Vampire Charging

[GrahamW42]

No, not a typo, this isn’t “vampire drain” – it’s the opposite.

I do most of my charging at home, using a normal 13 Amp 3-pin plug.

The other evening I was charging and stopped exactly as 70% was reached. I slid the “limit” slider down to 50% (as I like to control when charging occurs).

The next morning (+14h) the app showed 69% - so I had lost between 0.001% and 1%. No surprise there, it could hardly be anything else.

I next checked in the evening (+26h) hoping for 68% (so between 1% and 2% for day) but expecting/fearing 67% (between 2% and 3%). The app said … 70% !!!

I went out to the car to get the story from the horse’s mouth (as the app has been known to differ from the car by a single % before). I found confirmation of 70% on the screen and dashboard. The limit slider was still on 50 % too. So the car has taken on a charge of 2% or 3% “by itself” !

I’ve never seen this behaviour before in a year of ownership.

I thought that I must have accidentally thumbed “start charging” on the app – but you can’t do that when the limit slider is below the current SoC. It’s simply not an option.

So why did it charge “by itself” ?

And why did it stop at 70% ?

Has anyone ever seen something similar ? Please tell me I’m not going mad !

 

[ewoodrick]

Battery voltage varies with temperatures, could be that simple. You are also looking for levels of precision where they don't exist.

Most importantly, stop worrying so much about the battery. Severe range anxiety.

 

[GregRF]

Happens all the time. Battery is constantly trying to calibrate and balance. I'll see large jumps in range after long periods of short commutes (~10% battery) and then a long drive where I drive to below 50% of the battery range. Suddenly the battery reports an extra 10 miles of range.

 

[tsh2]

I don't know if anyone already checked this, but it would be perfectly _possible_ for vampire drain to be satisfied through the charging circuits when plugged in. In the same way a laptop does.

There would be a hardware cost, but it would allow a car to be powered in the workshop with the battery removed - maybe.

 

[Silicon Desert]

Battery voltage varies with temperatures, could be that simple. You are also looking for levels of precision where they don't exist.

Most importantly, stop worrying so much about the battery. Severe range anxiety.

Amen on that comment !

 

[Silicon Desert]

Happens all the time. Battery is constantly trying to calibrate and balance. I'll see large jumps in range after long periods of short commutes (~10% battery) and then a long drive where I drive to below 50% of the battery range. Suddenly the battery reports an extra 10 miles of range.

yes, reminds me of what happened when I got back from a long trip last week. The battery was reporting I had 90 miles remaining when pulling in the garage, and then 4 hours later, it reported 79 miles (and same percentage change as well). Afterward, I didn't drive the car for 3 days and it still reported about 77 miles (same percentage as well). So sure, it can jump around at times, but over a period of several days, it lost about the amount I expected due to quiescent system draw.

Somewhere out there is probably a psychiatrist making a ton of money from people with severe battery anxiety

 

[doghousePVD]

First, stop oversweating the charging.

Second, I suspect the change to 50% charging was not "recorded" since there wasn't any discharge, no driving, and probably no disconnect from the charging cable.

Also, energy capacity numbers are inherently not very accurate. Unlike a fuel tank, measuring the amount of available energy in a battery is not an exact science. Batteries can appear to charge from resting, temperature changes, and, who knows, phases of the moon.

 

[arg]

I don't know if anyone already checked this, but it would be perfectly _possible_ for vampire drain to be satisfied through the charging circuits when plugged in. In the same way a laptop does.

The car only does this when it is "on". When the car is "off", the traction battery is disconnected with all remaining loads being powered by the 12V system (with the car occasionally turning itself "on" to recharge the 12V battery). The only output of the AC charger is at traction battery voltage, so it is possible to float across the battery when the car is "on" and achieve no net charge/discharge and all power drawn from the AC, but there is no route from AC to the 12V system without going through the 400V system. More recent cars have a low-power 12V output from the traction battery to reduce wear on the 12V battery, but still no direct route from AC to 12V.

Above is all for Model S/X (which presumably the OP on this thread has since he's had it a year). I don't have detailed info for Model 3, but everything I have read suggests it is the same in this respect, though with everything integrated inside the pack so it is harder to tell what is going on.

 

[animorph]

TeslaFi reported my X was charged to 227.12 RM/80% at 10 PM Friday. It was at 226.80 RM/80% at 10 PM Saturday. It was at 226.32 RM/81% on Sunday at 11:59 PM. So I lost a fraction of a rated mile but gained 1%! It stayed at 81% but went to 226.88 RM at 9 PM Monday. After that it reported 80% with 226.86 RM at 9:20 PM Monday. Currently it's sitting at 226.32 RM/80%. The car has been parked this entire time, of course.

Not sure how the API is calculating the percent numbers. Maybe it's a fluctuation in the estimate of a 100% charge rather than a fluctuation of the actual charge level. But it does fluctuate, and with very little change in the rated miles number. Could be a rounding thing as well since percentages are only reported to the nearest 1%, though in my case that can't be the only explanation.

Although we have had reported cases of actual unexpected charging, OP is most likely just minor charge calculation fluctuations.

 

[GrahamW42]

Hello everyone – OP here. Thanks for all the contributions,

For the record – I don’t suffer from anything like “battery anxiety”. However I am interested in how this stuff works.

I am well aware of the “estimated” nature of these things – so I never display range miles in order not to sweat about the odd mile going missing (or coming back).

In about a year of ownership I have only ever displayed battery % and have stored the car in a garage.

In hot summer weather and cold winter weather I have seen the car lose slightly over 2% per 24hr – presumably as fans/heaters cut in.

In milder spring and autumn (like now) it loses a bit less than 2% per 24hr.

I rarely let a complete day go by without a glance at the SoC. I have never, ever, ever, seen the SoC %age inexplicably go up by even a single %. Not once. This recent “incident” seems to suggest an increase of 2% (from my estimate of 68% back to 70%).

I suppose either I had my first ever vampire recharge or my first ever noticeable battery-rebalance complete with SoC re-estimate.

Either is odd.