charging

[i3forandy]

I have a question for the more seasoned i3 owners out there...

My daily commute is about 30 miles total, so I don't need to charge every day. I have also read that some suggest keeping the SOC roughly between 20-80% if you don't need the full range of the battery. However, in order to precondition the battery the i3 needs to be connected to a charger.

So... In colder weather, am I better off performing smaller daily (overnight) charges with battery preconditioning, or waiting until the SOC falls below 15-20% to recharge the battery, this would mean that the battery would be operated cold on days where there is no charging performed.

I am wondering what other folks are doing to limit battery capacity loss over the life of the battery. Obviously the preconditioning improves the battery performance over a cold battery, but I am wondering if I would be taking away capacity in the longer term by more frequent shallow charging sessions...

Thanks
Andy

 

[i3an]

There has been quite bit of back & forth here on this topic, and the general argument that appears to have the upper hand is that it is no longer of any benefit to strive keep the traction battery within a certain range of charge, that BMW has done their homework, have refined their battery-management algorithms, has a pretty good warranty, and already limits one's access to well within the theoretical upper and lower bounds of the battery, so just relax & plug & play. UNLESS you are to be away or otherwise not driving for a while, in which case leave it unplugged with a partial charge. I plug in automatically out of habit, and check the battery (13.09 Batt. Kapa. Max) after pre-conditioning once a month, and it is showing negligible signs of decline—certainly less than my own!
In your case, with your commute, best to pre-condition and keep fully charged.

 

[i3an]

Correction: make that 13.08 Batt. Kapa. Max

 

[epirali]

There has been quite bit of back & forth here on this topic, and the general argument that appears to have the upper hand is that it is no longer of any benefit to strive keep the traction battery within a certain range of charge, that BMW has done their homework, have refined their battery-management algorithms, has a pretty good warranty, and already limits one's access to well within the theoretical upper and lower bounds of the battery, so just relax & plug & play. UNLESS you are to be away or otherwise not driving for a while, in which case leave it unplugged with a partial charge. I plug in automatically out of habit, and check the battery (13.09 Batt. Kapa. Max) after pre-conditioning once a month, and it is showing negligible signs of decline—certainly less than my own!
In your case, with your commute, best to pre-condition and keep fully charged.

The only things I would add (and I think i3an is pretty much spot on) is that if you want a little "insurance" and you have a daily departure time that is fixed set for low cost charging and set the low cost time to be around 5-6 hours before your preconditioned departure time (2-3 hours to charge assume L2 and at low charge, 3 hours for preconditioning). This way you will delay the charging/high charge state over time by a few hours a day. Say you arrive home at 6pm and leave at 8am you can plug in but not start charing until 2-3am with low cost charging, shaving 8-9 hours of idle full charge. Again it should make much of a difference but over many years its just something you can do.

 

[jadnashuanh]

FWIW, if you are going to not use the car for a period of time, read the manual...it must have enough charge to keep the battery from becoming too discharged, which will damage it more than leaving it attached, IMHO. Keep in mind that the logic in the car and the way the EVSE works, it physically disconnects the acv input...it's not like a typical thing like a cellphone charger where the acv remains attached and power going in. The car will only reactivate the EVSE once the level drops a moderate amount, and that might only be once a week or so. The standby losses are not huge, but if you start out low, will deplete the battery. Keep in mind, the 12vdc battery is what runs things when not plugged in...that battery will be destroyed if the charge gets low, and lead-acid batteries self-discharge a slight amount and are more affected by cold weather than the LiOn ones. The 12vdc battery only gets recharged when the car is either in READY state, or you have the EVSE attached.

 

[alohart]

Keep in mind, the 12vdc battery is what runs things when not plugged in...that battery will be destroyed if the charge gets low, and lead-acid batteries self-discharge a slight amount and are more affected by cold weather than the LiOn ones.

