Calculating Battery Capacity / Degradation

[rdrcrmatt]

Math is hard! If you input the right number, the post above becomes:

"So, if 55.1kwhr = 78%, then 100% = 70.64kwhr usable capacity.

So going with that chart, I'm at 93% of my original capacity. So, I've lost 7% (70.64/75.9)"

Yea, I fixed it and got those same numbers.

 

[islandbayy]

I came across a Youtube video by kmanAuto. I can't find it at the moment, he has a lot of videos. He was seeing his battery capacity drop when he went through a period where he had to charge up his battery to 100% several times in a month and he ran it down very low during the same time period. He saw his battery capacity come back. At the time of his video, his rated capacity was 1 mile more than when he got the battery.

He goes into a lengthy explanation what he thinks is going on there. It's more than I can explain from memory at the moment.

I suggest going through his YouTube videos and see if you can find the video. I think it was posted in the last year and he was going through a car wash to get some gunk off his car from parking under a tree when he recorded it.

Edit: Didn't read the original post closely enough. You might want to disregard what I said above if it's just lead foot driving...

I think these are the ones:

Followed by the explanation:

And you could tell I was a bit burned out by live as of the making of that video..... And yes, I went through the car wash twice. Sue me... No water shortage in Wisconsin What comes from Lake Michigan... Goes back Into Lake Michigan...

 

[mknox]

Huh? I thought all loads are measured on the trip meter while driving anyway. Seems like every load that gets turned on or off, such as HVAC shows up in consumption meter while driving. What loads are not calculated?

I have a separate sub-meter on my car's charging circuit and can assure you, there are things not accounted for by the car. The total energy reported by the car is significantly less that what I measure going in. Some of this is stuff like charger inefficiencies, cooling pumps running when charging and so forth. The "vampire losses" when the car sits are also not accounted for. As an analogy, it's like spilling gas when you fill an ICE car, and having a leaky gas tank. You still pay for all that gas, but the car's trip computer does not account for what you spilled at the gas station, and what leaked out of your tank in the driveway.

 

[apacheguy]

Vampire drain is not accounted for in the trip display. But this isn't really an issue if someone begins their journey shortly after charging completes. While the car is on, all loads are accounted for.

 

[liuping]

Vampire drain is not accounted for in the trip display. But this isn't really an issue if someone begins their journey shortly after charging completes. While the car is on, all loads are accounted for.

Has the accounting for the computers systems on the trip meter really been verified?

It the computers are running off the 12V system then it seems like they may not get counted in the trip display whether the car is moving or not.

I would think the vampire draw is always there, we just don't think of it as a vampire load when using the car. Also, it would be the fully awake "vampire" draw we had before sleep worked, so it could easily account for a kWh or two of power in a day trip.

 

[apacheguy]

When the car is on, the DC DC converter kicks in and supplies all 12 V loads directly. So yes it accounts for all ancillary draw in this instance.

 

[bonnie]

The title of this thread doesn't seem to be accurate anymore unless I'm missing something.

Seems like this is a great discussion on determining battery capacity. Could it be renamed to 'Calculating Battery Capacity / Degradation'? Easier to find during a search ...

 

[Zextraterrestrial]

the 70.XX seems pretty close to what I am seeing at around a 300Whr /mi usage trip
projected 100% numbers from this summer's longer legs (all over 60kWhr used)
70.845 70.952 70.933 71.735(used 98% for 234.4 mi in hills, pretty ideal temp...a touch of tail wind leaving petaluma then headwind into the humbay area)

the most I have used is 75.9 kWhr in 2013 (firmware 4.4) was at 266 rated at 100% then, currently seeing 253-255

 

[wraithnot]

Vampire drain is not accounted for in the trip display. But this isn't really an issue if someone begins their journey shortly after charging completes. While the car is on, all loads are accounted for.

