Model s at 40,000 Miles

[qwk]

Maybe not. Let's say there are two cells that are out of balance and when charging one of them gets to 95% full and the other to 80%. Each trip the lower cell is more deeply discharged because it starts at a lower SOC, and also because it's operating at a lower voltage it has to put out more current to produce the same power as the other cell. That cell sitting at a lower SOC is being worked harder all the time. In reality of course the differences will be small and you have many parallel cells working as one, but I'm not sure that fully balancing a pack occasionally is more damaging than driving around with an out of balance parallel sheet in relation to the others.

And to add to this, lets say one charges to 50% because they want to maximize battery life. Since a Tesla battery will get out of balance by doing this, one will have to range charge in order to bring it back into balance. Is this better than somebody that always charges to 90% and keeps their battery balanced better, therefore not needing to do range charges in order to bring the pack back into balance? Personally I think we are splitting hairs here, as the battery life difference between the two above scenarios will be small.

 

[Ampster]

Maybe not. Let's say there are two cells that are out of balance and when charging one of them gets to 95% full and the other to 80%. Each trip the lower cell is more deeply discharged because it starts at a lower SOC, and also because it's operating at a lower voltage it has to put out more current to produce the same power as the other cell. That cell sitting at a lower SOC is being worked harder all the time. In reality of course the differences will be small and you have many parallel cells working as one, but I'm not sure that fully balancing a pack occasionally is more damaging than driving around with an out of balance parallel sheet in relation to the others.

It is easier for me to get my head around what you are saying if I think of the actual capacity of the two cells (or group of parallel cells). When I have a weak cell in my pack, it means that it will accept less amphrs than the other cells, so it gets to the top sooner than the others. It also gets to the bottom sooner on discharge. As a percentage of its own capacity it would be charged to a higher SOC and discharged to a lower SOC than its sister cells. That weak cell would have a limiting effect on the capacity of the pack. Are we saying the same thing?

 

[djp]

It is easier for me to get my head around what you are saying if I think of the actual capacity of the two cells (or group of parallel cells). When I have a weak cell in my pack, it means that it will accept less amphrs than the other cells, so it gets to the top sooner than the others. It also gets to the bottom sooner on discharge. As a percentage of its own capacity it would be charged to a higher SOC and discharged to a lower SOC than its sister cells. That weak cell would have a limiting effect on the capacity of the pack. Are we saying the same thing?

You're thinking of a degraded cell, which is different than being out of balance. When out of balance, all cells may have the same capacity but the levels are offset from each other. Think of it as a bundle of sticks with the ends out of line.

One cell will reach full charge first, which limits the rest of the pack. That cell will have extra capacity when the pack is fully discharged. A different cell will reach bottom sooner, which limits the pack on the downside.

I don't think being out of balance puts too much additional stress on the pack since the limits are on the highest and lowest cells, which keeps the rest of the cells in a narrower range. The main downside is that you don't have the range when you need it, and it could take a few weeks of balancing to restore the range (as Odenator found).

I think Tesla still has work to do to improve the Model S balancing algorithm - it seems to work much better in the Roadster.

 

[Ampster]

Yes, that was my example and I didn't distinguish between the two. Also unlike my conversions where I am dealing with packs of single cells in series, the Tesla has lots of single cells in parallel. That alone should reduce the likelihood of a group (sheet?) getting out of balance.

Other than limiting range an out of balance pack could be run that way for a long time without a need to balance. That is assuming the BMS keeps those cells from hitting bottom or top. I have heard keeping the pack in the middle is the best guarantee of long life.

 

[JRP3]

And to add to this, lets say one charges to 50% because they want to maximize battery life. Since a Tesla battery will get out of balance by doing this, one will have to range charge in order to bring it back into balance. Is this better than somebody that always charges to 90% and keeps their battery balanced better, therefore not needing to do range charges in order to bring the pack back into balance? Personally I think we are splitting hairs here, as the battery life difference between the two above scenarios will be small.

Probably. The higher SOC causes issues because the higher voltage speeds electrolyte solvent breakdown to some degree, running out of balance causes different issues as I described. I can't say which is potentially more significant, but I'd say if you time your charging mostly so that your 90% charge is reached near the time you intend to drive that's probably better than keeping it at 50% most of the time.

