There is an easy way to eliminate the vampire error in the number you are trying to calculate. Just take a continuous drive over a decent amount of miles. Then take the kWh consumed shown on the dash(should no or very low vampire loss) divided by rated miles consumed for the same drive. Then take that number and divide by the rate at which you charge rated miles back into your car. For your car it should be around 295 Wh/mi (but that is for U.S. rated miles). For comparison, my final number comes out to 94% efficiency.
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Car’s energy consumption (lack of) accuracy
- Thread starter RuiP
- Start date
Seems like you're seeing mostly battery internal resistance. The car measures the input energy and output energy but can't really account for internal battery losses. And those losses are higher at higher Wh/km power draws, as your chart shows. That all seems reasonable.
I'm not sure I'd like an energy consumption Wh/km reading that included vampire losses. That would make it kind of hard to figure out how many kWh it takes to drive your commute each day. It would vary depending on how many days/hours the car sat parked. Then we'd have a lot of posters complaining it was inaccurate. I also don't think using the BMS SoC number would be very accurate.
Actually I like your chart. We always suspected the reported Wh/km (or mile) was low when compared to expected battery capacity. It may simply be that we could get the full claimed battery capacity if we discharged the battery at 1 W. Your 94% at 170 Wh/km is not too bad.
The car can measure battery output power (or input), and that should be fairly accurate. It can't measure battery internal losses, though it could probably apply a correction curve and make up a number. And that might come close to matching the energy that was input to the battery during charging. But that seems like a more derivative number than simple power out.
Given that you have the charging energy number, you can calculate your own Wh/km using that number. And that charging energy should give a much better indication of true battery capacity. But letting the car sit all week and not traveling any kilometers might give an (exactly) meaningless number for Wh/km. I'm glad that's not how it's calculated.
A few members have installed a separate electric meter just for the car. That would be the best way to measure your energy usage, including charging losses.
nwdiver
Well-Known Member
That's nearly impossible and definitely not worthwhile. Every module in the car has a BMS board that keeps the cells in balance to prolong the battery life by ensuring no cell is undercharged or overcharged. It does this by bleeding current through a resistor to drop voltage in cell groups that are higher than average. That's lost energy... you would need to track the current dump on each BMS board. What would the benefit of this be?
Parasitic losses, vampire drain and charging inefficiency are just part of the trade from ICE to EV. You save ~600wh/mi with the electric motor vs ICE, and pay ~30wh/mi in electrical losses... seems like a incredible deal to me. I never assumed the energy meter in the car was the energy used from the wall. How could it be? It was always my understanding that this was energy pulled from the battery. If I wanted to know how much it likely used overall I would just add ~10%....
You not testing the accuracy of the car... you're testing the 2nd Law of Thermodynamics... looks to me like it's real. Was there any doubt?
???? No.... that's 100% right... anything else is physically impossible... 2nd Law check SAT. You're not filling the battery with electrons... you're storing energy chemically. Chemical reactions require energy. You don't get all of it back.
.... only the ones that failed high school physics....
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jschwefel
Tesla fan/TSLA, Model Y and Cybertruck owner.
I tend to believe the whr/mile algorithm Tesla uses does not include the energy used by other systems in the car, i.e. heat, a/c, battery heating and cooling, etc. The algorithm seems to do a decent job when I charge, drive for a while and then recharge to the level I started at. On these trips, the energy it takes to reach the state of charge when I started the trip has been within 4% of what the car says it used.
While all of this is interesting, I don’t let it detract from the thrill of driving the car. It is by far the best, most enjoyable car I’ve ever owned. I’ll never own another ICE vehicle as long as there’s Tesla.
nwdiver
Well-Known Member
If I leave the car parked with the AC on for an hour or so I definitely see it on the energy meter. That's simply a measurement of energy pulled from the battery for any use.
It occurs to me that future BEVs might have a cycle counter that's essentially the BEV version of an odometer. Personally if I'm purchasing a used EV I'm much more interested in how many cycles are on an EVs battery than how many miles it's traveled. They could even make it sophisticated algorithm where cycling from 75%=>25% counts as 25% of a cycle (shallow cycles are far more gentle) while 90%=>10% counts as 80% of a cycle.
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jschwefel
Tesla fan/TSLA, Model Y and Cybertruck owner.
Agreed. The battery will show the usage, but will it show up in the whr/mile number?
nwdiver
Well-Known Member
The odometer kWh meter does not show any usage if the car is not moving. I think this has been verified several times by different people. That's what the vampire loss is. The battery reading will show the loss in rated miles or percentage. When the car is moving, all usage, AC, heating, etc., gets captured and displayed. That's why you will see the Wh/mi kick up when the heating or AC is turned on.
nwdiver
Well-Known Member
It doesn't UPDATE until you're moving but if I park w/o AC on my first 0.1 mile is ~400wh/mi. If the AC has been on for a while with the car parked it's >1500wh/mi...
hacer
Active Member
Besides the fact that the SOC display is already available to you, the SOC does jump around some, especially as the car ages. When it detects discrepancies due to observed voltage changes, it smoothly adjusts the SOC over time to compensate just to prevent the user from freaking out about it jumping it around. There are many reports of inaccuracies at the low end of SOC where the car still shows 10, even 20 miles of range but will not operate, similarly there are reports of reading zero but being able to drive another 20 miles. The fact is that it is hard to accurately estimate the SOC of even a single cell, let alone a battery with thousands of cells. If you can achieve 1% accuracy under all conditions then I think you'd be able to sell that for a few million dollars so go ahead and work on it.
