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Weird battery level behavior

drklain

Active Member
Dec 17, 2016
1,052
1,050
Scottsdale, AZ/Fairfax, VA/Brussels, BE
This is a new one to me in four years of Tesla ownership. I'm used to the projected range numbers on the battery from my TeslaMate data tracker getting lower in winter as the battery is less efficient and spends more time heating the battery, but yesterday I saw something very odd. Took it to a supercharger to do a full battery charge and balance the cells. From the time the charge indicator indicated it was at 100% until it actually stopped charging was 40 minutes (when it hit 100% it was at 8KW of charging and slowly dropped over the 40 minutes). I've seen that behavior before. What I had not seen before is that on the trip meter, the battery level indicated 100% for almost 18 miles after I left the supercharger (see picture below). Then when it did start to drop, it stayed at 100% until the battery was at about 97% according the graph scale...and then it jumped to 99% (see second picture below). About 10 miles later the percentage readout matched the scale.

My first thought was that the computer was recalculating what "100%" is and it would normalize out, raise the projected range data, but it did neither. In four years of ownership, I've never seen it sit at 100% (or any other percentage for that matter) for such an extended period. Note this was all highway driving at around 65mph so I have a good feel for how the battery line drops at those speeds. The behavior was almost like an ICE car that has been topped off such that there is fuel in the fill pipe leading to the tank and the tank float stays at "full" for a while until that fuel all gets used up. Obviously there is no equivalent of storing excess energy in a BEV, but I am at a loss to explain this behavior. My guess is that the BMS calibration is off.

So my questions for the wise experts on this group: Have any of you seen this behavior before? Is it normal or a sign of a BMS issue? Is there a way to correct/recalibrate it? I'm looking at taking my car in for the four year service next month (yes I have the prepaid service plan and am debating whether to cancel it and get the refund or force the service center to do everything that is on the plan contract...year four is quite extensive). If I need to get the service center to do something regarding this potential BMS issue, that would be the time to do it so would appreciate thoughts/feedback/suggestions.

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PCMc

Member
Jul 1, 2016
76
55
Columbus, IN
I've seen similar behavior. The time the car continues to charge after hitting 100% can be widely variable. I've not tracked the max I've ever experienced, but it can be in the 10s of minutes easily and watch the charging rate is a good indicator of how close it's getting. I've tended to mentally associate, maybe falsely, the the more the pack or BMS is out of balance the longer it can sit in this state. I've also see it happen where I could drive some number of miles with the car continuing to show on 100%. 18 miles is a bit longer than what I recall experiencing, but have seen 5-6 miles at highway speeds. I've also had a few cases where the percent capacity line on the trips display has suddenly jumped or dropped. That's necessarily only been after a 100% charge, but I've had it in a couple other cases.
 

PCMc

Member
Jul 1, 2016
76
55
Columbus, IN
Regarding your 4 year service plan and "forcing" them to do everything on the contract. You may want to take a close look at the agreement. I was in the same situation a couple years ago and when I looked it says they will performed the required service, not a specific list of items. So there is nothing that holds them to do items that may have been on the maintenance schedule when you bought the plan that are no longer on the plan.

You may also want to look closely at the cancellation terms. When I did this it was based on prorated time or miles, not on what services were performed. So in my case, I was about 2,000 miles away from the last 50k mile of my four annual services and so they would only have refunded a small amount. What i determined is it was a better deal in my case to have they go ahead and perform, although reduced, services per the plan than to cancel.

I would encourage you to discuss with them; like most things, others have had experiences different than mine. If they would have refunded 25% of my cost, I probably would have done that, but that was not made available to me and per the language in my maintenance agreement, I couldn't force them to do that.
 

ucmndd

Well-Known Member
Mar 10, 2016
6,291
11,730
California
(when it hit 100% it was at 8KW of charging and slowly dropped over the 40 minutes).
In my experience a battery actually approaching 100% SoC will be charging nowhere near 8kw… more like 1-2kw. So yeah, something was off.
I doubt it’s a cause for major concern. I also bet if you did another range charge right away it wouldn’t happen a second time.

I assume you charge to 100% infrequently? How long since last time?
 

drklain

Active Member
Dec 17, 2016
1,052
1,050
Scottsdale, AZ/Fairfax, VA/Brussels, BE
I do charge to 100% very infrequently ...only for long trips or long driving days where I need the extra range. Normal charge level on a daily basis is 70%.

I'm wondering if I should do the "run the battery as low as possible and then charge to 100% to reset things" drill. Thoughts from anyone?
 

PCMc

Member
Jul 1, 2016
76
55
Columbus, IN
Personally, I've never bothered much with the intentionally run my battery low drill.

I do a decent amount of longer range travel where I'll naturally cycle the battery into the 20-30% range, but generally not much lower than 15%. I have on a couple occasions gotten down into the 6-9% range, but that's on the order of 3 times total over about 5 years and 60k miles. I will charge to 100% more frequently in the past couple years before I leave on a long trip. That's to help reduce the time of my first charging stop. I'll generally time my charging before a trip to allow for about and extra 30-60 minutes than what I'd think it will take just to help buffer how long that last 1% takes. Yeah, that means my car may sit at 100% for upwards of an hour, but I don't consider that to violate the "don't let it sit at 100% for long" category.

For the first couple years I owned my MS90D I would charge nightly. The last 1.5 years I've started to go several days without charging so that my battery will cycle through a larger range, down to around 40-50% and back to 80%.

While the run it really low, let it sit for a couple hours, then charge it all the way up drill might yield some benefits, I find that my run the battery through a wider range during normal operation does well enough. I've decided that it's not worth actually forcing extra stress on the battery just to try and eek out some small benefits.

Just my two cents worth.
 
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drklain

Active Member
Dec 17, 2016
1,052
1,050
Scottsdale, AZ/Fairfax, VA/Brussels, BE
Great feedback. I have a weird usage pattern due to my work. When I am in town, the car is either driven about 20 miles per day (to/from our offices) or about 100 miles per day (to/from where I keep my boat). I used the keep the car at a 60% charge...driving to the office and back would drop it to the low 50's, recharge that night and repeat. With travel to the boat, I now keep the car at a 70% charge which lets me drive to the marina and back arriving home at a 30% charge. That approach seems to keep the car in the 70-30 range that is generally considered "no damage" (or "less damage") to the battery cells from a degradation perspective.

My challenge is that I am traveling 2-3 weeks a month and when traveling the car is parked at the airport. I generally get there with around a 60% charge and then turn off all my apps and let the car deep sleep. Depending on how long I am gone, the car is generally in the 40-50% range when I return (losing about 1-2% a day). While IAD does have electric charging spots, they are generally taken and I don't feel good about blocking one for several weeks when I really don't need the charge to make it home and there are plenty of Bolts, I3s and the like who need the charge more than me. I often see Teslas parked in those spots who don't even bother to plug in, which strikes me as discourteous, but that is a whole other thread.

In any case, I'll try running the battery down and recharging to see what happens. Thanks for the thoughts!

P.S. - I agree that charging the battery to 100% in advance of a long trip, having it sit that way for an hour or two and then leaving for the trip does not violate the "don't keep it at 100% for extended periods" rule. My sense is the issue with Lithium Ion batteries is when it is plugged into a charger and basically left sitting at 100% for extended periods (meaning weeks/months). Kind of like the issue with old laptop batteries...
 

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