How is the remaining battery capacity computed?

[alechenne]

Dear All,

I'm the happy owner of a Tesla model S P90D Ludicrous since end of December 2015. It has now a little bit more than 12'500 km. Two days ago, I drove through Switzerland for a "long" trip and I decided to charge it at 100% the night before. The estimated km was 400 (which is correct based on the fact that I got between 363 up to 368 km when charged at 90%).
As the battery capacity is 90 kWh, the estimed km is based on 225 Wh/km.
When I returned back home at the end of the day, on the trip counter (dashboard --> see screenshot), it was indicated: 261.3 km, 57.6 kWh, 220 Wh/km and a remaining distance of 83 km. On the big display, it was indicated 21% remaining battery capacity.

I dont understand how it's computed; from my point of view, the remaining capacity is 90-57.6=32.4kWh --> 36% of a full capacity of 90lWh. And the remaining distance (at 220Wh/km) has to be: (32.4*1000)/220 =147 km, which is making a theoritical total distance of 147+261=408 km, which seems also loigical as the mean Wh/km of my trip is 220 and less than those estimated in the begining of the day (225).

Is my logic wrong?

Thanks for your feedback and imputs to help me to understand that logic.

Kind regards

P.S. Unfortunately English isn't my mother tongue, nevertheless I hope you can understand my question.

 

[GHammer]

The biggest problem with your calculation is that the entire 90kWh of the battery is not available for use. Some of the capacity is reserved to protect the health of the battery; by some estimations as much as 11kWh.

 

[alechenne]

The biggest problem with your calculation is that the entire 90kWh of the battery is not available for use. Some of the capacity is reserved to protect the health of the battery; by some estimations as much as 11kWh.

Hi Hammer,

Thanks for your reply. If I apply your logic (which seems me totally ok), I would find that the "spare" capaciy if about 14 kWh (36-21% of 90 kWh).

Kind regards

 

[David99]

As Hammer said, the total usable capacity isn't 90 but a little less. Not sure exactly how much on the 90, though. On a new 85 it is 79.7 kWh. That information isn't presented to the user, but you can get it (among a lot of other things) get at the CAN bus where the service people hook up their laptops.
The total usable capacity also depends on temperature and a few other things. It is not unusual that the total capacity changes when the battery gets warmer or cools down. I have a little app that shows me this (and a few other things) and I see that number go up and down a little 'randomly'.
Interesting is also there are two percentages. One that the user gets to see and then the total battery capacity that you can only read when looking at the CAN bus. When I charge to 100% both show 100%. When the car shows 0 miles left and percentage also shows 0 but the total percentage shows 5% left! In other words, the car hides about 5%. Now don't get too excited, those extra 5% are not automatically usable. Some have been able to drive 24 km after the car showed zero, others had the car shut down right at zero or even a little before. I have always been able to drive a little passed zero. But again, don't count on it.

BTW, the trip counter on the dash board only counts energy while the car is moving. If you stop, even just at a stop sign or red light or for a break, the counter will not count energy, even though the car still uses energy! So the total kWh number might be off.

 

[alechenne]

As Hammer said, the total usable capacity isn't 90 but a little less. Not sure exactly how much on the 90, though. On a new 85 it is 79.7 kWh. That information isn't presented to the user, but you can get it (among a lot of other things) get at the CAN bus where the service people hook up their laptops.
The total usable capacity also depends on temperature and a few other things. It is not unusual that the total capacity changes when the battery gets warmer or cools down. I have a little app that shows me this (and a few other things) and I see that number go up and down a little 'randomly'.
Interesting is also there are two percentages. One that the user gets to see and then the total battery capacity that you can only read when looking at the CAN bus. When I charge to 100% both show 100%. When the car shows 0 miles left and percentage also shows 0 but the total percentage shows 5% left! In other words, the car hides about 5%. Now don't get too excited, those extra 5% are not automatically usable. Some have been able to drive 24 km after the car showed zero, others had the car shut down right at zero or even a little before. I have always been able to drive a little passed zero. But again, don't count on it.

BTW, the trip counter on the dash board only counts energy while the car is moving. If you stop, even just at a stop sign or red light or for a break, the counter will not count energy, even though the car still uses energy! So the total kWh number might be off.

Dear David99, thanks for adding information to Hammer reply. The way Tesla is counting energy is surprising, because they should have to take in account the global energy and not only those of the electrical motors. Of course, to do that, they also need to have the right sensors, but I couldn't beleive they don't have a current measure on the main battery. Anyway, both of you have replied to my question.

