Proper battery hygene and observations about charge capacity

[cduzz]

I've got a March 2016 S90D (pre facelift, non-performance); I've had it for 8ish months and 7000 miles. In the time that I've had it I've seen the battery slightly improve in "rated" capacity.

When I first got the car, it had at most a rated range of 276; I filled it almost all the way up once and otherwise was careful to keep the charge below 80%.

Lately I've given up on nannying the car -- if I wear out the battery so be it; I've got access to a free charger at work (30a/208v) so I often (at least once a week) charge the car to the point where it starts throttling the amperage. I will take it right off the charger and start driving it, and typically drive it enough after to get it down to 90% before I park it for long periods of time. I have occasionally let it get down to 100 miles of range (35-40%ish) but that's uncommon. I never charge it overnight.

In the time that I've switched from "good hygiene" to "bad hygiene" I've seen that the rated range has gone from 272-276 to 277-280 according to the guessometer. This morning it reported before charging that 100% would be 272, but once on the charger the 100% went to 275 the 277 then 280 and that's where it's stayed for the rest of the charging session. The throttling, too, seems like it has come later and later in the charging sessions, where at first it'd throttle back to 20a at even 92%, now it seems to go up to 95% before it starts to throttle back. In fact, for the first time ever, the esti-meter is saying that 100% is 281...

Over the winter I didn't drive it long distances, but the rated range was somewhat accurate relative to highway driving (it says I can go 250 miles, I easily drive 200 miles and have some cushion remaining, etc).

Is my filling it to near the top often relatively often helping the BMS calibrate the true remaining range / balance the cells to a degree not seen in cars that never brush against the top?

 

[cookie99]

is it getting warmer where you live? winter battery range is always lower

 

[animorph]

Exercising the full capacity range of the battery does allow the BMS to gather more data about the battery's behavior. As far as I know, 0% and 100% are always the same battery voltage. That and temperature are about the only external battery measurements the BMS can see. Beyond that it needs to see charging/discharging behavior, how many kWh's are input/output for a given change in battery voltage? And that will vary with SoC and of course age. So using the full range of the battery allows the BMS to observe all that data, at all SoC's, and provide a better estimate of the energy in the battery.

 

[cduzz]

is it getting warmer where you live? winter battery range is always lower

I had the car over the summer last year (got the car in june) -- at the time 90% was usually 250 and 80% was usually 221; now on a 70 degree day 100% is (projected to be) 281 (from 277) and 90% is 251 and 80% is 222.

My car has the 1st gen 90 pack, which others have observed as being as durable as a green banana. But after 3 months of doing almost the opposite of general charging guidance, my car seems to be getting "more" range not less.


I'm wondering how many others charge to 95-97%, then drive it 90% for overnight, then when the car gets below 70% they charge it back to 95%?

 

[ekendahl]

It seems the discussion is about two things

1. BMS' ability to report an accurate range
2. Actual Battery degradation.

Charging to a high state of charge will improve the first but, over time, increase the second. Staying at a low state of charge will confuse #1 but slow down #2. Staying low doesn't mean your actual range is lost - it's just that the BMS starts reporting inaccurate numbers. If you charge the car full then run it low a couple of times then BMS should be able to recalibrate.

I think what you are seeing is the result of letting the BMS get a better understanding of what's actually going on but if you keep doing it then your battery will degrade slightly faster. How much? I have no real idea! I suspect the increased degradation is lower than people expect.

 

[Chaserr]

Higher states of charge (I've heard over 93%) will slowly let the battery rebalance imbalanced cells (via some super slow "power bleed" process I don't understand). When you are imbalanced, the car stops charging sooner so none of the individual cells go over 4.2v. There's a big thread on tesla capping packs that is a whole different discussion, but one of the ancillary topics that came up there was that tesla's BMS lets packs get pretty far out of balance sometimes and that situation can appear to be degradation though it isn't. It seems to happen more to batteries that are regularly charged to lower states of charge over long periods of time,but can probably happen to anyone since it's a BMS function.

 

[cduzz]

It seems the discussion is about two things

1. BMS' ability to report an accurate range
2. Actual Battery degradation.

Charging to a high state of charge will improve the first but, over time, increase the second. Staying at a low state of charge will confuse #1 but slow down #2. Staying low doesn't mean your actual range is lost - it's just that the BMS starts reporting inaccurate numbers. If you charge the car full then run it low a couple of times then BMS should be able to recalibrate.

I think what you are seeing is the result of letting the BMS get a better understanding of what's actually going on but if you keep doing it then your battery will degrade slightly faster. How much? I have no real idea! I suspect the increased degradation is lower than people expect.