Yes, that was my example and I didn't distinguish between the two. Also unlike my conversions where I am dealing with packs of single cells in series, the Tesla has lots of single cells in parallel. That alone should reduce the likelihood of a group (sheet?) getting out of balance.

All those cells in parallel should behave as a single large cell. Large prismatic cells are really just layers of cells paralleled together in a single casing.

Other than limiting range an out of balance pack could be run that way for a long time without a need to balance. That is assuming the BMS keeps those cells from hitting bottom or top. I have heard keeping the pack in the middle is the best guarantee of long life.

It would seem to be if the cells stayed balanced. The large format LiFePO4 cells seem to stay in balance more than the NCA cells Tesla is using, I don't even use active balancing with mine.

 

[AmpedRealtor]

I do not have an email discussion with Tesla. I had discussed this issue with Justin, the service manager at Bellevue SC, who then discussed it with the Tesla engineers after parsing my logs. Since they could not find anything wrong with my battery, they suggested keeping my battery ALWAYS topped off at 100%. It has something to do with the computer sensing a lower voltage from the battery and calculating a lower estimated SOC. Supposedly keeping the battery at 100% for a period of time resets the computer detection of the battery voltage. But I'm no electrical engineer so my layman's interpretation might be completely off.

I had planned to stop range charging when I hit 255 rated miles, but when it shot up to 258 rated miles, I decided to keep going to see if I could get back to 263 rated miles on a range charge. I know some people think this is damaging to my battery, but I think this fear of battery degradation is somewhat exagerated for the period of time that I have been range charging. I look at it like smoking. I know its bad for me, but if I only smoke for 3 months, my overall risk of developing emphysema/COPD/Lung cancer is still low. Just my 2 cents. And I really do need every mile on my battery driving from Olympia to Portland and back for TrailBlazer games. When my battery showed a max of 231 rated miles, I couldn't make it back (to the Centralia SC) without stopping for an extra hour at a j1772 on the I-5 (including a 3 hr Blink charge at the Moda Center).

Should your battery prematurely degrade due to your keeping it charged to 100% all of the time, please be aware that Tesla does not cover battery degradation at all in its warranty. The battery warranty covers battery "defects", not degradation. Degradation of the battery is specifically excluded from the warranty, as follows:

The Battery, like all lithium-ion batteries, will experience gradual energy or power loss with time and use. Loss of Battery energy or power over time or due to or resulting from Battery usage, is NOT covered under this Battery Limited Warranty. See your owner documentation for important information on how to maximize the life and capacity of the Battery.​

Just like people who don't have cancer, it's easy to say "my risk is low". Yes, until you get cancer. The same is true here. You think you are fine and have a low risk, but then if you discover your battery degraded, you will not be so quick to dismiss it and will likely be upset. Again, Tesla has excluded battery degradation from its warranty very clearly and explicitly. If in a few years your battery capacity drops by 30%, you will find no love from Tesla given your usage pattern. I'm just trying to save you from future heartache. There is a reason Tesla warns owners to max charge only "occasionally".

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Probably. The higher SOC causes issues because the higher voltage speeds electrolyte solvent breakdown to some degree, running out of balance causes different issues as I described. I can't say which is potentially more significant, but I'd say if you time your charging mostly so that your 90% charge is reached near the time you intend to drive that's probably better than keeping it at 50% most of the time.

Ownership recommends to keep the battery at a 50%-60% state of charge in order to maximize the long term life of the battery. This came from Ownership and from two different people. This also lines up with the recommendations of those who understand the engineering side of these Panasonic cells. I am taking Tesla's advice. I keep the car charged to 50% and bump up the charge depending on where I'm going that day. I have an HPWC and so I can charge at 67 MPH without any issues.

There is definitely a lot of speculation in this thread. My methodology is based upon Tesla's excluding battery degradation from the battery warranty. As such, I am taking every precaution that I can in order to maximize battery life. Being out of balance is not something to be concerned about and will not cause any degradation. Balancing affects only the range displayed, but not the actual state of charge available for driving.

 

[mnx]

Balancing affects only the range displayed, but not the actual state of charge available for driving.

Ummm what do you think the range displayed is based on?

