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Battery Degradation Scientifically Explained

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On the subject of Lithium plating when charging a cold battery, does anyone know what happens when connected to shore power running the battery & cabin heaters? As far as I can tell, the battery, on one hand, isn't charging, but either (a) the current from shore power must be flowing through it (?) or (b) the inverter is powering the heaters directly, bypassing the battery?

I usually warm the battery using shore power in the winter, but if all power flows through the battery, that may not be the best plan ...

Anyone?

I do not think it goes "through" the battery. I'm not an expert, that's just my thought. You did ask for "anyone" :D

Evidence to support my theory ... if you are charging at L2, and increase accessory use (turn heat full blast) you will see your charge rate drop (mi/hr or km/hr) because some of the "shore power" is being diverted from charging the battery to powering the accesories.

Now, I suppose it *is* possible they are just misleading you with the display, and showing you the net add charging speed (subtracting off the accessory usage) ... but ... I don't see why they'd do it that way :)
 
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Evidence to support my theory ... if you are charging at L2, and increase accessory use (turn heat full blast) you will see your charge rate drop (mi/hr or km/hr) because some of the "shore power" is being diverted from charging the battery to powering the accesories.

After thinking about it, the heaters run on DC, not AC, and the contactors close, so the battery is involved. The proof of that would be that if you connect to a low power charger (say 3kW) and turn on all the heaters (12kW), they will draw from the battery pack to make up the difference. That implies that even if you are not technically charging, the DC to power the heaters is coming via the battery and you are really charging the battery at the rate required to power the heaters. So when the battery is really cold, perhaps better to just use the battery and not bother with the shore power unless you actually need it.

This falls squarely into the "thinking about this more than you need to category", but the info on lithium plating got me thinking about what you could do in the best interests of the battery when it is really cold.
 
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After thinking about it, the heaters run on DC, not AC, and the contactors close, so the battery is involved. The proof of that would be that if you connect to a low power charger (say 3kW) and turn on all the heaters (12kW), they will draw from the battery pack to make up the difference. That implies that even if you are not technically charging, the DC to power the heaters is coming via the battery and you are really charging the battery at the rate required to power the heaters. So when the battery is really cold, perhaps better to just use the battery and not bother with the shore power unless you actually need it.

This falls squarely into the "thinking about this more than you need to category", but the info on lithium plating got me thinking about what you could do in the best interests of the battery when it is really cold.

I thought after your first sentence you were going to say you agreed with me ... LOL.

AC to DC conversion doesn't mean the battery is involved. It just means DC is involved. The DC can be routed to charge the battery or to power the accessories. Just like it can be taken out of the battery to run the motor, or reversed and put back into the battery from the motor during regen. They can control the flow of power ... I don't see this point that AC-to-DC is happening as any proof that the battery is involved.

If you have 3kW available and need 12kW for accessories, you can (a) take 9kW from the battery and 3kW from 'shore', or you can (b) put 3kW "into the battery" while you are taking 12kW "out of the battery".

I don't think there's any proof one way or the other here.
 
Well, stop doing that! Leave the Leaf at 4-6 bars charge for in-town driving.

I'd like to do that... but sharing a one-stall carport for both our EVs. Logistically it is a pain to get the Leaf to stop charging at anything less than 100% (no way to set it to stop charging before 100%) and we need it to be at 90% in order to fit in our charging routine. C'est la vie.
 
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Many shallow charge cycles are better for lithium batteries than fewer deep ones and less time spent at 100% is better, especially in high heat environments. Apple's phoneOS 13 will reportedly stop at 80% overnight and top off to 100% in the morning.

How do we reconcile this shallow cycles guideline with cell balancing? It's been reported that daily shallow cycles may cause cell imbalance up to a point where SC intervention might be necessary, what would be the sweet spot as to minimize degradation at cell level while allowing the BMS to keep a proper cell balance?
 
How do we reconcile this shallow cycles guideline with cell balancing? It's been reported that daily shallow cycles may cause cell imbalance up to a point where SC intervention might be necessary, what would be the sweet spot as to minimize degradation at cell level while allowing the BMS to keep a proper cell balance?

Recent(ish) post by @wk057 said the BMS is constantly balancing now.

EDIT: Post in question #1157:
Tesla's 85 kWh rating needs an asterisk (up to 81 kWh, with up to ~77 kWh usable)

Worth an update here.

Tesla has changed the balancing algorithm many many times over the years. Originally this was a very dumb setup that would only kick in once a cell group reached a threshold voltage, usually around 90-93% SoC. This is no longer the case.