Well, to be fair, having a battery where you have to charge it in a weird way for 2 months in order to rebalance it *is* a degraded battery; the damage isn't permanent, but it does affect how you can actually use the car.

Once the weather warms up (more) I'll probably charge only to 80% as I did last summer, unless I've got a specific reason to charge more like I'm planning on taking a trip; partially because I believe the battery degradation at higher charge rates is faster when the battery is warm and partially because I'm more confident that the battery can be charged quickly with the battery warm, *if* I need the range for whatever reason.

 

[Battpower]

Well, to be fair, having a battery where you have to charge it in a weird way for 2 months in order to rebalance it *is* a degraded battery; the damage isn't permanent, but it does affect how you can actually use the car.

Once the weather warms up (more) I'll probably charge only to 80% as I did last summer, unless I've got a specific reason to charge more like I'm planning on taking a trip; partially because I believe the battery degradation at higher charge rates is faster when the battery is warm and partially because I'm more confident that the battery can be charged quickly with the battery warm, *if* I need the range for whatever reason.

I think the opposite is the case. At low temperatures and high (over 90 % ish) there is increased risk of formation of lithium metal in the cells which leads to those cells dropping out of your battery through individual fuses.

Never a good idea to go to 100% if you can reasonably avoid doing so, and never leave battery sitting for days or longer fully charged especially in low temps.

Exactly as stated above, the car needs to learn about the battery's real performance and it can only take the real performance over full charge range into account if it actually sees the battery operated over that range. That isn't really a bms function. The bms purely deals with maintaining balanced charge across all cells combined with safe, optimal operation.

 

[cduzz]

I think the opposite is the case. At low temperatures and high (over 90 % ish) there is increased risk of formation of lithium metal in the cells which leads to those cells dropping out of your battery through individual fuses.

Never a good idea to go to 100% if you can reasonably avoid doing so, and never leave battery sitting for days or longer fully charged especially in low temps.

Exactly as stated above, the car needs to learn about the battery's real performance and it can only take the real performance over full charge range into account if it actually sees the battery operated over that range. That isn't really a bms function. The bms purely deals with maintaining balanced charge across all cells combined with safe, optimal operation.

My somewhat unusual charging situation is that I charge at work; there are only a few shared charging stations and we're asked to charge only 4 hours at a session. I typically plug in after noon and unplug when I leave, so the car charges to 93-96% and I unplug and immediately drive and when I get home it is always at or below 90%. The couple times it's looked like I may get home with more than 90% I'll just run the heater higher on my way home.

So my car never sits with the battery between 90-99% except while it is actively charging or actively driving.

 

[Spacey73]

On an ebike battery(I used to build lots of them) the battery management systems wouldn’t start balancing until voltage was really high ...about 4.2v which is way too high for my liking. Basically it would stop charging if any cell hit 4.25v and bleed off the higher charged cells until all cells were 4.2v.

because the balancing rate is infinitely lower amps than the charge rate this could take a couple of days on a small 13 ah 48v battery. Why so long and why was the pack so out of balance...

Because when you run the battery below 10% the pack has a much greater charge of going out of balance...not all cells are exactLy the same capacity. It would be great to know when the Tesla BMS kicks in at what charge rate....is the 93% an actual figure or a guess?

 

[Battpower]

On an ebike battery(I used to build lots of them) the battery management systems wouldn’t start balancing until voltage was really high ...about 4.2v which is way too high for my liking. Basically it would stop charging if any cell hit 4.25v and bleed off the higher charged cells until all cells were 4.2v.

because the balancing rate is infinitely lower amps than the charge rate this could take a couple of days on a small 13 ah 48v battery. Why so long and why was the pack so out of balance...

Because when you run the battery below 10% the pack has a much greater charge of going out of balance...not all cells are exactLy the same capacity. It would be great to know when the Tesla BMS kicks in at what charge rate....is the 93% an actual figure or a guess?

I have been trying to understand this too. There are some users on here who clearly understand lithium battery charger design in detail, and there are pictures and other info about Tesla batteries and cell balancing.

I think the area of confusion is between maintaining a balanced pack within safe operating limits (role of BMS) and performing range calculations and predictions which is a separate function.

It appears likely that Tesla have modified their cell balancing system over the period since their first batteries were designed and I have not yet found info specific to latest rev batteries. Individual cells (not bricks of cells) are forced to balance with all the other cells in each brick because they are all in parallel.

Tesla specifically advise not bringing batteries up to a charge level where all cells / bricks are max'd out at 4.2v. When you set the max SOC you are effectively setting either the amount of energy to enter the battery or the required cell voltage that should correspond to a proportion of the cells / battery max energy capacity.