Kind regards

Alex

 

[David99]

Dear David99, thanks for adding information to Hammer reply. The way Tesla is counting energy is surprising, because they should have to take in account the global energy and not only those of the electrical motors. Of course, to do that, they also need to have the right sensors, but I couldn't beleive they don't have a current measure on the main battery. Anyway, both of you have replied to my question.

To avoid confusion, the car does count everything. Anything that uses energy is accounted for. The only odd thing is that it stops counting while the car is stopped. As long as you move, every electron is accounted for.,

 

[Boatguy]

If you display "Energy" or the percentage of battery, remaining, there is no calculation, it's just a measurement of the battery voltage applied to a table of state of charge (SOC) and voltage values.

The translation of SOC into rated range, projected range, etc. is where it gets more complicated. First, as others have noted, not all of the battery is available for driving.

Next, you can determine your car's assumed Wh / rated km with this formula (I'm guessing that the longest average Wh/km on your Consumption display is 50km; use the longest average consumption).

(50km avg Wh / km) / (rated range / projected range)

Projected range from the Consumption display and Rated range from the instrument display.

Now take the assumed Wh / km from above and multiply it by your 100% rated range and you'll know the usable kWh of your battery.

As others have pointed out, Tesla only includes power consumed while driving, but on a long trip vampire drain is irrelevant, until you park for the night!

 

[alechenne]

To avoid confusion, the car does count everything. Anything that uses energy is accounted for. The only odd thing is that it stops counting while the car is stopped. As long as you move, every electron is accounted for.,

David99, thanks for this clarification; this is now clear for me. Kind regards

 

[scaesare]

To avoid confusion, the car does count everything. Anything that uses energy is accounted for. The only odd thing is that it stops counting while the car is stopped. As long as you move, every electron is accounted for.,

To clarify: It's my observation that he car stops displaying the used energy while stopped, but I still believe it is accounting for it.

I say this because, the longer I sit in the car with it on in the morning prior to moving, the higher my initial average is once the 1st 1/10th of a mile clicks over. So the car was tracking that energy, even though it didn't display it until i started moving. I believe the same things happens during a driving session while stopped (i.e. at a traffic light), however that amount is not easily seen, as it;s relatively small in compared witht he overall driving average once you have a few miles on the trip.

This issue of not updating the displayed energy while stopped makes sense when you realize that the display is watt-hours used per mile traveled...

 

[David99]

I know what you mean when starting to drive in the morning, but I'm still not sure about the car counting the energy when stopped. Suppose you drive for 1 mile, then stop and blast the heater for 20 min. The energy used is not shown in their the energy graph nor the trip info. If it was actually accounting for it you would see a huge spike up once you start driving again. But it doesn't. When you start driving again after standing for a while with lots of energy used, there is no spike and the kWh number doesn't jump up. If it really was accounting for every energy it would have to add it in at some point, but it doesn't.

 

[scaesare]

Hmm... interesting point... have you tried that?

 

[David99]

Hmm... interesting point... have you tried that?

Yes many times. While waiting for my kids at their school or dance studio. I sit in the car with the AC running for a while. When I get back on the road there is no spike and the used energy number doesn't jump up.

 

[Boatguy]

I don't think there should be any debate that vampire loss is not included in any of the consumption Wh/mi displays. If my car sits in the garage for two days and loses 6-8 RM, the Wh/mi in the consumption display does not change, and when I get in the car and drive away the energy display does not spike up based on vampire consumption.

In fact because I live on a hill, after my first 1/2 mile of driving my "consumption" is negative!

 

[scaesare]

Vampire loss while the car is off, no. But the car does account for some initial startup energy, but apparently not necessarily all when not moving...

 

[Boatguy]

Vampire loss while the car is off, no. But the car does account for some initial startup energy, but apparently not necessarily all when not moving...

I've observed in my i3 that the car spends some energy heating the battery back if the ambient temperature is less than about 75F, and that makes initial consumption much higher. If it's a cold day and the cabin heat is also on, that makes it worse. So in that sense, "startup" consumption can be much higher than steady state. I've seen this in the MS also where the first mile can have much higher Wh/mi than the 10th mile.

I think overall we're all in agreement here.

 

[scaesare]

I think overall we're all in agreement here.

And perhaps all slightly puzzled.

 

[hacer]

Yes many times. While waiting for my kids at their school or dance studio. I sit in the car with the AC running for a while. When I get back on the road there is no spike and the used energy number doesn't jump up.

I think the distinction is that the power is not counted if the car is parked. If you try waiting for your kids at school with the car in D and the foot on the brake, you may get a different result.