First, let me point out that Tesla's BMS software has come a LONG way... I'd consider it a work of art now. Lots of genius in there. It's absolutely amazing and full kudos where kudos are due here.

One thing they're now able to do is to calculate out the capacity of individual bricks of cells (96 in the 85/90/100, 84 in the rest) based on a ton of factors and compute this in near real time, in a full range of conditions, with almost magical accuracy. They're basically running physics simulations (similar to how they calculate out unmeasurable metrics in the inverter firmware, like rotor temperature) of the entire pack based on measured power usage/charge, balancer usage, temperature, temperature delta based on coolant flow and coolant temp, predicting temperature gradients, and probably 100 more variables. This is the holy grail of proper balancing for safety and longevity for a battery pack. This is not a dumb system anymore by any means. Knowing the actual capacity of the individual bricks allows them to know exactly which ones need cell bleeders enabled, and for exactly how long. With this data, they can balance on the fly at any SoC, and just use top and bottom SoC windows for fine tuning, validation, and calibration.

The car balances all the time whenever its needed. It knows when a cell group will need balancing before it's even out of balance... which is really freaking weird when you think about it, especially if you're watching a playback of the pack balancing and voltages and see it engage a balancer on a cell group that doesn't look out of balance at all, and watch it fall completely in line still at the end of a charge or discharge cycle. It keeps track of which groups will need it, which wont, how long they'll need it, how much they've been balanced, etc.

It really is an epic setup now.

The short answer to the balancing question: It balances any time it needs to balance.

As for SoC shenanigans, yes getting closer to 100% or 0% will give it a chance to tune things better... but it's not needed anymore. Just charge like you need to, and drive.
 
You're spot on with everything you say - 100% indicated SoC is not 100% absolute SoC, and for short time periods is absolutely fine.

I have a question for clarification on this. Is it the act of charging to 100% that damages the cell or leaving it set that? Think of it like a balloon, if you blow it up to "100%" then it'll stretch the rubber, but say you want a smaller balloon at 80% size, the rubber will be slightly loose and "damaged".

I've only ever charged to 100% SOC three times, but left immediately as it was done charging. I normally do 90% on trips now.
Would charging to 100% and leaving immediately damage the battery less than if I charged to 90% and it sat there for 8 hours all night until we left? (That being said, I still leave immediately after charging to 90%)
 
I have a question for clarification on this. Is it the act of charging to 100% that damages the cell or leaving it set that? Think of it like a balloon, if you blow it up to "100%" then it'll stretch the rubber, but say you want a smaller balloon at 80% size, the rubber will be slightly loose and "damaged".

I've only ever charged to 100% SOC three times, but left immediately as it was done charging. I normally do 90% on trips now.
Would charging to 100% and leaving immediately damage the battery less than if I charged to 90% and it sat there for 8 hours all night until we left? (That being said, I still leave immediately after charging to 90%)

I think you’re doing it “right” by choosing 90 and also leaving just after it hits 90. I don’t know the answer to the question of whether sitting at 90 for a long time is worse than sitting at 100 momentarily before leaving, but I think I would guess going to 100 is worse. Tesla lets you set 90 as a daily charge target. This means they expect many many cars to sit overnight at 90 daily and be fine. Who knows though :)

My typical max is like 93-94. Daily max is 80 or 90. It’s only worth 100 at start of trip if the first SC is not reachable without 90% of your capacity, otherwise you are just saving yourself a couple minutes at your first super charging spot anyways, so personally I wouldn’t bother going past 93-94.
 
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I absolutely love you video on battery degradation mechanics from the electro-chemical perspective. I've spoken for various battery engineers/researchers on the subject but I've never seen such a detailed and succinct summary of degradation as yourself.

Question regarding degradation at 100% SOC. I've done a number of charges to 100% SoC for several reasons but I'm concerned about harming my battery unnecessarily. The reasons for doing so include
  • Getting an accurate capacity measurement
  • Provide an opportunity for the BMS balance the pack where module voltages show its greatest deltaV
  • Occasionally because the energy was free and I needed to avoid an idle fee....(fewer times than digits on my hand =))
I've heard that charging to 100% even once might have significant negative impact but I'm not entirely sure why. Previously, I was under the impression the danger was not getting to 100%, but rather long duration storage at 100%. From your video, however it sounds like lithium plating and mechanical stress at very high SOC are additional concerns beyond increased rates of electrolyte decomposition (my historical understanding) so I should really avoid charging to 100% if I can avoid it (of the dozen or so times I've done it I only really needed it once). Would that be your advice or do think I'm overly worried about 100% SOC?