Put another way, if a battery is very out of balance and says it has a range of 300km, is it reasonable to expect that I can drive 400km?

 

[Johan]

There is definitely a lot of speculation in this thread. My methodology is based upon Tesla's excluding battery degradation from the battery warranty. As such, I am taking every precaution that I can in order to maximize battery life. Being out of balance is not something to be concerned about and will not cause any degradation. Balancing affects only the range displayed, but not the actual state of charge available for driving.

Sorry but I disagree with your last sentence. If you do keep the battery at 50-60% it's probably good for battery life, but only if you drive very little. If you drive more than little, you will probably often be near 0 miles which also causes wear on the battery. Also I would speculate that this wear will be uneven when it comes to the individual cells/modules, since if you're only ever charging to 60% your pack will become more out of balance over time hence creating kind of a "spotty" wear on the battery as described by other poster above where the cells with the lowest voltage will be the ones always being the most deeply discharged, thereby increasing wear on these particular cells, making the even more out of balance (larger voltage difference between the best and worst cells) which over time creates a viscious circle. I would argue that the economics would favor wearing out the pack evenly, with all cells ageing at the same rate as opposed to "spotty" wear where the pack as whole reports poor capacity but you have to go through it section by section and perhaps switching some cells but not all which would again leave you with an unbalanced pack.

My guess is (just a guess guys) for maximum longevity charge to 50-60%, or whatever lowest percentage you can charge to without ever going down in to the last 10% SOC, but every winter do a two-week period of balancing where you range charge all the time and let the car sit for most of the time. Pure speculation on my part.

 

[AmpedRealtor]

Sorry but I disagree with your last sentence. If you do keep the battery at 50-60% it's probably good for battery life, but only if you drive very little. If you drive more than little, you will probably often be near 0 miles which also causes wear on the battery. Also I would speculate that this wear will be uneven when it comes to the individual cells/modules, since if you're only ever charging to 60% your pack will become more out of balance over time hence creating kind of a "spotty" wear on the battery as described by other poster above where the cells with the lowest voltage will be the ones always being the most deeply discharged, thereby increasing wear on these particular cells, making the even more out of balance (larger voltage difference between the best and worst cells) which over time creates a viscious circle. I would argue that the economics would favor wearing out the pack evenly, with all cells ageing at the same rate as opposed to "spotty" wear where the pack as whole reports poor capacity but you have to go through it section by section and perhaps switching some cells but not all which would again leave you with an unbalanced pack.

My guess is (just a guess guys) for maximum longevity charge to 50-60%, or whatever lowest percentage you can charge to without ever going down in to the last 10% SOC, but every winter do a two-week period of balancing where you range charge all the time and let the car sit for most of the time. Pure speculation on my part.

You must have missed this part of my post: "I keep the car charged to 50% and bump up the charge depending on where I'm going that day." I always add mileage depending on where I expect to drive, so I never go below 60 miles of remaining range. For instance... right now my battery is at 50%. In three hours I pickup a friend from the airport, which is about 70 miles round trip. I add about 45 miles of range before I leave (under 45 minutes with HPWC) and will still have about 90 miles or more of remaining range when I get home. I typically try to keep my usage between 20% and 80% state of charge.

 

[Johan]

You must have missed this part of my post: "I keep the car charged to 50% and bump up the charge depending on where I'm going that day." I always add mileage depending on where I expect to drive, so I never go below 60 miles of remaining range. For instance... right now my battery is at 50%. In three hours I pickup a friend from the airport, which is about 70 miles round trip. I add about 45 miles of range before I leave (under 45 minutes with HPWC) and will still have about 90 miles or more of remaining range when I get home. I typically try to keep my usage between 20% and 80% state of charge.

But you make sure to never ever range charge? Because in that case there might be a risk that your pack over time becomes out of balance. And as far as I understand the range charge is the best way to balance the pack but just charging to the standard point of just below 90% also does an almost as good a job at balancing the pack.

And the what you said:

Balancing affects only the range displayed, but not the actual state of charge available for driving.

This is, AFAIK, just plain wrong. The worst cell will be the limiting factor for whatever range you can squeeze out of the pack. Let's say the average cell has 3.4V of voltage when the car shows only 10 miles remaining but the worst cell has only 3.0V. In theory maybe 90% of the cells still have a lot of charge but if you run the pack further down the worst ones will come below 2.9V or whatever the limit is where they actually do become bricked (the cells, not the whole pack). That would be permanent loss of those cells, which is why the car won't let you go further.

