How accurate is the "since last charge" info? Troubleshooting excessive consumption

[Neon001]

Got my wall charger installed Monday afternoon, and set my charge threshold to 80%. Charged overnight and drove the car 115.4 miles since Tuesday morning (just over 48 hours). Arriving at work this morning my battery SoC was 30%. That's 50% SoC used over 115 miles, which doesn't seem completely ridiculous given that it's a Performance model, but it also indicated my total energy consumed at just 30 kWh, and average consumption rate at 251 Wh/mi.

This just doesn't add up. I've read the battery capacity without reserve is something like 71-72 kWh, so the 50% consumption for 30 kWh of use seems way off. Assuming it's 71 kWh total, 30 kWh used should be more like 42%. I did sit in the car idle with AC on for about 45 minutes yesterday at lunchtime, but I noted the battery only indicated a drop of about 1% the entire time. I find it hard to believe vampire drain would account for the remaining 6-7%, especially over only 2 nights. I do not leave AC on when the car is parked, and it's been very mild weather here since Monday (mid 70s).

For all I know, the "since last charge" info captures all usage since last charge (it should!). I doubt it, but it's possible. If it did, it should capture the vampire plus idle/parked usage too.

Can anyone explain this?

 

[Zaxxon]

The in-car trip computers capture only usage while the car is moving.

 

[AlanSubie4Life]

115.4mi * 251Wh/mi = 28.97kWh

For P3D:
28.97kWh/ (230Wh/rmi) = 126 rated miles (40.6% for a new LR AWD battery; unknown in general since % is a quantity that changes - it doesn't represent a fixed amount of energy)

So that's what you would have lost if you had done all that driving at once with no time spent in Park.

This is in line with your calculations. You lost 50%, so there is about 9% missing. Expect 1% per day for vampire/phantom.

As mentioned above, the in-car computers do not count usage when the car is in Park.

Other possible usage in addition to vampire/phantom:
Sentry mode (~24 rated miles per day)
Overheat protection (usage depends on the temperature & whether you have the AC mode set to on).
Car not sleeping for whatever reason (pretty rare but it happens).

 

[Superspectral]

I own a P3D+ and a Chevy Bolt. The Bolt uses virtually no charge when shut down. A/C and heat don't start until you press the start button. The Bolt also tracks energy usage whether you are driving or not. The Tesla P3D+ starts the AC as soon as you open the door. It runs a lot of fans and pumps whenever. It has a big computer. And as said before, it only tracks energy in drive. If I drive just a little bit each day, the Bolt and P3D+ have similar range. *However* if I drive the P3D+ in a single long distance trip it has 50+ more range. And, if you do one sitting you will get 70+KWH from 100% to 0%. If you do many short drives, then only 60kWH. It does seem at least 10% is used sitting around.

 

[lskarp]

I am out in South Florida and it is getting HOT. I see that my AC runs here and there to prevent cabin overheat (I have that setting, set). I got a sun screen for the windshield and it helps a ton when I am at work. Bottom line though, while I want to see great "mileage", it is pennies per day of usage, so I have to be better about getting over it. When I drive distance, I am getting great "mileage" and oftentimes better than the Trip Computer originally estimates by 3-5% over 150+ miles.

 

[darth_vad3r]

The in-car trip computers capture only usage while the car is moving.

Super duper slight tiny clarification — I believe it is actually only when in drive (or reverse), and not park. Not sure about neutral

So if you are stopped behind a 10-minute railcrossing while in D, you may see the trip meter update.

How it works I think is based on internal odometer and ‘wattometer’ which get snapshotted when you shift out of park, and then again when back into park. Then a list of these is combined for longer trips like “since last charge”, A, and B, while the since h:mm trip always resets when you shift out of park.

 

[Neon001]

Charged back to 80% at a supercharger yesterday, and the charging window told me the car took 42kWh of energy to do this. The since last charge info just prior to charging said I had used 32kWh. So 10kWh in non-moving usage...seems like a lot. I believe I had driven about 130 miles at that point, so my net consumption was 323 Wh/mi. Not great, but it could be worse I guess. That was only just over 48 hours though, so it still seems really high to me given that vampire should only account for about 1.5kWh of that.