Fortunately, a U.S. i3 isn't equipped with a standard flooded-cell lead-acid 12V battery which does self-discharge over time. Instead, a U.S. i3 is equipped with an absorbent glass mat (AGM) lead-acid 12V battery which does not self-discharge appreciably over time. In addition to our i3, we have another car that we store 6 months/year. Both have AGM 12V batteries. As part of my storage preparation, I disconnect the 12V battery and fully charge it if necessary. When I return 6 months later, the resting open-circuit voltage of these 12V batteries indicates that they have remained fully-charged.

So a U.S. i3's 12V battery won't self-discharge significantly over time, but if it remains connected to the car's electrical system, it will be discharged over time by vampire loads such as the keyless entry system, burglar alarm, etc.

I don't know whether i3's in other markets are equipped with AGM 12V batteries. A U.S. i3 includes option 570, "Stronger Electrical Supply". This might be an AGM 12V battery, but I don't know.

If our i3 operated in a cold climate where battery pack preconditioning were advantageous, I would reduce the charging power so that the indicated SOC of the battery pack would be ~90% after battery pack preconditioning at the end of the charging session unless I needed the full range. Our JuiceBox EVSE allows the charging current to be adjusted dynamically in 1 amp increments, so adjusting the charging power is easy.

While BMW's battery management is likely to ensure less than 30% battery pack degradation during the 8-year warranty period thus avoiding most battery pack degradation warranty claims, I want to minimize the degradation of our battery pack even more because I hope to keep our i3 longer than 8 years. I avoid charging to an indicated 100% unless I need the full range, and I don't allow our battery pack to remain at a low SOC. An indicated 100% SOC is actually 97% of the theoretical full charge which is quite high in terms of longevity according to what I have read about Li-ion battery cells. Unless the SOC's of the 96 individual battery cells are very accurately balanced, those cells with higher SOC's could easily be charged to near the theoretical 100% which isn't ideal. The current 60 aH i3's battery pack management system makes 10% of the theoretical capacity unavailable at the low end of the charge range but only 3% unavailable at the high end, so there is very little margin for error. Apparently, the new 94 aH battery pack management system increases the battery pack's capacity unavailability by 5% (18% vs. 13% for the 60 aH battery pack) which could increase the battery pack's longevity.

I realize that most i3 owners lease, so they don't plan to keep their i3's more than 3 years, they don't want to be bothered with such "fine tuning", and they treat their i3's like their mobile phones which they plug in nightly and charge to an indicated 100%. This is quite convenient. The battery packs in these i3's will be fine during the original owners' lease periods, but I wonder how the experience of subsequent i3 owners will be after their battery pack warranties have expired.

 

[jadnashuanh]

After the vehicle attains what it thinks is fully charged, you might notice it turning the EVSE back on again a few times over the next few hours which is to do charge balancing which evens out the charge on those cells that might be lower than ideal. The adjacent cells tend to bring those lower up once the EVSE is disconnected, and the overall effect ends up being, a slight decrease in overall charge, which is why the EVSE turns back on again to bring those back up.

I'm not worried. While not exactly the same, my sister has one of the original Prius's, and its batteries are still functioning fine (over 200K miles and LOTS of years)...we've learned lots in battery management since, and the cells are more robust.

 

[michaelbmw]

I have a Ford Focus Electric. People on the Myfocuselectric forum say the same kind of rah-rah thing....the design is wonderful, the car has thermal management for the battery, just plug it in, don't worry, etc etc etc

Well let me tell you...the car is going back at the end of a three year lease with 52,000 miles. The battery is down approximately 22%, and it is a drag. Trips that I could have taken when new, now I can't

DO worry about treating your battery well. DO NOT fully charge it when you don't need to do so.. DO NOT depend on BMW to be so clever that their batteries defy the laws of physics. Your battery WILL degrade, they all do...do what you can to minimize this.

