BMS weirdness

[seenhear]

So yesterday I had a lot of driving to do, so started out the morning with a 100% charge.
I finished the day back home at about 8% charge. I needed to do some work in the garage, so had to park the car on the street for about an hour, maybe 2, before plugging it in inside the garage. When I finally moved the car from the street, the battery was reading ZERO%. That seemed like a lot of drain, but I didn't think anything of it, especially since it didn't have an issue driving the 30 feet into the garage.
Plugged it in and that was that... except I forgot to change the charge level, so it went up to 100% again that night. Well, sort of.
This morning, I checked the charge level, and it was indeed set for 100%, but the SOC reported was only 97%, despite the car reporting "charging complete." I was curious why it said charging complete when it was at 97% and the charge level was set to 100%. The day before it charged to 100% no problem. I tried unplugging it and then plugging back in, but it just immediately stopped charging and went to "charge complete" status with it still at 97%. I had to do some driving so wasn't able to look into it any further. It's now down around 70%.

I'm curious though if anyone has an explanation for why it stopped charging at 97%? Does this have anything to do with the methods for "resetting" the SOC % that the BMS reports?

Thanks...

EDIT: the car is a S75, Oct 2017 build. 110k miles. Typical "full" 100% SOC is around 211, but I forgot to check what the estimated range was this morning (or yesterday). I do have ScanMyTesla if that might help.

 

[Jillmark99]

The car prob charged to 100% then stopped charging and gave the charging complete message. Then vampire drained down to 97%

 

[brkaus]

I frequently see the car at 2% below the charge target.

 

[ewoodrick]

It's probably a temperature thing. The batteries are now cooler than when the charging completed.
After charging is complete, the unit cycles as needed. If you have sentry on or something pulling energy, it will normally go down a few percent before charging back up.

 

[ucmndd]

If you park the car below ~10% and let it sit without charging, you’ll “lose” like 5-8% charge as the battery cools. My car does this every time, without exception, as recently as yesterday.

I don’t think it’s actually gone, as in it’s not real vampire drain - just BMS drifting or locking out some capacity as the battery cools.

I suspect the charging complete at 97% is related too. But I also suspect if you run it down again and charge it back up everything will be fine.

 

[aerodyne]

It could be your pack is unable to change to 4.2 V on each string, and the BMS cannot balance the pack sufficiently.

Keep in mind that 97% is somewhere between 96.5 and 97.4%.

I'd install SMT or equal, and look at the voltage and CAC imbalance. It's been reported that a CAC imbalance of 20% will shut down the pack.

If you pack is starting to go you want to get it replaced under warranty.

It is my opinion that packs won't last much more than 8 years on the older cars.

 

[ResHacker]

I have experienced exactly same behavior while car left at SOC <10%. When charging then on SC the power against SOC was behaving just like I have 5% capacity more. So it is in real I believe some BMS SW capacity locking mechanism to push you to charge the car ASAP. But to be honest, even with those 0% it was absolutely no problem to drive few kms to nearest SC, even no other limitation has occurred.
Regarding the top SOC problem, I used to have it on 85kWh battery. On 100kWh it is still possible to charge to 100% or a little "above" as the BMS number is probably getting a bit pessimistic with the time while not topping it up in the meanwhile

 

[seenhear]

The car prob charged to 100% then stopped charging and gave the charging complete message. Then vampire drained down to 97%

Yeah, I've seen that before. However usually you can tell it to start charging again. This time I couldn't, there was no option to start charging. It just said "charge complete" at 97%. I also unplugged it for a few minutes, then plugged it back in, and it wouldn't start charging, it just went immediately to charge complete notice. This has never happened.

 

[seenhear]

It could be your pack is unable to change to 4.2 V on each string, and the BMS cannot balance the pack sufficiently.

Keep in mind that 97% is somewhere between 96.5 and 97.4%.

I'd install SMT or equal, and look at the voltage and CAC imbalance. It's been reported that a CAC imbalance of 20% will shut down the pack.

If you pack is starting to go you want to get it replaced under warranty.

It is my opinion that packs won't last much more than 8 years on the older cars.

Thanks. I'll take a look.
There are SO many metrics to look at in SMT. Can you me more specific on your instructions? What is CAC? Which voltage do I compare to CAC? Also, do I need to do this with the battery at 100% or any SOC?

 

[seenhear]

It's probably a temperature thing. The batteries are now cooler than when the charging completed.
After charging is complete, the unit cycles as needed. If you have sentry on or something pulling energy, it will normally go down a few percent before charging back up.

Yes, I've seen that before, but never that it won't let me start charging again. Usually if it's at 97% or whatever from drain, I can initiate charging again manually. Not this time

 

[Stefanv1993]

Ive had this same thing happen parked with 11% and returned an hour later to the car being at 4% only. Barley made it to the supercharger while driving several miles at 0%


Also had limit set before to 100% and it stopped at 97% saying complete and could not start charging again as it would just say complete.


Mine is a 2017 P100D for reference with 80k miles

 

[aerodyne]

Thanks. I'll take a look.
There are SO many metrics to look at in SMT. Can you me more specific on your instructions? What is CAC? Which voltage do I compare to CAC? Also, do I need to do this with the battery at 100% or any SOC?

Start with max min voltages under the battery tab. If more than 50mv at rest I'd be concerned. Less than 15 is a good sign.

