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Charging Past 80%

34K views 352 replies 65 participants last post by  imrj 
#1 ·
I have only been charging to 80% since I got my car back in August pursuant to the BMW recommendation in the manual. Does anybody know why BMW recommends this, and what would I be risking if I charged to say 85%? That extra 5% or so would allow me to make two round trips to work with ease, so I have been considering doing it. Thanks!
 
#2 ·
There is, I believe, a lot of evidence that 80% is best but I don't believe the extra 5-10% will make a huge difference. I am only basing that on the fact that Polestar suggest a 90% limit. Now it may be they have a bigger buffer at the top end but some simple maths probably suggests not. I try to stick to 80% but wouldn't inconvenience myself to achieve this magic figure. You also have to think about how long you are going to keep the car and if it is leased or PCP, why do you care.
There is my non scientific answer. Luckily there are far cleverer people on here that will give you a better answer 😉
 
#10 ·
There is, I believe, a lot of evidence that 80% is best but I don't believe the extra 5-10% will make a huge difference. I am only basing that on the fact that Polestar suggest a 90% limit. Now it may be they have a bigger buffer at the top end but some simple maths probably suggests not. I try to stick to 80% but wouldn't inconvenience myself to achieve this magic figure. You also have to think about how long you are going to keep the car and if it is leased or PCP, why do you care.
There is my non scientific answer. Luckily there are far cleverer people on here that will give you a better answer 😉
When I owned my Tesla Model S it was recommended to charge 90% which i did almost every year for 6 years on 2 year old vehicle with occasionally going to 100% when needed. The battery new was rated 265 and when i got rid of it, it was 254 which was better than most 2014 Model S that was 8 years old. Oh, I also used the superchargers about a dozen times per year charging to about 80% each time. So going to 90% didn't effect the battery much.
 
#3 ·
There have been quite a few discussions about this in some length. Wish I could remember the posts and link you to them. Might be worth having a search and seeing what you can find.

The short answer is that charging a battery reduces its capacity a tiny bit each time you do it. The proportion of the capacity reduction gets disproportionally greater the closer you get to 100% (its a curve, not a straight line), so, the recommendation is to try and keep SOC (state of charge) to between 20% and 80% to get the best life out of the battery. This isn't just EVs but any rechargeable battery. The same applies to your mobile phone for example.

It's not something to get hung up on though. If it would make a significant difference to your charging schedule, I wouldn't worry too much about going to 85% on a regular basis.
 
#5 · (Edited)
The way I see it is this: If it doesn't bother you, don't exceed 80%, but if it's a lot more convenient to you to exceed that figure, I'd do so comfortable knowing that the car has an 8 year 160000km warranty on the battery (at least in Canada). Regardless, I'd look up charts to show battery degredation vs. capacity utilization.

If the degradation after X years of charging to 100% daily is less than 20%, where X years is your expected length of ownership, then I wouldn't worry about the 80% rule, since you'll lose the 20% gradually over the course of ownership vs. restricting yourself to an artificially decreased capacity from day 1.



Why do people keep spouting this myth? It takes a second of Googling to understand that capacity utilization and charging velocity are two SEPARATE factors that both determine degradation. Charging to 80% daily via DCFC might do more harm than charging to 100% daily with a really slow AC charger (this is probably not true, but just using it as an example). Another (yet again) separate factor is temperature, and there are many others.
 
#6 ·
NMC batteries don't like to go to high states of charge and really don't like to sit at high states of charge. 80% is a compromise to let you use a lot of the battery but to also try to not hurt the batteries. From what I understand the ideal for NMC batteries is 30-50%. So if you can do your daily driving with only 50% capacity, consider lowering it further.

LiFePO4 batteries, on the other hand, don't mind going all the way 0-100% and sitting at 100%. And they're cheaper. Which might make you ask "then why do we use NMC batteries?!l" Power density is a big one and another is that LiFePO4 batteries don't like to be below freezing.

There's a lot of material out there about maintaining NMC batteries if you're curious.

