Battery % increasing without charge

[Inky38]

Ok so the other night I put my M3LR on charge with a charge limit of 80%

When I got in my car today, it said that the battery % was 82%

I thought I may have inadvertently caught the slider either on the screen or within the app, so I reviewed my logs in TeslaFi.

Sure enough, my charge completed at 80%
It then went into parked/idle for 41 minutes and gained another 2% of charge (5.52 miles of range added with an increase of 1.23 Kw) before then going to sleep.

anyone else seen this?

I’m guessing the bms is doing something here

 

[ucmndd]

I’m guessing the bms is doing something here

Yes. It happens.

 

[Phlier]

Any chance that the battery pack got warmer due to an increase in ambient temperature while the car sat?

I've seen my car's state of charge go up a percent or two when the battery pack got warmer after it was done charging.

Temp change? BMS doing stuff? Both?

Yes.

 

[jmaddr]

This is just another example of how the BMS is not exact. We read of users who are concerned over a 5 mile loss or a 1% drain when it can happen in your favor. In most cases, it's temperature related (charges to 80% when cooler at night and when the battery heats up due to ambient temperature it raises the used capacity up a bit). In both cases just know that the BMS charge reading is just an estimation.

 

[jmaddr]

This is a great writeup of how the BMS "estimates" battery charge:
How I Recovered Half of my Battery's Lost Capacity

 

[camalaio]

There are multiple ways this can happen.

• Temperature effects
  • Before the snowflake even appears, I've seen up to 3% "locked out" that's otherwise there, but not reported until the battery is near 25C/77F.
  • A charge while cold and then later warmed up could explain this, for example.
• Reduction in usable battery capacity post-charge
  • After the charge completes, the BMS may figure out you have less capacity remaining than it previously thought. In certain cases, this means whatever current capacity you had is actually a larger portion of your max capacity. That is, you didn't gain energy, your max energy at full just moved down, thus you have a bigger percentage.
• Increase in apparent usable remaining capacity
  • Similar to the previous point, but something changes regarding the estimation of the current remaining capacity. This could be a result of balancing, for example, if one group of cells is determined to be a less weak link of the chain than before.

• Impacts of rounding
  • The numbers are rounded, of course. 80% could've been 80.49%, and the 82% could be 81.5%. So potentially only just over a 1% spread to show as 2%, which makes either theory above even more plausible.

And there's probably even more ways, unfortunately none involving energy being magically added the battery.

As to how TeslaFi saw and measured that, I'm not sure. It's worth noting that TeslaFi is probably seeing a change in the number of miles and converting that to kWh. If that's the case, it discredits my second point, and credits both the first or third options (with the fourth point always being a factor).

 

[GtiMart]

I see that too sometimes after a charge. Read the writeup that jmaddr linked and don't stress about it