Battery Math Concerns

[OhLookItsRandy]

All,

I bought a Model s 100D. Looking at the Trip, I started doing some math, and I'm wondering if anyone else has similar calculations, or explanation.

I charge my battery to 80% every night. Upon entering the car, it is immediately at 79%. I'm attributing that to the fact it charges at 80.00%, and after getting in the car, it goes to 79.99%.

After my drive to work and back, I checked the trip. It said my use since last charge was 13.5 kwh. I would expect this to be 13.5% (100 kwh battery). However, my % left was 63%, meaning it used 17% (which would be 17kwh for a 100 kwh battery). could the radio/hvac really use 3.5% after only 38.6 miles? Doing the math with just miles, 13.5/17= .79 kwh/%. multiple that by 100, and the math would make the battery 80 kwh.

Thoughts? Ways to Check? I called support, and they said someone would have to call me back... 2 days ago. All the while my car is still out of alignment, I still have a mystery rattle, my windshield is scratched, and my GPS sporadically freezes me in a random spot.

 

[roblab]

The display does its own calculations. It has just a little to do with reality, and you learn to mentally calculate. It does try to figure elevation changes, which I don't do so well.

And anybody can do alignment. Really.

 

[OhLookItsRandy]

The display does its own calculations. It has just a little to do with reality, and you learn to mentally calculate. It does try to figure elevation changes, which I don't do so well.

And anybody can do alignment. Really.

Even the kwh used? You would think that would have little to do with calculations, and more to do with looking at the power leaving the battery.

Also, I understand everyone can do an alignment, but I wouldn't expect a 120k car to be delivered out of alignment. Not the point of the thread, just pointing out the lack of attention to finishing a product properly. Wondering if it could be an incorrect battery.

 

[mal_tsla]

The kWh is measured. The % remaining is estimated. It could well drive as far as your math says it should. Or it could not. The car is making its best estimate.

Don’t take those numbers so seriously. Drive with a 10% range cushion and you should always be fine.

Lithium ion batteries don’t work like a tank of gas. You’ll know how much energy was available in it when it stops providing it. Anything else is an estimate. Heck, even the low voltage cut off is an estimate of the battery’s chemistry. If you really want every last electron available from the battery, you’ll need to drain it until it catches fire. I’m sure nobody wants that in their $100k car.

 

[NeverFollow]

May be there is a problem with unbalanced cells?

 

[dhrivnak]

Unfortunately with most EV's the numbers do not quite add up. For example the new R80 pack in my Roadster has only 73 KWh. So is Tesla generously rounding up or are including the unusable buffer? But it is not only Tesla as my Volt has an 18.3 KWh battery but I only get to use 10.4 KWh.

 

[Johann Koeber]

It is not correct to assume the 100 in the 100D means it has 100 kWh of usable battery capacity. The battery capacity can be more or less than 100 kWh initially and will degrade over time. The degradation will depend on several factors including

- time (how old is the pack?)
- number of cycles
- depth of cycles
- number of extreme SOC (completely empty or completely full, and how long this prevails)

The Tesla battery reserves some kWh on the top end of the battery and some kWh at the bottom end - to protect the battery. The usable kWh are thus less than the nominal number on the car's designation. Also this number is probably rounded.

To find out, how much usable capacity you have, you could:

1 charge the car to full (100 %)
2 drive to deplete the charge as much as possible; to be safe you might want to discharge only to 10 %
3 check the 'trips' screen in the center console to see how many kWhs you have used
4 extrapolate to 100 %

 

[kavyboy]

It seems that the initial consumption is super high for the first mile or so, although 3.5% is pretty high. I often see an initial spike at 600-900+ Wh/mile in the first few minutes. I'm guessing it's battery maintenance, but it could be AC or heat. As noted above, the numbers are estimates, however the estimates are much less accurate for shorter trips (where it doesn't matter so much) and get much better for long range, where it actually counts.