Normally, for lithium, it should / will be the latter, and a charge cycle could include an initial faster charge rate while all the cells are well below the selected / required voltage, but then taper off to a much lower rate as cells reach the selected maximum. The longer the time allowed as the charge rate tapers off, the better balanced the cell bricks will be and the better they will perform in use.

As far as I can see, the only way to have a well balanced battery and also know its actual maximum energy capacity is to follow what Tesla also advises. That is to (gently) discharge the battery to somewhere around 10%, then charge slowly to 100% allowing the car to sit on charge until the power drawn from your charge outlet drops to near zero. I imagine there will always be some quiescent power use once charging is complete. Even when the SOC indicates 100% the balancing phase (which is still adding energy to the battery) could continue for some time.

By repeating this a few (3 or 4) times, the energy / SOC calculations will become more accurate and better aligned to the reality of your specific battery. It is probably best to perform this with moderate ambient temperatures, and to not leave the battery longer than needed at >90% SOC.

If you then go back to a 20% to 80% charging regime you will protect your battery, but gradually lose accurate range information.

 

[cduzz]

I have been trying to understand this too. There are some users on here who clearly understand lithium battery charger design in detail, and there are pictures and other info about Tesla batteries and cell balancing.

I think the area of confusion is between maintaining a balanced pack within safe operating limits (role of BMS) and performing range calculations and predictions which is a separate function.

It appears likely that Tesla have modified their cell balancing system over the period since their first batteries were designed and I have not yet found info specific to latest rev batteries. Individual cells (not bricks of cells) are forced to balance with all the other cells in each brick because they are all in parallel.

Tesla specifically advise not bringing batteries up to a charge level where all cells / bricks are max'd out at 4.2v. When you set the max SOC you are effectively setting either the amount of energy to enter the battery or the required cell voltage that should correspond to a proportion of the cells / battery max energy capacity.

Normally, for lithium, it should / will be the latter, and a charge cycle could include an initial faster charge rate while all the cells are well below the selected / required voltage, but then taper off to a much lower rate as cells reach the selected maximum. The longer the time allowed as the charge rate tapers off, the better balanced the cell bricks will be and the better they will perform in use.

As far as I can see, the only way to have a well balanced battery and also know its actual maximum energy capacity is to follow what Tesla also advises. That is to (gently) discharge the battery to somewhere around 10%, then charge slowly to 100% allowing the car to sit on charge until the power drawn from your charge outlet drops to near zero. I imagine there will always be some quiescent power use once charging is complete. Even when the SOC indicates 100% the balancing phase (which is still adding energy to the battery) could continue for some time.

By repeating this a few (3 or 4) times, the energy / SOC calculations will become more accurate and better aligned to the reality of your specific battery. It is probably best to perform this with moderate ambient temperatures, and to not leave the battery longer than needed at >90% SOC.

If you then go back to a 20% to 80% charging regime you will protect your battery, but gradually lose accurate range information.

Thank you this is very informative.

There are people with "85" batteries that appear to "never" get to 100% -- if the pack is severely imbalanced could it be that it could take more than 24 hours (or more) to properly balance the pack?

 

[Battpower]

Thank you this is very informative.

There are people with "85" batteries that appear to "never" get to 100% -- if the pack is severely imbalanced could it be that it could take more than 24 hours (or more) to properly balance the pack?

The trouble is that the notion of 100% capacity is not as straightforward as it might seem! A manufacturer could take a battery that can store 50kwh if cells are max'd out to 4.2v and De-rate it to 45kwh hence only charging cells to 4.15v (guessed figures) the claimed 100% for that battery of 45kwh has enough headroom to charge and balance at that level without too much risk to the battery but at the cost of not extracting the largest performance possible from the battery.

I have not studied in detail how far Tesla tries / tried to push their batteries. One thing is certain though, that Tesla opts to manage battery temperature etc to extract highest performance.

My reading of the chargegate / batterygate issue is that Tesla realised that they had a number of batteries that were not responding very well to the demands placed on them by owners (even though use was within Tesla's claims for the car's capabilities). By adding a small amount of extra headroom (limiting max cell voltage and tighter control over charging regime) Tesla are probably hoping to keep those batteries viable past the end of warranty albeit at a lower energy capacity. The batteries can 'lose' capacity in 3 ways. Gradual and even deterioration across all cells predicted by energy throughput. Capping of max charge voltage. Invidual cells within bricks going bad either high resistance = heat and less energy stored or internal short which causes the cell to drop out of the pack by an individual fuse blowing.

The first case (gradual and even use-aging) and last (cells going bad) are somewhat inevitable and you would expect Tesla's warranty to recognize acceptable levels beyond which the pack has failed prematurely and should be replaced. I don't think their wty comes close to doing this, but recent software has apparently set limits at which a battery is considered fubared and Tesla have fitted some replacements.

