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What happened? Odd charging behavior at 100%.

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I’ve owned a 2023 Model 3 RWD with the LFP battery for about a year now, and charge it to 100% every couple weeks, but experienced some weird behavior I had never seen before the other day, and I’m wondering if anyone knows what was going on.

I always let the car sleep for about 3.5 hours after charging to 100% to make sure the BMS can take a calibration reading there. Normally, when I wake it up from this sleep, it doesn’t ever start charging again.

But the other day, when I woke it up from the app, it said it was at 100% with a range of 264 miles (1 mile less than the last few 100% charges, but not unexpected), and then it started charging at the max AC rate of 32 amps / 7.7 kW. My first thought was “ok, maybe it didn’t quite get to true 100% due to cold or something and is topping up now.” But after a few minutes, when it was still at 7.7 kW, I started thinking A) it wasn’t cold, it was 77 F / 25 C in the garage that night, and B) the car normally drops down to 3 or 1 kW in the last 5 minutes of charging when it’s at 99-100%, so the full 7.7 kW when it’s already showing 100% seems a little excessive. After about 10 minutes, it was still going, and I hit the stop charging button in the app to stop it, because I didn’t want the battery to get damaged if this was some kind of bug.

The range did go up from 264 to 265 during this little freak session, but when I look at the charge stats, it says that 1.01 kW was used, but only 0.13 kW was added to the battery for a terrible 13% efficiency.

Any ideas what happened here? Was this just balancing cells and should I have let it go until it finished on its own? If it was cell balancing, why wouldn’t it have happened on the initial charge to 100% a few hours before, and what’s the deal with it pulling in max power but barely any of that going into the battery? Where was that 87% charging loss going?
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I don't think I've read a single case of a battery overcharging in a Tesla, the BMS really seems to do a great job. I would try to find another reason for all that power being used. Maybe the AC or heat was running for example which might spend a few kWs, and those would potentially be pulled from the wall so the battery SOC doesn't reduce.
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