There's at least two schools of thought around this whole topic. One is, I care about my car and want it to last, so I do the things that will help it last. Another is that I have an 8 year, 100k mile warranty on the battery so screw it. The problem is BMW has no minimum capacity in the warranty. So if you run it down hard, you may not get a free replacement. I plan to take care of mine.
 
#17 ·
NMC batteries don't like to go to high states of charge and really don't like to sit at high states of charge. 80% is a compromise to let you use a lot of the battery but to also try to not hurt the batteries. From what I understand the ideal for NMC batteries is 30-50%. So if you can do your daily driving with only 50% capacity, consider lowering it further.

LiFePO4 batteries, on the other hand, don't mind going all the way 0-100% and sitting at 100%. And they're cheaper. Which might make you ask "then why do we use NMC batteries?!l" Power density is a big one and another is that LiFePO4 batteries don't like to be below freezing.

There's a lot of material out there about maintaining NMC batteries if you're curious.

There's at least two schools of thought around this whole topic. One is, I care about my car and want it to last, so I do the things that will help it last. Another is that I have an 8 year, 100k mile warranty on the battery so screw it. The problem is BMW has no minimum capacity in the warranty. So if you run it down hard, you may not get a free replacement. I plan to take care of mine.
Do you have any sources for the claim that NMC batteries don't like to sit at high states of charge? Because all the battery research I've read says that storage at 60% and above is more or less the same, you really need to be lower for it to make any difference at normal temperatures. See the below picture from one of the papers on it. Calendar aging is largest during the first year and slows down a lot after that.

As for charging I agree, higher state of charge is worse when charging.

 
#8 ·
My friend works at a car manufacturer that builds EVs and tells me it’s all a load of rubbish. The manufacturers build in safe guards to protect the batteries so 100% isn’t really 100% anyway and that this only becomes a problem after 5 years +.

He concluded with this (and i agree), that if you can afford an EV like this, 9/10 you change your cars every 3-4 years anyway so cutting your charge limit to 80% in order to help the next man is a waste of time and makes your personal ownership experience worse overall.
 
#12 ·
It's not a load of rubbish but it is a long term thing. Some people have a problem thinking long term. If something slowly does damage they can't see day to day then it doesn't matter to them. It is still happening though.

80% on a modern EV is still a lot of range evey day. I don't see why you'd charge higher every day unless you really need more range every day.
 
#14 · (Edited)
The iDrive8 message states exactly the following: "A charging target of 80% is recommended for quick charging, optimal battery life and the fastest charging route". There is no mention or differentiation between AC and DC charging when it comes to the charging target of 80%.

Also, this can be cross referenced with the Owners manual that also clearly states: "To optimize the service life of the high-voltage battery, note the following: (2) Keep the charge level between 10% and 80% if possible, for instance by setting a charging target to 80%". Also, no differentiation between AC and DC charging when it comes to the 80% recommendation.

I'll just stick with what's in the manual and not come up with my own recommendations :).

This is similar to saying the engine oil should be changed every 20,000 miles instead of 10,000 miles as written in the manual of an ICE car, just because someone on the internet said they do it this way and it's fine. Or why inconvenience yourself for the next owner anyway since this won't impact the life of the engine that much? Crazy right? How is this any different?
 
#20 ·
I think comparing this to oil changes is a stretch. Losing some 5 to 10% of battery capacity over 8 to 10 years vs potentially destroying an engine doesn't equate to me.
 
#16 ·
I am jokingly saying "secretly". First time it happened on my home charger, i had a morning 220 mile business trip, only to notice 50% charge in the morning. My engineering problem solving brought me to the charge limit screen, haha. Today it happened again, I noticed it on the EA fast charger screen (50%), I knew exactly where to go.
 
#29 ·
There's another aspect of OP's question that should be considered:
what would I be risking if I charged to say 85%? That extra 5% or so would allow me to make two round trips to work with ease
By charging to 85% instead of 80%, he/she may be able to charge less frequently i.e. every other day rather than every day. That may in fact decrease the average SoC of the battery and hence may actually be better in the long run. Just a thought.
 