The middle case of capping the battery to extend its life is really hard to justify. Effectively they turn your 85 into a 70 or whatever, certainly not for the owner's benefit as the owner then has a car with a lower capacity battery. The battery may be in good shape and be capable of use at a higher energy capacity, but owners are denied access to that storage just to give the battery an easier time.

Giving any battery / bms time to nudge up the voltage of all cells to the configured max voltage will pack the highest possible energy into the battery, but it's likely a case of diminishing return as you spend ages charging, increase chance of lithium metal deposits forming inside the cells and likely waste energy during the extended balancing period.

Staying within the 20% - 80% soc range is a good idea to preserve your battery, but your indicated range will probably not be so accurate.

 

[Peter Lucas]

I've got a March 2016 S90D (pre facelift, non-performance); I've had it for 8ish months and 7000 miles. In the time that I've had it I've seen the battery slightly improve in "rated" capacity.

When I first got the car, it had at most a rated range of 276; I filled it almost all the way up once and otherwise was careful to keep the charge below 80%.

Lately I've given up on nannying the car -- if I wear out the battery so be it; I've got access to a free charger at work (30a/208v) so I often (at least once a week) charge the car to the point where it starts throttling the amperage. I will take it right off the charger and start driving it, and typically drive it enough after to get it down to 90% before I park it for long periods of time. I have occasionally let it get down to 100 miles of range (35-40%ish) but that's uncommon. I never charge it overnight.

In the time that I've switched from "good hygiene" to "bad hygiene" I've seen that the rated range has gone from 272-276 to 277-280 according to the guessometer. This morning it reported before charging that 100% would be 272, but once on the charger the 100% went to 275 the 277 then 280 and that's where it's stayed for the rest of the charging session. The throttling, too, seems like it has come later and later in the charging sessions, where at first it'd throttle back to 20a at even 92%, now it seems to go up to 95% before it starts to throttle back. In fact, for the first time ever, the esti-meter is saying that 100% is 281...

Over the winter I didn't drive it long distances, but the rated range was somewhat accurate relative to highway driving (it says I can go 250 miles, I easily drive 200 miles and have some cushion remaining, etc).

Is my filling it to near the top often relatively often helping the BMS calibrate the true remaining range / balance the cells to a degree not seen in cars that never brush against the top?

You are mistakenly assuming, as many do, that the cars display of full charge range reflects battery condition or battery degradation. Many are learning that like my car, there can be significant battery degradation even though the cars display of full charge range remains like-new. The battery in my early 2016 Model S 90D has a usable capacity of 72-73 kwh. This is far less than the 83-85 kwh that these batteries are supposed to have. Despite this, the full charge Rated Range continues to display around 275 miles (and my real range is 200-220 miles). The displayed 275 miles is just a few miles less than when these cars were new. Among users of Tezlab, Model S owners get an average of 69% of rated miles. So I wouldn't read much into small changes in the displayed full charge range. May be random. Or who knows.

 

[cduzz]

You are mistakenly assuming, as many do, that the cars display of full charge range reflects battery condition or battery degradation. Many are learning that like my car, there can be significant battery degradation even though the cars display of full charge range remains like-new. The battery in my early 2016 Model S 90D has a usable capacity of 72-73 kwh. This is far less than the 83-85 kwh that these batteries are supposed to have. Despite this, the full charge Rated Range continues to display around 275 miles (and my real range is 200-220 miles). The displayed 275 miles is just a few miles less than when these cars were new. Among users of Tezlab, Model S owners get an average of 69% of rated miles. So I wouldn't read much into small changes in the displayed full charge range. May be random. Or who knows.

I recently charged the car to 100% and it reported that it had 274 miles of range; I drove it about 200 miles at 75-85 over hilly ground (Boston to Berlin Vermont) and ended the trip with about 25 miles of displayed range. I the efficiency was between 340 and 370 the whole way. It was fully charged overnight on a 16a/240 circuit and it was quite cold. It supercharged at 110 at peak. To me the most critical feature of the battery is that I can supercharge at high rates -- there are very few cases where I'd be driving to the maximum capacity of a battery and then charging on an AC charger overnight.


Very recently I've stopped driving to work and have been charging the car once per week when it gets to 50% and charge to 70% and my maximum range seems to be between 274 and 279 according to the guessometer's projection.

 

[Electric700]

...
Staying within the 20% - 80% soc range is a good idea to preserve your battery, but your indicated range will probably not be so accurate.

Good advice. If you go under / over that occasionally, e.g. for long trips, I don't think that will cause any issue.