Also I would point out that most ICE vehicles don't display the energy (fuel) consumption at all, let alone always displaying it perfectly. With a typical car, you have to manually take odometer readings and record the fuel you add to the tank. You're free to do this as well with the Tesla.
The first reading when you start driving doesn't mean much. 1500 Wh/mi for 0.1 mile is only .15 kWh. But if you are parked for hours with AC on, your car will be losing rated miles, but then when you start driving, it won't suddenly show 1kWh usage on the odometer, or whatever the energy equivalent is for those rated miles. It's just not tracked when the car is not moving, but it is tracked internally by the BMS.
I don't use AC, but my Wh/mi usually starts over 1000, but very quickly recovers to a normal range.
nwdiver
Well-Known Member
1500Wh is 1.5 kWh; And I only see it jump that high if energy has been used while sitting in park.
It's not reading 1500 in Wh, the reading is in Wh/mi, so 1500 Wh/mi for 0.1 mile = .15 kWh.
Are you saying your car shows 1.5 kWh used immediately after you start driving? I would doubt that.
nwdiver
Well-Known Member
That's precisely what I'm trying to say... the AC was consuming energy in Park. That consumption is reflected as 1500wh/mi when the car starts moving and the trip meter updates.
Yes, but what does the actual kWh usage say? That's the reading I am talking about.
nwdiver
Well-Known Member
I dunno... that part of the trip meter only displays miles, time, and wh/mi. But; if AC consumption isn't accounted for then why is my efficiency >1500 wh/mi for the first tenth of a mile when the AC was on but <500 when it wasn't?
hacer
Active Member
The "efficiency" for the supercharging you've shownn is false. (1) It is not considering the input energy to the chargers (and thus their efficiency) and (2) It presumes that all the energy flowing into the battery is stored chemical energy which is wrong, especially at the high currents that supercharging provides. Even so, supercharging might still be more efficient than 120V charging because the on-board charger isn't great when supplied with low voltage. It's certain that supercharging is less efficient than 240V charging because the superchargers are made from many parallel chargers equivalent to the on-board charger and the higher charging currents are guaranteed to have greater heating of the battery (which is lost energy) and because of that the cooling system must run to remove that heat which also uses energy. But it is way faster so those inefficiencies are worth it when you are travelling.
I completely believe the point in the OP's original post. I know about vampire drain and the fact that pre-cooling the car is not factored into the numbers. However, when I watch my usage on a trip when those aberrations are not a factor, the numbers simply never add up. To get rated miles my 85D should use 290 wH/mile. The real number appears to be somewhere between 270 and 280. I know this because if I watch the kWh used since last charge and the corresponding mileage, those are the numbers I need to be able to add the miles traveled since last charge to the current rated range to get back to the miles I left the charger with.
Also, and I know this is not scientific, my electric bill went up about $125 to $150 a month since I got my Tesla if I compare year over year. I pay about 23 cents a kWh and drive about 1,700 miles a month. My rough calculations are that this car cost between 5 and 7 cents a mile in electric. It is better than a comparable gas car, but not half, like Tesla advertises. In fact, I drove a Prius before this and I think it is an even swap on the cost per mile for fuel. Compared to a comparable S class you are definitely saving, but they are exaggerating how much.
In short, I think there are "optimistic" algorithms built in to make the operating costs seem cheaper than they really are.
Also, and I know this is not scientific, my electric bill went up about $125 to $150 a month since I got my Tesla if I compare year over year. I pay about 23 cents a kWh and drive about 1,700 miles a month. My rough calculations are that this car cost between 5 and 7 cents a mile in electric. It is better than a comparable gas car, but not half, like Tesla advertises. In fact, I drove a Prius before this and I think it is an even swap on the cost per mile for fuel. Compared to a comparable S class you are definitely saving, but they are exaggerating how much.
In short, I think there are "optimistic" algorithms built in to make the operating costs seem cheaper than they really are.
In your observations I believe your also witnessing the (normal) losses during charging, which are covered in this topic.
And yes, the ACTUAL cost of driving a Tesla (or any EV?):
wh/mile x cost of electricity
x 105% to 110% to account for charging losses
x 105% to 115% to account for discharge losses
x 105% for vampire drain (if you drive some 15k miles / year)
x 105% to 110% to account for charging losses
x 105% to 115% to account for discharge losses
x 105% for vampire drain (if you drive some 15k miles / year)
(and this is not accounting for pre-heat consumption during winter)
So, you need to add some 20% to 30% to the car's reported consumption, and THEN multiply by your electricity rate.
It could account for them by difference. Like this.
As for the rest of your post, including your doubts regarding the inclusion of vampire drain in the Wh/km reading, yes, I 100% agree with you. It could add some confusion / difficulty to compare or conclude anything.
Still, it would be nice to have the vampire drain accounted somewhere.
Irrespective of vampire drain, I think it should be imperative to account for internal resistance losses. (using a for-difference calculation; an approximation algorithm, or any other way).
I mean, right now Tesla shows us this (Total Energy) and that's definitely not correct. It's not Total Energy. It's "The Part of the Energy that Got to the Motors and Other Systems, but Not The Energy That Got Lost":
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