#347 ·
There's another aspect of OP's question that should be considered:

By charging to 85% instead of 80%, he/she may be able to charge less frequently i.e. every other day rather than every day. That may in fact decrease the average SoC of the battery and hence may actually be better in the long run. Just a thought.
Nope, shorter charges are better, so recommendation is to charge to the SoC you need instead of charging to 80 all the time. Saw this interesting video in one of the threads on this forum, but can’t remember which it is.
 
#31 ·
Just so I can understand the graps properly, a DST cycle is the equivalent of going from 100 to 0%? So if you go from 50% to 40% and back you have done one tenth of a cycle?
Also, all of the talk above fails to mention buffers on our battery. Not sure if BMW publish the size of the top and bottom buffers.
 
#32 ·
So if you go from 50% to 40% and back you have done one tenth of a cycle?
Yes. If there were no other factors involved (which as we have discussed, there are), there would be no difference between charging 10 times and using 10% of the battery each time, and charging once and using 100% of the battery. In both cases you would use one charge cycle. As has been shown though, because of the other factors involved, the former is better for the battery than the latter, if the 10% you are using is in the 'middle' of the capacity of the battery.

Not sure if BMW publish the size of the top and bottom buffers.
Not that I have seen. The only thing that seems to be published is that the total capacity is 83.9 kWh and, of that, 80.7 kWh is usable. There doesn't seem to be anything published as to how that 3.2kWh is used, or where.
 
#35 ·
That is my understanding from reading different research papers on cycle aging at least. It makes a difference but when talking about 80k miles we are debating maybe getting 6% degradation instead of 3% in the most optimal charging case or something like that (not completely accurate comparison ofc but the differences are quite small in actual degradation). So for me that will drive the car for a maximum of 30k miles I will charge to whatever is most convenient for me and not worry if I charge to 100% at a public charger and let it sit for a week if it makes my life easier (I don't have a charger at my garage spot so I use public charging either at other places or just outside my garage).
 
#41 ·
I think if you're only gonna keep the car for 3-5 years and only care about what uhappens while you own it, yeah you can do whatever you want and it won't make much difference to you. I want mine to still be in as great of shape as possible when I am done with it, so I am gonna be gentle on the battery. I also plan to keep mine until I can replace it with an EV Cayman, which might be like 2028 with Paint to Sample taking as long as it is. So I'll be driving mine for 5+ years.

We are only talking about an additional 4-5% degradation most likely. Except in the case of regular DC fast charging to 100%. I think that might be more. But still not enough that you probably care if you are only gonna keep the car 3 years.
 
#47 ·
I pointed it out in my post but would reiterate, the example given if for some general research and is not specific to the i4. The figures are there to illustrate the principal of charging to different SOCs not what you can expect from your i4. The i4 may hold up better or worse than those figures.

I think, like everything in life, its all a case of balancing different priorities and deciding what is best for you. The point of sharing knowledge about this is so you can make informed decisions.

Personally, charging once a week suits me and most weeks I only need to charge to 60-70% to leave me with 20%, so that is what I do. Very occasionally I do a long trip and then I charge to 100% before I leave.

To those who aren't planning to keep their cars long, and think "It will be the next person's problem", I'm sure dealers will have a way of checking the residual capacity of batteries and will value the car accordingly.
 
#49 ·
The method and speed of charge to 100% absolutely matters.

DC rapid from 80% to 100% is definitely worse than L2 which is worse than L1.

Charging generates heat and stress on the battery.

The 20 to 80 part of the charge curve generates the least heat in all the different ways to charge.
Charging on AC requires converting to DC so the batteries always charge with DC and we are talking about the current used to charge.

DC 400 will deliver maximum charge rates provided you have enough current. That maximum is 200kW; 500A and 400V

L2 will deliver a maximum of 50A x 220V = 11kW but converted to 400V is 27A - inverter losses of about 10% so 24A

L1 will deliver a max of 15A x 120V = 1800W
1800/400 = 4.5A

To think that the charge rate and current to charge have no effect on cell wear is incorrect.

