Range Loss Over Time, What Can Be Expected, Efficiency, How to Maintain Battery Health

[3sr+buyer]

Fortunately, Tesla builds the packs with very well matched original cells, so balancing is almost not even needed aside the occasional nudge to the higher cell groups. Generally deltas of less than 10mV in healthy packs. This is why hack jobs like module replacements and shops on YouTube claiming to "repair" modules by doing insane things like clipping cell fuses are 110% full of crap. The Tesla BMS has no ability to cope with modules that don't perfectly match up with the rest of the pack... as in, they need to be built and used together from day 1.

How does Tesla refurbish used packs that were presumably failed packs that have been returned after warranty replacement?

Since the non-failed cells in those packs have some degradation, would dropping new cells in for the failed cells result in too much imbalance for the BMS to handle? If so, do they look for cells from other returned packs with the exact same amount of degradation to build refurbished batteries?

 

[MP3Mike]

Since the non-failed cells in those packs have some degradation, would dropping new cells in for the failed cells result in too much imbalance for the BMS to handle?

Yes.

If so, do they look for cells from other returned packs with the exact same amount of degradation to build refurbished batteries?

If the are any failed cells, or group of cells under performing, the entire pack is sent for recycling.

The only packs that can be refurbished are ones that have fixable failures, like contactors, BMBs, etc. but the cells are all still good and balanced.

 

[IceMan8247]

I’ve seen both of those but question their source material. There is a lot of advice and guidance available, but it is hard to discern what is accurate. Some is generic and not specific to the Tesla BMS. Others are for different vehicle configurations or are outdated based on hardware or software revisions. Other advice is provided by Tesla service techs with questionable interpretation or translation from documentation.

There are only a few people on this forum that have the credentials (@wk057), usually gained from deep testing or reverse engineering, that I’d trust beyond Tesla documentation. And Tesla documentation is not infallible either as it may refer to out-of-date configurations.

Bottomline, I didn’t think the Model 3 2170 battery pack had a means of applying selective current to specific bricks to null pack imbalance via charging. I thought it could only use the bleed resistors to remove charge from individual bricks. I’d love to learn more if someone has firsthand knowledge.

Hey Zoomit - FYI I pulled the video down from Youtube due to the inaccuracy you brought to my attention yesterday. Thank you for getting @wk057 involved in the discussion. You guys really helped me out.

 

[Mechsmoto]

I'm at 259 miles 90% SOC. 22 M3P w/26000 miles and a dozen or so supercharges. All other charging done at home via wall connector. Just tried the single digit let it sit 3 hours, then charge to 100%. I didn't want to wake the car, and am thinking I took it off charge before it could balance the pack. Do I need to drain all the way back down to single digits and repeat to see a difference? Currently it's 2 miles less than it was before I tried the "fix". SMT shows balancing like this.

 

[KenC]

I'm at 259 miles 90% SOC. 22 M3P w/26000 miles and a dozen or so supercharges. All other charging done at home via wall connector. Just tried the single digit let it sit 3 hours, then charge to 100%. I didn't want to wake the car, and am thinking I took it off charge before it could balance the pack. Do I need to drain all the way back down to single digits and repeat to see a difference? Currently it's 2 miles less than it was before I tried the "fix". SMT shows balancing like this.

Scanning quickly, I could only see an 8mV differential. Doesn't seem all that imbalanced to me. Mine usually shows around 6mV imbalance, but I haven't tried balancing in several years.

 

[Mechsmoto]

Scanning quickly, I could only see an 8mV differential. Doesn't seem all that imbalanced to me. Mine usually shows around 6mV imbalance, but I haven't tried balancing in several years.

I saw another post you made about having extremely low degradation after 4 years and 50k miles!! Hats off to you on that sir!!!

You're right, usually it's around 6mv... I think I had the climate control running in that pic, so it was jumping between 6-8mv.

Honestly, I'm just hating that my first car had 76-77kwh at the same mileage, and this one that I very reluctantly paid even more money for.... is now sitting at just a hair over 73kwh. Was really hoping there was something I could "fix" to improve it.

 

[Candleflame]

Yes.

If the are any failed cells, or group of cells under performing, the entire pack is sent for recycling.

The only packs that can be refurbished are ones that have fixable failures, like contactors, BMBs, etc. but the cells are all still good and balanced.

that was my impression too. There's people who tried to be clever and replace modules on the Model S with exactly the same degradation with the hope it would avoid the issue- and the cells still went massively out of balance after a few months.

There is no real solution to module replacements except for the car to be able to shunt charge certain modules more or less....