I don't think the "trip meters" and "since last charge" numbers account for energy use when the car is on, but in park. I was stuck on the Richmond/San Rafael bridge for two hours with the climate on a couple of months ago and I watched the rated range decrease, but there was no change in the values for the "trip meters" and "since last charge". I have yet to do a similar experiment when the car is in reverse or neutral . . .

 

[rdrcrmatt]

When the car is on, the DC DC converter kicks in and supplies all 12 V loads directly. So yes it accounts for all ancillary draw in this instance.

I don't think this is accurate. The service center said one of the reasons the cars have been eating 12V batteries is that they let the voltage of the 12v drop some amount that causes the batteries to end up cycling out rather quickly. Perhaps they've changed that behavior in newer firmwares.

 

[Schlifey]

I really appreciate others pitching in to help me understand this. From my own knowledge, the Wh/mi shouldn't affect my energy usage vs percentage. I tried to stay away from the "distance" indicator because there are no real numbers to work with, just miles rated. The point I was trying to make, and I know it's not solid evidence with 60% of my battery, but I drive 45 miles, each way, per day for work. I used to be able to go to work, back home, to work, and back home again without charging. Lately Ive noticed that after one trip back and forth, I'll be around 35-40%% max (with a 90% charge).
I watched it again today and the low degradation others are getting just doesn't make sense to me. All the power used should be accounted for on the "energy used" tab. Every % should be 1.25 kWh. When my % is going down much quicker than it should, according to the simple math, it raises questions. I just don't think miles rated is accurate because it could say 350 and you couldn't really tell if that is correct because everyones driving is different. I hope I am wrong but the numbers are all I have to go on in my car.

 

[HankLloydRight]

I really appreciate others pitching in to help me understand this. From my own knowledge, the Wh/mi shouldn't affect my energy usage vs percentage. I tried to stay away from the "distance" indicator because there are no real numbers to work with, just miles rated. The point I was trying to make, and I know it's not solid evidence with 60% of my battery, but I drive 45 miles, each way, per day for work. I used to be able to go to work, back home, to work, and back home again without charging. Lately Ive noticed that after one trip back and forth, I'll be around 35-40%% max (with a 90% charge).
I watched it again today and the low degradation others are getting just doesn't make sense to me. All the power used should be accounted for on the "energy used" tab. Every % should be 1.25 kWh. When my % is going down much quicker than it should, according to the simple math, it raises questions. I just don't think miles rated is accurate because it could say 350 and you couldn't really tell if that is correct because everyones driving is different. I hope I am wrong but the numbers are all I have to go on in my car.

With all due respect, I think you should go back and re-read what people have posted, as most of your answers are there. Your "simple math" makes too many inaccurate assumptions to make any valid conclusions.

To summarize: First and foremost, you need to rebalance your battery to restore the full charge capacity.

All the power used should be accounted for on the "energy used" tab.

It's not.

Every % should be 1.25 kWh.

It's not. This depends on many factors (balanced battery) and assumptions (what is a "full charge" in kWh available to you? -- nobody really knows for sure -- it's not 80kWh).

When my % is going down much quicker than it should, according to the simple math, it raises questions.

I think people already gave you many suggestions as to why this might be the case, the least of which is severe battery degredation.

I just don't think miles rated is accurate because it could say 350 and you couldn't really tell if that is correct because everyones driving is different.

I think you're being misled thinking that using percentage is more accurate than rated miles --- but it's not because the number "90%" you're using is a moving target -- the actual rated range that 90% represents is going down due to an unbalanced battery, not battery degradation.

As I have suggested, try charging your battery to 100% a few times (and really to 100%, not 99% or close -- let the charging totally finish), and then drain down to 5% or so and repeat a few times. You'll see your rated range increase substantially (but 100% is still 100%). Charging to 100% and checking the rated miles is independent of driving habits, and it's the best indicator of battery capacity/health. Like I said, no "simple math" or assumptions on available battery capacity needed. Just charge to 100% and see what your rated miles are. It's probably down to 240 or probably less. Rebalancing a few times should bring that back up to 255 rated miles or more.