Lithium plating and solid electrolyte film growth increase as charge rate and charge depletion rates increase. I can point to engineering papers and lots of math, but let's skip that and go right to the results.

As temperature increases the effect is the same as increasing charge rates.

So charging slower is beneficial for battery life. It is preferable to charge with L2 vs rapid DC charge. L1 is to slow to be effective, but no matter how often or to what state of charge with L1/2 you are not going to generate the same stresses that DC charging at max current will do.

Also from that all out track run in that is detailed in another thread you can see that high discharge rates generate lots of heat and is extremely stressful on the cells. Think of those brutal acceleration runs as very close to a short circuit on the battery as you can get. Short circuits or low resistance circuits are never good for batteries. The amount of energy lost in heat is dramatic.

Anyway, I say if you are L1 or L2 charging go for 90%, but DC charging should only be done when necessary and only to 80%.

Tanzanite over tartufo M50 in transit.
 
#50 · (Edited)
The method and speed of charge to 100% absolutely matters.

DC rapid from 80% to 100% is definitely worse than L2 which is worse than L1.

Charging generates heat and stress on the battery.

The 20 to 80 part of the charge curve generates the least heat in all the different ways to charge.
Charging on AC requires converting to DC so the batteries always charge with DC and we are talking about the current used to charge.

DC 400 will deliver maximum charge rates provided you have enough current. That maximum is 200kW; 500A and 400V

L2 will deliver a maximum of 50A x 220V = 11kW but converted to 400V is 27A - inverter losses of about 10% so 24A

L1 will deliver a max of 15A x 120V = 1800W
1800/400 = 4.5A

To think that the charge rate and current to charge have no effect on cell wear is incorrect.

Lithium plating and solid electrolyte film growth increase as charge rate and charge depletion rates increase. I can point to engineering papers and lots of math, but let's skip that and go right to the results.

As temperature increases the effect is the same as increasing charge rates.

So charging slower is beneficial for battery life. It is preferable to charge with L2 vs rapid DC charge. L1 is to slow to be effective, but no matter how often or to what state of charge with L1/2 you are not going to generate the same stresses that DC charging at max current will do.

Also from that all out track run in that is detailed in another thread you can see that high discharge rates generate lots of heat and is extremely stressful on the cells. Think of those brutal acceleration runs as very close to a short circuit on the battery as you can get. Short circuits or low resistance circuits are never good for batteries. The amount of energy lost in heat is dramatic.

Anyway, I say if you are L1 or L2 charging go for 90%, but DC charging should only be done when necessary and only to 80%.

Tanzanite over tartufo M50 in transit.
Techwhiz1, you are in transit!

Congratulations! You must be very happy.

😍😍😍
 
#52 · (Edited)
I don't have any issue charging beyond 80%, especially when at home (Level 2) and when heading out on a road trip. My hunch is that the 80% recommendation is to avoid having owners charging to 100% and leaving it unused in the garage for the weekend. Charging to 100% and leaving within an hour or two of full charge is just fine.

My strategy at home (when charging to 100%) is to set the car to charge to 90% overnight and then manually increase it to 100% when I wake up (usually about an hour before I leave). That way I minimize the time spent at 100% and the battery is a bit warmer when I leave. This works well when traveling too (at a hotel Level 2 plug).

For DCFC, I generally avoid charging past 80%, because I'm a firm believer that fast charging to full is not a good idea. Besides, there is almost never a reason to hang out that long on a public DCFC station anyway.
 
#61 ·
I would never bought a used EV without getting a report on the battery, Either a simple readout from the OBD or a paid service like

Regarding charging 80 / 100 percent. BMW says :
For the i3 - charge to 100 percent
For the iX3,iX,i4, iX1, i7 - Charge to 80 percent.

The i3 has much bigger buffers than the new cars.
I dont listen to what other says - if they dont consider the very small buffers in the i4.
 