Keep your battery at mid charge levels. Do not charge beyond what you need to complete your trip with a reasonable reserve. Practice just-in-time charging so that the battery is at peak charge for a minimal amount of time.

I'm tellin' ya. Be smart. You car is a $10,000 battery with wheels. Do everything practical to treat it well.


P.S. Before someone says, "Yeah, but Ford isn't smart like BMW...". Look at the testing performed by Idaho National Labs. AVT.INL.GOV Over the first 4000 miles, I3 batteries degraded 3-4%, almost identical with Ford batteries. This is what batteries do...they fade with use.

 

[jadnashuanh]

http://bmwi3.blogspot.com/search?q=battery+capacity

After 50K miles, and two years, his battery has about 94% of the original capacity. Almost in the noise, and it appears he does nothing special and charges to full regularly.

Tom, correct me if I've misinterpreted your blog posts.

 

[alohart]

After 50K miles, and two years, his battery has about 94% of the original capacity.

I'm pretty certain that Tom is depending on the Batt. Kapa. Max value displayed by the hidden service menu. Do any of us really understand what this value means? Do we know whether BMW is reducing the unusable capacity buffer as battery pack capacity degradation occurs this keeping the useable capacity constant? Batt. Kapa. Max has varied by almost 2 kWh in our i3 with no up or down trend, so it seems very difficult to use this value to measure battery pack capacity degradation over time.

There's a finite and fairly small unusable capacity buffer, so if BMW is reducing the size of this buffer to maintain a constant usable capacity, this reduction will have to end with capacity degradation then becoming noticeable. It's just a matter of distance driven and time. Fortunately, BMW seems to have done a good job of minimizing the degradation rate, maybe better than Ford.

I trust the Idaho National Labs to accurately measure battery capacity degradation. Also, based on electrochemistry, battery cell degradation is unavoidable after repeated charge-discharge cycles and even the passage of time. Statistical thermodynamics guarantees that any electrochemical reaction can't be 100% reversible.

 

[i3an]

Art says "Batt. Kapa. Max has varied by almost 2 kWh in our i3 with no up or down trend", but this is not my experience, which has been quite steady with a very small downward trend. I haven't actually cross-referenced it with the battery temperature, but I check the Batt. Kapa. Max about once a month, and always in the morning at full pre-conditioning and full charge, so I am guessing the temperature is pretty much the same, which otherwise could introduce variable readings.

 

[alohart]

Max about once a month, and always in the morning at full pre-conditioning and full charge, so I am guessing the temperature is pretty much the same, which otherwise could introduce variable readings.

I certainly haven't been rigorous controlling the conditions when I've read Batt. Kapa. Max. Temperatures in our Honolulu parking structure parking space don't vary much throughout the day and the year. But I might have read Batt. Kapa. Max at significantly different battery pack temperatures (e.g., just after driving vs. after being parked overnight). To get the most consistent results, the battery pack temperature, another hidden service menu value, should probably be read at the same time as Batt. Kapa. Max. With this temperature information, it might be possible to correct Batt. Kapa. Max for different temperatures.

Another variable might be how similar the SOC's of each battery cell are. The more variable these SOC's, the lower the battery pack capacity. I'm pretty certain that the i3's battery management system equalizes the SOC's of the cells, but I don't know how or when this occurs. One way the SOC's of the cells could be equalized would be at the end of a charging session when the cell with the highest SOC has reached full charge (apparently 97% of the actual full charge). Charging at a lower power level could continue with the fully-charged cells being taken out of the charging circuit so that those with lower SOC's could continue charging to their full state (called "top balancing"). If the i3 does this, one would need to leave the EVSE connected for some time after the battery pack appears to be full.

The owner's manual of our Mitsubishi i-MiEV recommended a monthly top balancing procedure of running the SOC of the battery pack down below, I believe, 20%, fully charging the battery pack, and then leaving it charging for several additional hours, but I don't recall reading anything about doing something similar for our i3.