Although I don't have hard numbers on the subject, I can comment living in Southern California that I definitely hear my vehicle cycle pumps/coolant when the ambient temperature is especially high (>85-90F) hours after I've parked the vehicle.



I think there is important point to be made that "Fast Charge" by SAE definition is clearly defined from some earlier charts. To a battery however, "fast charging" is really a function of its capacity. Charging a 24 kWh pack at 50 kW (~2C) is REALLY fast charging. Charging a 100 kWH pack at 50 kW is not really that bad (0.5C). From my perspective, a charge rate at or faster than a 1 hour charge time is considered fast charging to the battery perspective.

I'm glad you found it useful and clear!

I don't think you should worry about occasionally charges to 100%, primarily because 100% indicated SoC isn't 100% real SoC. I can't think of any reason a single charge to 100% would have any significant impact, so I wouldn't worry too much, and primarily try to avoid when temperatures are very high.

Charging to 100% *may* be more of an issue as the battery degrades more, however this depends upon how smart the Tesla BMS algorithms are, and whether they can account for something called stoichiometric drift, which I'll perhaps explain more in another post.

Thank you for the info about battery over-temperature protection!

Well that's very interesting! I was already doing most of the recommendations in this thread, but this one is new to me. I live somewhere it is below freezing for months at a time. What temperature does the plating start to become more significant? I'll have to change my current charging regime for winter months (I currently have a timer delay so the car sits as little as possible above 80%).

Anecdotally, I should add that my first EV is a Leaf. For the first 4.5 years it was my daily driver, and due to the charging limitations, I couldn't set it to stop at 90% (or any other value). So I used delay charging to have it reach 100% just before I left for work, then charged at work to 100% again before driving home. Once I got the Model 3, the Leaf has now had several months of minimal daily driving, but it often sits at 100% for upwards of 12 hours. It is degrading at a similar rate now driving only 7,500 km/yr as it was at >35,000 km/yr!

Interesting to hear of your anecdotal evidence about increased rate of degradation - clearly a very powerful effect!

On the subject of Lithium plating when charging a cold battery, does anyone know what happens when connected to shore power running the battery & cabin heaters? As far as I can tell, the battery, on one hand, isn't charging, but either (a) the current from shore power must be flowing through it (?) or (b) the inverter is powering the heaters directly, bypassing the battery?

I usually warm the battery using shore power in the winter, but if all power flows through the battery, that may not be the best plan ...

Anyone?

I'm not sure of exactly what happens, as I'm not sure what the electrical architecture is within the battery, though I'm 95% sure the power from the charger will go directly to the heater and bypass the battery.
 
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I thought after your first sentence you were going to say you agreed with me ... LOL.

AC to DC conversion doesn't mean the battery is involved. It just means DC is involved. The DC can be routed to charge the battery or to power the accessories. Just like it can be taken out of the battery to run the motor, or reversed and put back into the battery from the motor during regen. They can control the flow of power ... I don't see this point that AC-to-DC is happening as any proof that the battery is involved.

If you have 3kW available and need 12kW for accessories, you can (a) take 9kW from the battery and 3kW from 'shore', or you can (b) put 3kW "into the battery" while you are taking 12kW "out of the battery".

I don't think there's any proof one way or the other here.

Agreed - The power definitely enters the battery, but is highly likely to bypass the cells and directly go to the battery heaters, if indeed battery heaters are present within the pack. Given how well the Tesla pack is designed, I'd be extremely surprised if the energy was transferred via the cells. We'd need to look at the electric HV architecture to confirm.

@Zoomit implied heat was being generated using the motor rather than a separate PTC heater.
 
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I'd like to do that... but sharing a one-stall carport for both our EVs. Logistically it is a pain to get the Leaf to stop charging at anything less than 100% (no way to set it to stop charging before 100%) and we need it to be at 90% in order to fit in our charging routine. C'est la vie.


Sounds like you've found the real world limitations of carmakers trying to make things 'simple' for the customer....
 
I have a question for clarification on this. Is it the act of charging to 100% that damages the cell or leaving it set that? Think of it like a balloon, if you blow it up to "100%" then it'll stretch the rubber, but say you want a smaller balloon at 80% size, the rubber will be slightly loose and "damaged".

I've only ever charged to 100% SOC three times, but left immediately as it was done charging. I normally do 90% on trips now.
Would charging to 100% and leaving immediately damage the battery less than if I charged to 90% and it sat there for 8 hours all night until we left? (That being said, I still leave immediately after charging to 90%)

Getting occasionally upto 100% won't be too much of an issue if you don't leave it there for significant periods of time - this about it as the likelyhood of an undesired reaction increasing rather than an irreversible balloon stretch.
 