You don't need to go to be 100%

Go to the SMT thread or contact the vendor about getting CAC values, not sure how that works.

 

[seenhear]

I repeated the same kind of situation last night/today, as I have another big day of driving ahead, so wanted to start with 100% (I rarely do charge above 70%, FYI).
Last night I ended up down at zero again: after letting the car sit for 30 minutes at 7%, then when I went out to pick up my kid from a meeting a couple miles away it was at 2%. Not a problem I figured, trip should be less than 4 miles round trip. I hit zero % after picking him up and drove about 1.5 miles on zero. Got home, plugged in with limit set to 100%. This morning it was at 94% with "charging complete" notice.

This time, I unplugged, drove kid to school, came home, and plugged in again at 92%, and the car decided it was still at "charging complete" and wouldn't accept any more charge.

Also the app indicated only 50kWh were added in that charging session... from zero to 94% ... should be much more than 50kWh.

 

[seenhear]

Go to the SMT thread

Which thread is considered "THE" SMT thread? I see many discussions involving SMT. Not sure if I see one that seems like a master thread for SMT stuff.

 

[aerodyne]

I repeated the same kind of situation last night/today, as I have another big day of driving ahead, so wanted to start with 100% (I rarely do charge above 70%, FYI).
Last night I ended up down at zero again: after letting the car sit for 30 minutes at 7%, then when I went out to pick up my kid from a meeting a couple miles away it was at 2%. Not a problem I figured, trip should be less than 4 miles round trip. I hit zero % after picking him up and drove about 1.5 miles on zero. Got home, plugged in with limit set to 100%. This morning it was at 94% with "charging complete" notice.

This time, I unplugged, drove kid to school, came home, and plugged in again at 92%, and the car decided it was still at "charging complete" and wouldn't accept any more charge.

Also the app indicated only 50kWh were added in that charging session... from zero to 94% ... should be much more than 50kWh.


View attachment 944455

Sound like a failure is imminent. How much time left on pack warranty?

 

[aerodyne]

Which thread is considered "THE" SMT thread? I see many discussions involving SMT. Not sure if I see one that seems like a master thread for SMT stuff.

Vendor - Scan My Tesla, a CANBUS reader for Android

I have a refresh Model S that I would like to use Scan My Tesla on. Exactly what is a "CAN Server"? Is there a cable like there used to be for the older Model S that allows you to plug in the OBDLink? Thanks. CANserver here: CANserver and manual: CANserverManual · joshwardell/CANserver Wiki...

teslamotorsclub.com

 

[ewoodrick]

I repeated the same kind of situation last night/today, as I have another big day of driving ahead, so wanted to start with 100% (I rarely do charge above 70%, FYI).
Last night I ended up down at zero again: after letting the car sit for 30 minutes at 7%, then when I went out to pick up my kid from a meeting a couple miles away it was at 2%. Not a problem I figured, trip should be less than 4 miles round trip. I hit zero % after picking him up and drove about 1.5 miles on zero. Got home, plugged in with limit set to 100%. This morning it was at 94% with "charging complete" notice.

This time, I unplugged, drove kid to school, came home, and plugged in again at 92%, and the car decided it was still at "charging complete" and wouldn't accept any more charge.

Also the app indicated only 50kWh were added in that charging session... from zero to 94% ... should be much more than 50kWh.

Has it been doing anything similar when you were charging to 70%?
Have you had a software update that may be impacting it?
What if you just charge to 90 or 96%.

100% is kind of a magical number and things can act a little differently.

But what could be happening is that if you rarely charge to 100%, then the BMS is getting the ability to recalculate and you are just caught up with the parameters resetting themselves.

Taking it to 90-95% periodically is recommended to allow the BMS to reset.

 

[brainhouston]

Does sound like CAC imbalance so BMS adjusts % display accordingly...
If in warranty, try Supercharging few times, ppl said it sometimes pushes the pack to fail...

PS, BMS won't allow any brick to go over 4.2V so thats why top % drops...

 

[seenhear]

Sound like a failure is imminent. How much time left on pack warranty?

As long as it doesn't pose a safety risk or strand me, I would love that. LOL
My DU and battery warranty are up Oct. 4th, 2025... so I have a lot of time left. Unlimited miles.
Incidentally I am scheduled for a DU replacement. Had to push the appointment back due to work travel this week. Was supposed to drop it off today, now next week.

 

[seenhear]

Has it been doing anything similar when you were charging to 70%?
Have you had a software update that may be impacting it?
What if you just charge to 90 or 96%.

So now that it's been a few days, and I've charged to various levels less than 100%, I can say that yes, definitely, it is not charging to the level I set the past few times. I've set it to 80% and 70% on different days, and in the morning it's always 3-4% below the target, and will NOT take any more charge; it says "charge complete."

100% is kind of a magical number and things can act a little differently.

But what could be happening is that if you rarely charge to 100%, then the BMS is getting the ability to recalculate and you are just caught up with the parameters resetting themselves.

I thought it might be in somesort of recal or reset, but it's been a few days now, and as stated above, not getting to the full target charge the last few times, even at less than 90%

Taking it to 90-95% periodically is recommended to allow the BMS to reset.

I tend to charge it to 100% several times per year, maybe slightly less than 1x/month. I do this out of necessity, not to try to force any BMS reset or anything; rather just because I'm going to need that range that day.