#62 ·
Good comments. As a software developer I usually am lazy about the hardware understanding at times ;) But it was interesting knowing the differences and all the data really pointing to multiple variables of how long do you plan on owning, what degradation is acceptable to you, and will the battery fail at some point. I never even knew 80% was recommended till reading this for home charging. I have been usually charging rarely on Electrify America's 350 Kw DC to about 80~ give or take. And at home I don't charge but once a week or two and let it charge over night to 100%. I think based on what I am hearing I will charge on level 2 AC to 90% instead. I hadn't checked but that's in the manual right to auto turn off when a level charge is reached for both AC and DC I thought? I have only owned the car for a few months but I plan on owning it 5 to 10 years so probably want to get the most bang for the buck out of it.
 
#63 ·
I've been driving electric cars for a few years now, and that includes long-distance trips. I currently own the BMW and a Porsche Taycan, which is a really fast charging car. 10-80% goes with an average of 200 kW, in time about 18 minutes. You can select "battery-saving fast charging" in the Taycan, then the peak power does not go to 270 kW, but stays at 200 kW. I don't. I charge to 100% before every long haul, drive fast (it's Germany) and charge fast. I like to drive the battery down to 1% or 0% on the road.

In connection with the topic of the thread, it is important to mention that you can read the battery on the Taycan („Car Scanner“), so also SoH and cell drift. The values are both very good after not quite 20,000 mls, the cell drift is even like when new. I am not surprised. It doesn't matter if you charge to indicated 100% or drive to indicated 0%. What is important is that I charge by timer directly before driving from 80 to 100% and immediately recharge the battery at 0%.

Because it is only important to stay briefly in these extreme states. This is because the cell has two poles and the ions are either more at the anode or the cathode depending on the state of charge. Only at 50% is there balance between the anode and cathode. But the problems do not grow linearly. At 80% displayed, the imbalance is still ok. The same is true for displayed 20%. But you should only go beyond this for a short time.

Cell Voltage Deviation Taycan: 0,01 V
Brown Rectangle Amber Gold Wood
 
#298 · (Edited)
I've been driving electric cars for a few years now, and that includes long-distance trips. I currently own the BMW and a Porsche Taycan, which is a really fast charging car. 10-80% goes with an average of 200 kW, in time about 18 minutes. You can select "battery-saving fast charging" in the Taycan, then the peak power does not go to 270 kW, but stays at 200 kW. I don't. I charge to 100% before every long haul, drive fast (it's Germany) and charge fast. I like to drive the battery down to 1% or 0% on the road.

In connection with the topic of the thread, it is important to mention that you can read the battery on the Taycan („Car Scanner“), so also SoH and cell drift. The values are both very good after not quite 20,000 mls, the cell drift is even like when new. I am not surprised. It doesn't matter if you charge to indicated 100% or drive to indicated 0%. What is important is that I charge by timer directly before driving from 80 to 100% and immediately recharge the battery at 0%.

Because it is only important to stay briefly in these extreme states. This is because the cell has two poles and the ions are either more at the anode or the cathode depending on the state of charge. Only at 50% is there balance between the anode and cathode. But the problems do not grow linearly. At 80% displayed, the imbalance is still ok. The same is true for displayed 20%. But you should only go beyond this for a short time.

Cell Voltage Deviation Taycan: 0,01 V
View attachment 23634
Agree, I use my Taycan (and my i4) similarly. I mean charging to max 80% on regular basis, but when needed(!) above with a timer. Never let the car sit below 20% (although driven frequently below) or above 80% for more than an hour. I accept manufacturer's recommendations but still not want to limit myself to 60% of the available capacity as first priority. I do not want to think about my car as a less then 100 miles capacity one.
On the other hand the huge diff and advantage of Taycan is a really big buffer. Taycan's almost 10kWh (93,4/83,7kWh), i4 less than 3kWh (83,9/81,5kWh). Although we do not know how much up/down both cases but it is there somewhere still. Additionally i4 timer is rubbish against to Taycan's perfect one.
 
#70 ·
I am glad to see it in writing.
California treats BEV batteries the same as hybrid and that is 10/150.

Having to argue about it and bring the law into the dealership would suck.

Tanzanite over tartufo M50 in transit.
 
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