 

[Mechsmoto]

Is 22mv imbalance acceptable during use? 75% SOC... idle.. ac on manual level 1.

 

[Mechsmoto]

Is 22mv imbalance acceptable during use? 75% SOC... idle.. ac on manual level 1.

 

[wk057]

> 20mV is borderline, depending on SoC. If you're at > 20mV no-load at 50% SoC, that's usually an indicator of an issue or pending issue. Not always, but these apps don't get enough info to truly diagnose anything at all.

 

[Mechsmoto]

> 20mV is borderline, depending on SoC. If you're at > 20mV no-load at 50% SoC, that's usually an indicator of an issue or pending issue. Not always, but these apps don't get enough info to truly diagnose anything at all.

Thank you sir! I read in another one of the posts your one the members who really knows these cars. I'm struggling with my 21 M3P having 76-77kwh available at same mileage, and somehow this 22 is only showing just shy over 73kwh.

I've had this sickening feeling that there is a bad module... but not sure how to confirm it. Can I let Tesla know I have SMT? Or will they blame it for my issue ifff the pack does indeed need to be replaced?

It's now at 59% soc with SMT showing 14-16mv imbalance. I wouldn't be so concerned if the modules matched. But I see atleast 2 models that have fairly big discrepancies between cells in the same module.

I hope I wrote that in a way that makes sense. Thanks so much again for your help!

 

[MP3Mike]

I've had this sickening feeling that there is a bad module... but not sure how to confirm it. Can I let Tesla know I have SMT? Or will they blame it for my issue ifff the pack does indeed need to be replaced?

Tesla won't touch it until it fails, or reports >30% degradation. (And they won't trust anything you provide from SMT.)

 

[wk057]

Tesla won't touch it until it fails, or reports >30% degradation. (And they won't trust anything you provide from SMT.)

This mainly.

If you're < 20mV delta at 40-60% SoC, then most likely there's no issue.

Pack capacities vary significantly, and tend to settle around their true initial values after a few months of use. For example, the S 100kWh packs have a wide range from about 93 kWh to 103 kWh of real capacity.... and Tesla considers all of these normal.

As @MP3Mike said, Tesla won't care and won't bother even looking at anything unless there's a failure, you lose > 30% from new (or rather, 30% from advertised EPA range), or there's a BMS error showing that needs attention.

 

[Mechsmoto]

Tesla won't touch it until it fails, or reports >30% degradation. (And they won't trust anything you provide from SMT.)

10/4 on that! Thanks so much again for jumping in to help fellas! Really do appreciate you guys!

 

[KenC]

I don't really look that often at SMT, when I drive, but I did notice that the imbalance can shoot up to 18mV. It fluctuates.

 

[wk057]

I don't really look that often at SMT, when I drive, but I did notice that the imbalance can shoot up to 18mV. It fluctuates.

You can't rely on readings from SMT and similar tools while pulling any load (driving, HVAC, etc) or charging. They take a full ~5-10 seconds to get the voltage readings from each cell group, and they'll always differ in that time period if not idle. So kind of useless under load/charge.

 

[Candleflame]

> 20mV is borderline, depending on SoC. If you're at > 20mV no-load at 50% SoC, that's usually an indicator of an issue or pending issue. Not always, but these apps don't get enough info to truly diagnose anything at all.

the problem is that thats all irrelevant unless tesla replaces the battery. and they only do this on an imbalance alert and to get there you need afaik around 100mv imbalance.

 

[Mechsmoto]

Thanks so so much again for all the input fellas!! You guys are awesome!! I LOVE the forums for this!! Being able to bounce different ideas around, and read through different thought processes... it's so much better than just overthinking it all in my own tiny little brain!!!

Today, I was parked when I was checking it, with a/c on manual 1... when I noticed it at 22mv. Then checked while driving to see what/if any difference there would be.

As far as I could tell it spent most of the day somewhere between 14-22mv of imbalance. It's charging up now. But I know KenC and I bounced some stats back and forth the other day and I think it was 6-8mv when charged... I'd expect it to be about the same in the am when I check.

Thought I had read about the 100mv difference triggering a replacement in another thread. Thanks for the confirmation on that Candleflame!!

 

[Vol16]

2018 LR RWD 91k miles
296 miles shown @ 100%
310 new, so ~4.5% loss
I’m very pleased

 

[Candleflame]

ill pick up the car on the weekend but new battery is in.

Seems to be refurbished pack. No clear if it is a 82kwh heavily degraded pack or just a standard 78.8kwh pack with 10%ish degradation.
Will see what scan my tesla says when i got the car back. The nice thing about a degraded 82kwh battery is maybe that it wont degrade that much further.