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I' m curious where you got the 94% number from? I posted something on Reddit a while back and supposedly a Tesla Engineer replied and gave me a lot little things on keeping the battery healthy. He said that below 94% SOC that they start seeing large drop off in diminishing returns in terms of battery damage.
It's near the point where regen disappears. For what that's worth.
 
I' m curious where you got the 94% number from? I posted something on Reddit a while back and supposedly a Tesla Engineer replied and gave me a lot little things on keeping the battery healthy. He said that below 94% SOC that they start seeing large drop off in diminishing returns in terms of battery damage.
It's near the point where regen disappears. For what that's worth.

Ya, I just picked some small amount over 90. I might charge to 90 and it’s sitting there for a while and I then know I’m about to leave “soon” so I slide it up to 94-ish and let it charge until I actually leave. I don’t choose 100 because I want to avoid that and I figure if I “forget to leave” for some reason the car will stop at 94.

I still get some regen, and I know the extra 4% will get “burnt off” pretty quickly on my trip, even if it’s just my commute home from work.

I also remember reading something about the Roadster or early S being limited to 92 or 93 or 94 or something like that for normal charging, so I figured 93-94 is safe, especially since when I do that I am typically leaving immediately and only staying above 90 for 15-20 minutes tops.

Was your reddit exchange from the pre-model-3 roadster and S days? Or more recent?
 
This is old and before they had 18650 cells let alone the newer 2170 cells, but it is one thing I had definitely read in the past that guided my 93-94 choice:

From Tesla’s blog, Nov 2006! A Bit About Batteries

The other factors affecting cycle life are tied to how the cell is used. In particular:

  1. Avoiding very high and very low states of charge. Voltages over 4.15V/cell (about 95 percent state of charge [SOC]) and voltages below 3.00V/cell (about 2 percent SOC) cause more stress on the insides of the cell (both physical and electrical).

  2. Avoiding very high charge rates. Charging faster than about C/2 (two hour charge) can reduce the cell's life.

  3. Avoiding charging at temperatures below 0° C. (Our design heats the pack before charging at cold temperatures.)

  4. Avoiding very high discharge rates. (Our pack has been designed such that even at maximum discharge rate, the current required from each cell is not excessive.)
Now it could very well be that the newer cells are different wrt the limit AND that Tesla’s “100%” is now actually this true 95% anyways :D ... but they still try to keep us to dashboard 90% or lower, so I am conservative in how much over that I will go, and only once a week max on average.
 
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More interesting quotes from that blog entry directly referencing cell voltages:

There is a huge difference in cycle life between a 4.2V/cell charge (defined by the manufacturers as “fully charged”) and a 4.15V/cell charge. 4.15 volts represents a charge of about 95 percent. For this reduction of initial capacity (5 percent), the batteries last a whole lot longer. Unfortunately, further reduction of charge has a much smaller benefit on cycle life. Understanding this tradeoff, Tesla Motors has decided to limit the maximum charge of its cells to 4.15 volts, taking an initial 5 percent range hit to maximize lifetime of the pack. We also limit discharge of our battery pack to 3.0V/cell and will shut down the car when the batteries reach this level.

[...]

The other significant factor that affects calendar aging is the charge state of the battery during storage. At higher charge states cells lose capacity faster. This is a second reason why we have limited our maximum state of charge to 4.15V/cell instead of 4.2V/cell. We also offer the driver the option of charging to only 3.8V/cell (~50 percent) or 4.10V/cell (~90 percent) to further extend calendar life if the full vehicle range is not needed on the next few trips. We advise and encourage a full (4.15V/cell) charge only when it is needed.
... again, this is from 2006, with early Panasonic cells? ... but still. Interesting.

It does seem that the Roadster’s “max” was ‘true 95%’, and they allowed you to choose 50 or 90% (true SoC), with a 2% true shutoff.

If you take that 93% available (2% to 95%) and they allowed you to choose to charge to 90% true SoC, that would be 94.6% of the SoC they are making available to you (88 above 2%, 88 / 93 available = 94.6%).

I don’t know how the Roadster dashboard SoC worked at that time. Was 2% true shown as 0% dashboard and 95% true shown as 100% dashboard? If that’s how it was then and is now, then 94.6% dashboard is 90% true, and the dashboard line at 90% is actually a lower true SoC (85.7% true).
 
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