 

[JRP3]

To reiterate what others have said, balancing does affect actual total range, not just displayed range. If one group of cells is at 60% SOC and the other at 50% SOC you only have the range available from the 50% SOC cells since any further discharge beyond that would take them below zero and send them into reversal, which of course the car will not allow. Your range is always limited by the lowest SOC bank of cells.

 

[Trnsl8r]

So the "sophisticated" system that would just keep track of all this for us so we didn't have to worry about it, is not really there? We have to do a manual 2-3 week exercise of balancing the pack in preparation for a long road-trip to revive lost range if we've just used the car locally for a long time and haven't range charged? Why can't the pack balance itself if just left at a standard, non-range setting?

Not bashing, just trying to understand.

 

[Ampster]

I don't think there is a sophistcated system that can balance these kind of cells in the middle of their SOC. I believe the reason for that is for most of the charge/discharge curve the voltage is very flat. I know a lot about BMS' s available to the DIY market but very little about the Tesla BMS. Tesla prefers it that way. What little I know it is fairly sophistcated.

Others have suggested that there is some balancing that takes place on regular charges.

 

[JRP3]

That was my initial thinking but looking at some NCA charge curves they have a more constant slope than LiFePO4 cells, so voltage is a more accurate indicator for SOC throughout the charge curve. I think some balancing does occur at moderate SOC but it may simply be more aggressive at a higher SOC.

 

[Olle]

View attachment 40813

Everything the other posters have said is true. My wife and I love the car. We fight over who gets to drive it daily (and she never wanted to buy it in the first place). I have had minor issues such as loud panoroof noise at highway speeds, auto-popping door handles, constricting seatbelts, breezy window seals, wiper fluid jets aimed too low...and ALL were fixed by Tesla as warranty. My original 21" Conti tires lasted over 30k miles. The only real issue that I've had (other than be limited to 90kH at superchargers) was a very unbalanced battery when the car passed over 30k.

View attachment 40814

My 100% rated range actually dropped as low as 231 miles. But after keeping my car continuously fully charged, I am getting closer to my previous 263 rated miles. So I am pretty confident that I have not had any battery degradation.

View attachment 40815

Out of curiosity, How did you get the display to change from 21 to 19 inch wheels?

 

[jerry33]

So the "sophisticated" system that would just keep track of all this for us so we didn't have to worry about it, is not really there? We have to do a manual 2-3 week exercise of balancing the pack in preparation for a long road-trip to revive lost range if we've just used the car locally for a long time and haven't range charged? Why can't the pack balance itself if just left at a standard, non-range setting?.

My thoughts are:

1. the battery management system works just fine, but some folks over-think the issue.

2. 50% is great for long term storage, but for driving the best area for the battery is over 50% because there are fewer microcracks because the cells don't get as hot for a given power output.

3. There is some pack balancing all the time, but the pack can't really be balanced unless it's over 90%. This is just a function of the difficulty of measuring the SOC. It's far easier to do so when the pack is completely full or almost so. Doing a few range charges while on trips (two or three times a year) should keep the pack in balance without undue degradation.

 

[brianman]

There is a reason Tesla warns owners to max charge only "occasionally".

Nitpicker's corner: This "owner's manual" guidance has become somewhat separated from the official company stance.

More specifically...
Creating the World's Best Service and Warranty Program | Blog | Tesla Motors

Battery Warranty

The battery pack in your car is obviously very important and expensive to replace. In developing the Model S, we took great care to ensure that the battery would protect itself, always retaining a few percent of energy. If something goes wrong, it is therefore our fault, not yours.

Except in the cases of a collision, opening of the battery pack by non-Tesla personnel or intentional abuse (lighting the pack on fire with a blowtorch is not covered!), all damage is covered by warranty, including improper maintenance or unintentionally leaving the pack at a low state of charge for years on end. The battery will be replaced at no cost by a factory reconditioned unit with an energy capacity equal to or better than the original pack before the failure occurred.

The intent is to provide complete peace of mind about owning your Model S even if you never read or followed the instructions in the manual.