As someone mentioned, it was likely that I made several shorter trips during that time. Not that I'm saying its false advertising to claim consumption rates in the 250 range, but given that it really only if you do it all in a single drive, I believe more folks should understand how usage will affect overall consumption.

Regardless, I can't be too upset because it's till very inexpensive to operate. Those 10 wasted kWh only cost me a little more than a buck, after all.

 

[AlanSubie4Life]

Charged back to 80% at a supercharger yesterday, and the charging window told me the car took 42kWh of energy to do this. The since last charge info just prior to charging said I had used 32kWh. So 10kWh in non-moving usage...seems like a lot

Not that I'm saying its false advertising to claim consumption rates in the 250 range, but given that it really only if you do it all in a single drive, I believe more folks should understand how usage will affect overall consumption.

A few comments:

There are some small charging losses even during Supercharging. No one has ever tried to measure them, to my knowledge (I’m sure they have but does not appear to be well known). They are significantly less than the ones you have at home (10-11%). The AC charging losses are counted in the EPA estimate.

So you actually “win” over EPA when you use a Supercharger, due to lower loss.

How many miles did you add? Did you note that? If you added 56%/174 miles, that would be about 5% charging losses. (42kWh/174rmi)/230Wh/rmi

The losses you had are generally higher than I would have expected for a 3-day window. But it is possible to use quite a lot of energy if you are sitting for a long time with climate control. And the Sentry mode (1kWh every 4-5 hours), overheat protection will add significantly too.

“Claimed consumption” for the P3D with 18” Aeros (so the AWD) is actually 290Wh/mi, the EPA rating. Not 250Wh/mi. Wall to wheel, assuming AC charging is used. But it does not include vampire losses.

Because you have sticky tires you do about 30Wh/mi worse than that. Which works out to 35Wh/mi after charging losses. So 325Wh/mi is what I would expect. It is possible to do quite a bit better if you live in a reasonably flat area and you avoid using regen (and the brakes of course).

 

[animorph]

Basically you can't make these comparisons due to various losses that can't be captured. Not that that stops tons of threads about this subject.

The best indicator of energy consumption is the kWh added while charging. That should get you close to the battery capacity.

How you drive affects how much energy you can get out of the battery. Drive slowly and you will get close to full battery capacity kWh on the trip meter. Drive fast, and accelerate fast, and you'll have a big gap, with the trip meter showing less usage than you'd expect. Losses in the battery is one reason why. At high power output more energy is lost as heat in the battery. That loss is not measured by car. Thus it looks like you put 70 kWh into the battery but only got 60 kWh out. Your battery got hot with the remaining 10 kWh.

Car’s energy consumption (lack of) accuracy

 

[AlanSubie4Life]

Drive slowly and you will get close to full battery capacity kWh on the trip meter. Drive fast, and accelerate fast, and you'll have a big gap, with the trip meter showing less usage than you'd expect. Losses in the battery is one reason why. At high power output more energy is lost as heat in the battery. That loss is not measured by car. Thus it looks like you put 70 kWh into the battery but only got 60 kWh out. Your battery got hot with the remaining 10 kWh.

I'm not going to say this is incorrect, and I'm familiar with the thread you linked to, but in general I've personally found this to not be the case. I always get an extrapolated 71-72kWh of capacity for a 310-0 discharge (so not counting the reserve) when I check the numbers.

I've actually heard (but not verified) that the meter reads LESS accurately (reads lower) for very LOW power draws. Personally, all I can say is that over a fairly wide range of usage levels (200Wh/mi up to 300Wh/mi), I get pretty close to the 230Wh/rmi number (it may be as high as 235Wh/rmi), when I measure carefully (there are quite a few ways to introduce error inadvertently) and calculate. (Extrapolates to 71.3kWh to 72.8kWh)

I'd be happy to see data to the contrary, but it's actually quite difficult to gather accurately, opportunities to measure accurately are rare (requires 100-mile continuous trips or so with adequate pre-conditioning and consistent temperatures), and of course most people don't really care!

