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Battery Capacity Not Adding Up

Yesterday I charged to 99% and then drove 300 miles round trip without charging. I had 5% capacity left when I parked and my "Since Last Charge" trip meter showed 66 kWh used. Since I used 94% of the battery that day that leaves me to believe that my full battery capacity is 66 kWh / (99%-5%) = 70.2 kWh, which is less than I've read it's supposed to be. Granted, my actual 100% range would have been 319 miles, which is more than rated, but I'm curious what I'm missing from the capacity equation.

My trip start was at 300' of elevation and end was at 233', so 66' of net elevation changes there and I did climb and descend 7000 ft of elevation that day. The outside temperature varied between 55F and 85F throughout the day and I mostly ran the A/C in auto with a 70F set point. The car has 4000 miles on it and typically charges to 70% and discharges to 55-60% each day. I've charged it to 100% twice and supercharged 4 times from ~20% to ~70%.

Any ideas? Anyone else seen similar?
 

MP3Mike

Well-Known Member
Feb 1, 2016
18,055
44,490
Oregon
I had 5% capacity left when I parked and my "Since Last Charge" trip meter showed 66 kWh used.

Any ideas? Anyone else seen similar?

Yeah, the kWh used number is not all inclusive of power used. If you plugged in and charged back to 99% what did the number of kWhs added say? (It should be more accurate, at least on a L2 charger, since I think there is a known issue were it is reported inflated on Superchargers.)
 
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My guess is that Tesla's UI is conservative in terms of what the capacity/distance remaining is so that owners can have a little bit left in the pack to find someplace to charge when they "run out" of energy. If you could access the diagnostic menu that showed how much energy the car thought it had, that would tell you if there was a difference.

Pack capacity is also a function of the average discharge rate. The faster you pull energy out, the less energy you can get from the pack. If your rated round trip range was pretty close to Tesla's original EPA rating, I imagine you're not going to see much of a difference there, but if you were driving significantly faster, you might be able to get less energy out than if you were to drive slower.
 
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thefortunes

Active Member
Jun 14, 2013
1,095
1,346
Wisconsin
My guess is that Tesla's UI is conservative in terms of what the capacity/distance remaining is so that owners can have a little bit left in the pack to find someplace to charge when they "run out" of energy. If you could access the diagnostic menu that showed how much energy the car thought it had, that would tell you if there was a difference.

Pack capacity is also a function of the average discharge rate. The faster you pull energy out, the less energy you can get from the pack. If your rated round trip range was pretty close to Tesla's original EPA rating, I imagine you're not going to see much of a difference there, but if you were driving significantly faster, you might be able to get less energy out than if you were to drive slower.

NO NO NO. Many documented instances on this site of Teslas stopping right at zero (including my daughter) or even before zero. Please don't guess at things like this and propagate a fallacy.

ETA some have also driven past zero, but since it is NOT universal you should always treat zero as zero.
 
Not included in the total is:

1. All consumption while in "park". Mostly cooling/heating and standby losses.
2. Losses inside the battery pack, which increase with the average power output. (So, emptying the pack in two hours might give a capacity of 70 kWh, while emptying it in five hours might give a capacity of 75 kWh. Just to throw some numbers out there.)

Additionally, there *may* be unused available capacity under 0%. But it depends on the state of the BMS.
 
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If you ran it from 100% to 0, it would not use 75 kWh of charge. There is an upper and lower buffer that you can’t use to protect the battery. 70 is a reasonable number. On a Leaf, you only get 21 out of 24 kWh.
The Model 3 LR pack seems to have around 80 kWh, with 75-ish kWh available.

Also, there's no buffer at the top. If you charge to 100% you're actually charging to 100%. That's why recommended daily charge is 70-90%.
 
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SigNC

Active Member
Aug 23, 2017
1,620
1,527
NC
The Model 3 LR pack seems to have around 80 kWh, with 75-ish kWh available.

Also, there's no buffer at the top. If you charge to 100% you're actually charging to 100%. That's why recommended daily charge is 70-90%.

How do you know there is no buffer? Even if there was a few % buffer it would still be better to charger to 90% or under.
 
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How do you know there is no buffer? Even if there was a few % buffer it would still be better to charger to 90% or under.
I don't have any references handy, but when the 3 was first released a few people set to work on trying to figure every detail out as tends to happen with new products and the consensus at the time was that there was a bottom end buffer but not a top end buffer. This makes sense because going too low can rapidly cause irreparable damage to the pack while going to 100% is only minimally harmful and only when done repeatedly and for extended periods of time or when it's excessively hot.
 
NO NO NO. Many documented instances on this site of Teslas stopping right at zero (including my daughter) or even before zero. Please don't guess at things like this and propagate a fallacy.

ETA some have also driven past zero, but since it is NOT universal you should always treat zero as zero.
I don't think these two ideas are exclusive. Tesla can keep a certain amount of energy hidden from the user, and zero can still be zero when you hit it.

If Tesla decreases what you see as rated range/capacity compared to actual range/capacity, you'll still be out of juice when you hit 0, but the amount of energy in the pack at 5% may be more than 5% of the pack's actual energy. They can even include their estimate of battery capacity/range remaining as a function of your route if you're using navigation.

For example, if 5% on the screen is actually 10% of the battery's energy, 10% is 15%, 20% is 25% and so on, that could discourage the user from trying to push it too much in terms of range, but the car would still go dead if and when they hit zero.

That would also explain why the anecdotes I've seen suggest rated range drops more than most people expect it would as the pack approaches empty, except for certain situations like navigating to a supercharger. If Tesla doesn't know where you're going to drive/charge, and wants to minimize the chance you'll run out of charge, hiding more and more available battery energy as you use more and more energy driving around is a way to do that. If you're navigation destination is a supercharger, the system doesn't need to do that because it knows exactly where you're going and what you'll do when you get there. Or at least what you should do. ;)
 
It can go down quickly at end of charge because the tail end of the discharge curve for a lithium battery is abruptly steep and it's more difficult to get a reliable assessment of remaining capacity there. So while you may know you have 10% left right before you enter that portion of the curve, it's very hard to tell the difference between, say 7% and 3%. This would be another benefit to keeping a buffer at the low end - 0% "rated" charge could be established at a point prior to that cliff so that it's easier to predict remaining range before that point.

battery-discharge.png
 
How long was it charged before driving?
How long was it parked during your journey?
Any preconditioning used?

It reached 99% and stopped charging maybe 30 minutes before departure. The 300 miles spanned over 14 hours with about 4 hours of highway driving and 2-3 hours of low-speed mountain road driving with frequent stops for sightseeing. No preconditioning.
 

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