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And as far as I understand the range charge is the best way to balance the pack but just charging to the standard point of just below 90% also does an almost as good a job at balancing the pack.

Recent discussions in various threads have said otherwise. It has been stated confidently by many forum members that no balancing (or very little) occurs at anything below "99% charge" (in-car UI representation of 99%).

 

[cwerdna]

You are asking for a 40000 mile review of a car that has been out for about 18 months?

2500+ miles per month. ~600 miles per week. 100-150 miles per day.

Hmmm, there are a few owners like that. But only a few.

There are definitely very few... here's a car that's not a Tesla though that passed 285K miles in ~2 years.

Prius v: 200,000 miles club | Page 3 | PriusChat - 285K miles on a Prius v wagon
Prius v: 200,000 miles club | Page 2 | PriusChat - odo pic at 250K miles (usbseawolf2000 helps 2 fas 4 u post pics)
299,999+ Mile Club | Page 30 | PriusChat - purchase in January 2012

299,999+ Mile Club | Page 30 | PriusChat - previous car at trade time (465K miles on an 09 Prius in January 2012)
299,999+ Mile Club | Page 27 | PriusChat - some details

 

[brianman]

To reiterate what others have said, balancing does affect actual total range, not just displayed range. If one group of cells is at 60% SOC and the other at 50% SOC you only have the range available from the 50% SOC cells since any further discharge beyond that would take them below zero and send them into reversal, which of course the car will not allow. Your range is always limited by the lowest SOC bank of cells.

3. There is some pack balancing all the time, but the pack can't really be balanced unless it's over 90%. This is just a function of the difficulty of measuring the SOC. It's far easier to do so when the pack is completely full or almost so. Doing a few range charges while on trips (two or three times a year) should keep the pack in balance without undue degradation.

I still remain confused about this. I'll try to put my confusion into words.

Assumptions:
1. There are 5 cells.
2. When new, each cell is manufactured to store at least 10 kWh.
3. User sets charge slider to 60%.


Case A
If all the cells are brand new and not defective, then at charge completion the 5 cells each have at least 6 kWh stored. Total of 30 kWh, or 60% of original capacity.


Case B
Degradation results in the following capacities: { 9, 8, 7, 6, 5 } kWh.

Case B1 - Each cell is charged to the specified_limit
At charge completion, the cells have { 5.4, 4.8, 4.2, 3.6, 3.0 } kWh stored. Total of 21 kWh, or 42% of original capacity.

Case B2 - Weak cells are charged to cell_capacity, stronger cells hold some extra in a proportional fashion
10 * 0.6 = 6.0
Thus { a, b, c, 6, 5 }. Need to solve for a, b, and c.
(6+5) kWh = 11 kWh
60% * 10 kWh * 5 = 30 kWh
(30 - 11) kWh = 19 kWh
a = 9/(9+8+7) * 19 kWh = 7.125 kWh
b = 8/(9+8+7) * 19 kWh = 6.333 kWh
c = 7/(9+8+7) * 19 kWh = 5.542 kWh
At charge completion, the cells have { 7.125, 6.333, 5.542, 6, 5 }. Total of 30 kWh, or 60% of original capacity.

Notice the SOC relative to current capacity for the cells: { 79.2%, 79.2%, 79.2%, 100%, 100% }. I'm presuming this leads to accelerated degradation, especially for the 4th and 5th cell.

This leads me to believe B1 is preferred to B2. So let's run with that...

Revisiting B1:

At charge completion, the cells have { 5.4, 4.8, 4.2, 3.6, 3.0 } kWh stored. Total of 21 kWh, or 42% of original capacity.

The user asked for 60% but only got 42%. Seems like a rebalance might be in order. Assuming the user left the vehicle plugged in (and some additional set of criteria perhaps), why wouldn't the BMS want to rebalance these SOCs to ...
{ 4.2, 4.2, 4.2, 4.2, 4.2 }
... and the capacities to ...
{ 7, 7, 7, 7, 7 }


Is there some downside to doing this after every charge completion -- regardless of the slider setting?

 

[Johan]

Brianman: the balancing is an electrical phenomenon, not a BMS managed action as far as I can understand. If it were something that could be performed on demand by the batter management then this discussion would be moot.