One more reason you can't get 'rated range'

[David99]

I found an interesting behavior of the battery doing several tests on road trips. The BMS reports the energy stored in the battery on the CAN bus. I compared the numbers right before I left and at the end of a drive. You would think if you have say 60 kWh in the battery before you leave and use 50 kWh driving you would have 10 kWh left. In reality I found that this never happened. I consistently see less available than the numbers would add up to.
Here are two screen shots of the numbers at the beginning of a drive and at the end. 1.3 kWh got lost on the way. Top row is the energy used on the trip. 4th row is the remaining energy in the battery. (I did the first screen shot a little too late when I was already driving hence the 0.11 kWh starting number. It needs to be added the the remaining capacity at the beginning)

I charged the car to 100%, drove for a little over 2 hours and stopped at the next supercharger. The CAN bus tool I use (Scan My Tesla) counts *all* energy used. The missing energy isn't something else that wasn't measured or not accounted for.

So where is the energy lost? I have done this test several times and the amount of missing energy varies a little. Usually between 0.4 and 1.5 kWh. I believe that the energy reported by the BMS when fully (or partially) charged is correct. The energy measured at the end of trip that has been used is slightly lower because of battery internal losses. Depending on how you drive the losses from the internal resistance vary. Electric losses are 4 times higher at double the amperes. So if you drive with a heavy foot you are losing more. The above example was me driving pretty fast especially going up hill at 85 mph hence the rather high lost energy.

In the end its not a lot. Worst case it's 2% lost range. Just thought it was interesting to see those numbers.

 

[ewoodrick]

Are you driving within the parameters to get 'rated range'? You know, something like 65 mph?
Or are you thinking that you can get rated range when driving 80 mph?

In my Model 3, I find it quite easy to get rated range. Matter of fact, I think that the record is around 600 miles at about 40 mph.

It's not hard to get rated range.

 

[David99]

Are you driving within the parameters to get 'rated range'? You know, something like 65 mph?
Or are you thinking that you can get rated range when driving 80 mph?

In my Model 3, I find it quite easy to get rated range. Matter of fact, I think that the record is around 600 miles at about 40 mph.

It's not hard to get rated range.

Rated range is based ona certain consumption. Of course you can drive more conservative and get more miles than what is considered 'rated range'.

 

[ewoodrick]

I was responding to your title. You can get rated range.

Now as to your to your statements about lost energy, you are assuming that everything is measuring everything with 100% accuracy. Maybe it isn't

 

[David99]

Battery capacity can't be measured directly. The BMS is calculating the capacity from a lot of data. It is very accurate so is measuring energy going in and out. What the BMS can't predict is how you will be driving and what the internal losses will be. Hence the difference. If it was just inaccurate it would be off in both directions randomly. But each time I test it it's off coming in short.

 

[animorph]

Here's a more macro-level version of the same effect:

Car’s energy consumption (lack of) accuracy

The car can't measure internal battery losses, though it may try to estimate. They vary with how you drive, so predicting them precisely is nearly impossible. And given the nature of "losses", it always makes your range/measured consumption vs. battery capacity less than mathematically perfect, and always in the bad direction.

 

[ran349]

Battery capacity can't be measured directly. The BMS is calculating the capacity from a lot of data. It is very accurate so is measuring energy going in and out. What the BMS can't predict is how you will be driving and what the internal losses will be. Hence the difference. If it was just inaccurate it would be off in both directions randomly. But each time I test it it's off coming in short.

In terms of energy, the only precise value on your chart is the 55.8 kWh number. I assume that is equal to the 'since last charge' value on the dash, provided you did one continuous drive, but will also include energy consumed when the car is not moving, which does not show on the dash.
In your example, even if there was no missing energy as you showed there, you would have had to drive at about 278 Wh/mile to achieve rated miles, as opposed to the 295 'rated' value. That is close to a 6% difference.
So yes, rated miles is achievable for sure, but it requires better efficiency than some people realize to actually get there.

 

[David99]

Here's a more macro-level version of the same effect:

Car’s energy consumption (lack of) accuracy

I read the post. The difference is that it's a well documented fact that the car's trip meters do not count energy used when the car is in P. My car consumes approx 90-150 Watt being parked. When the car is keeping the cabin cool/warm or has other things to do it might be more.
The tool I used is measuring all energy used all the time. IOW some of the losses he noticed can be attributeed to the parking losses. But he also noticed the same effect. Hard driving has higher losses which can be explained by the ohmic losses.

Interesting is that before I left the BMS actually estimated that a little less is available (the 'expected remaining' value). At the end of the drive the battery was much warmer which means a lower internal resistance, hence the 'expected remaining' value us a little more than the 'nominal remaining'.

Anyways, I just wanted to share my findings.