I think what you're saying is probably to some extent true, but remember that heating losses in the battery are ALWAYS there to some extent. And even in the EPA test, they are there, yet they are still able to extract 78kWh from the battery (measured with calibrated instruments - the EPA discharge amount is measured to when the car literally can't move anymore - so no one ever gets to that point...or at least rarely). There is additional energy that is lost as heat during the discharge, that is never usefully accessible - it's just lumped into charging losses, essentially. Of course, for very high discharge rates (the EPA discharge rate isn't very high), I^2*R losses and internal resistance losses ARE greater, there is no getting around that. And those might well not be measured - but the meter error due to those unmeasured losses may in fact be less significant than meter inaccuracies for LOW draws (this is all speculation).

Personally, I think the most reasonable explanation is that the meter in the car reads a little low (and of course there is a substantial battery reserve below 0 rated miles). But the absolute value of the reading doesn't matter at all, as long as the vehicle you are driving is representative of what the EPA test article was (which is legally required). Your meter could read high or low by 500% and it wouldn't matter at all. My meter could read 100MWh/mi, and it wouldn't matter. You still have access to exactly the same amount of energy, and you are using it at a reasonable rate, consistent with the vehicle used for the EPA test. The meter is useful for relative performance comparison, and whether it is off by 500% or not, still allows it to be useful for that purpose. It's easiest to think of the meter as an arbitrary number generator that can be compared to itself, and used to make predictions about range (with prior empirical calibration for that vehicle), but nothing else (maybe can be compared to other users - depends on how well calibrated the meter is between vehicles).

The best indicator of energy consumption is the kWh added while charging. That should get you close to the battery capacity.

Mostly agreed - kWh added while charging, in the end, is what matters for cost, and is ultimately what determines your true efficiency (and you'll have lower kWh added at a Supercharger - but the per kWh price is a lot higher). However, can't be used to figure out the battery capacity in either case, because the charging losses are unknown (the EPA document suggests 11% charging losses or something - don't remember the exact numbers - and of course the losses are substantially lower when Supercharging).

 

[Neon001]

Just reading this as I was out of town for the last few days with no internet.

How many miles did you add? Did you note that? If you added 56%/174 miles, that would be about 5% charging losses. (42kWh/174rmi)/230Wh/rmi

I don't use the miles conversion. I prefer to use battery SoC. I will admit I'm new to the car, having only owned it a single month, however. I have checked the number of "miles added" using my app and seeing how many remaining free supercharger miles I have, and have calculated their usage there to assume a 400 Wh/mi consumption. This is great for me as I'm getting way more then 10k free miles from it, but only reinforces the "miles added" is just a calculation based on an assumed consumption. Obviously if I used it in the car it would use my own average consumption, but as we discussed, that does not factor non-rolling energy use...

The losses you had are generally higher than I would have expected for a 3-day window.

This is the crux of my point. And it wasn't a 3 day window, it was 2. It was almost exactly 52 hours from the time I stopped charging to when I plugged the supercharger in.

But it is possible to use quite a lot of energy if you are sitting for a long time with climate control. And the Sentry mode (1kWh every 4-5 hours), overheat protection will add significantly too.

I believe I mentioned that I did have a 45(ish) minute session sitting with AC on, but it was a very temperate day and I was mainly doing it to keep the bugs out of the car, so AC was probably hardly functional. In any event, it only showed a 1-2% drop in SoC.

Regarding the Sentry Mode loss, if that is indeed the power drain for sentry mode, that's ridiculous. 1kWh every 4-5 hours is 200-250 W of power to run sentry mode. That's absurd for 3 cameras and the data recording back end. Just thinking about that, if I park my car for 1 week at the airport with sentry mode on (I would definitely want it engaged in a parking lot), that would drain 50% of my battery capacity!

“...But it does not include vampire losses.

This is the heart of my issue. Full disclosure on overall consumption should come from someone. Obviously if there are minute losses from day to day vampire, I can see it, but losing 10 kWh relative to 32 kWh of driving use is NOT insignificant. And it's not like I had the thing "idling", AC blasting, stereo pumping, and lots of other accessories running for much of that time. I have sentry disabled at my home (where it is at least 60-70% of the day) and only "idled" the car with no stereo and AC on for approx 45 mins. Certainly not an unreasonable use.

 

[animorph]

This is pretty good, from Calculating the Battery Runtime - Battery University

So capacity can vary quite a bit with discharge rate (or driving speed), less capacity with higher speeds. Just generic Li-ion data, but does show a significant effect.