Oh, and BTW, it doesn't matter how you drive to drain down to a low state of charge -- you don't need to measure that energy usage at 300 Wh/m (or 290 or whatever). Just drive normally and ignore the "energy used". It's only the 100% range charged Rated Miles number that's important.

Once you balance your battery a few times, and rated miles is still well below 250, then yes, you may have some battery degradation (and I'm sure it's not 15%!). But that's the only way to accurately measure it -- not messing with kWh assumptions, percentages, and the "energy used" values.

 

[mknox]

As I have suggested, try charging your battery to 100% a few times (and really to 100%, not 99% or close -- let the charging totally finish), and then drain down to 5% or so and repeat a few times. You'll see your rated range increase substantially...

...Once you balance your battery a few times, and rated miles is still well below 250, then yes, you may have some battery degradation (and I'm sure it's not 15%!).

But just remember, that balancing "trick" doesn't work for everyone. It didn't work for me... in fact I lost an extra mile or two after doing exactly this.

However I agree, 15% seems extreme. I'm at about 9%. I'm coming up on 60,000 miles / 2.5 years. I see 218 Rated Miles @ 90% and 242 Rated Miles @ 100%.

 

[FredTMC]

Huh? I thought all loads are measured on the trip meter while driving anyway. Seems like every load that gets turned on or off, such as HVAC shows up in consumption meter while driving. What loads are not calculated?

How I have been keeping track of battery degradation is by:
1-With a well balanced battery, drive the car to a supercharger that is far enough away to get the car (battery) fully warmed up.
2-Charge the car to 100% at the supercharger until charging stops completely. Reset trip meter.
3-Immediately start driving the car. Drive as steady as possible. No hard acceleration or very high speeds. Flat roads as possible. Freeway driving as much as possible to eliminate start/stops. Try to target 280 to 300 wh/mi.
4-Do not stop driving until you reach your destination with as close to zero rated miles as your comfortable doing. I have been driving down to about 5 rated miles.
5-Take note of kwh extracted from pack (Total Energy on trip meter).
6-Repeat steps 1-5 a year later or whatever time scale you want. Helps to do this when the car is new in order to get a base line battery kwh capability. Take note on how much less kwh you are able to get out of the pack year over year. This is your actual degradation.

Although not completely accurate or totally scientific, it seems to be about the best way to do it. If you don't have a baseline kwh capacity of your pack when new, use someone else's. I believe you can get about 77.9kwh or so out of an 85kwh pack (base on Bjorn's latest run 450mile run). Older rev packs might not be exactly the same.

I have been doing this every year or so on my car. I have about 1.5% pack degradation after 2 years and 50K+ miles on my 60kwh pack. I can still get almost 55kwh out of my pack last time I checked.

The way the OP is calculating pack degradation is just not a very good way of doing it. Using kwh added back into the pack during recharging is a terrible way of calculating pack capacity. The added overhead during charging can very greatly from charge to charge depending on how much HVAC is need to keep the pack cool during charging and other things.

View attachment 92709

yep. This is the correct way to measure battery capacity (IMO). I've done exactly the same test and got results nearly same (~54kwh used).

It's the most accurate test I've seen of how much energy capacity your battery has.


Range (miles) is really a subjective measure (depends on if you drive fast or slow - Wh/mile)

 

[David99]

Vampire drain is not accounted for in the trip display. But this isn't really an issue if someone begins their journey shortly after charging completes. While the car is on, all loads are accounted for.

Turn on the AC full blast and see if the energy used goes up while the car is standing. It will not. Some people think it will show up once the car moves again in form of a spike on the energy graph, but that spike is just from acceleration. So the energy graph and the trip meters don't count energy usage while the car is standing.

 

[apacheguy]

Odd. I see no reason why Tesla should not include all power draw while the car is on in the trip displays.

 

[mknox]

Turn on the AC full blast and see if the energy used goes up while the car is standing. It will not. Some people think it will show up once the car moves again in form of a spike on the energy graph, but that spike is just from acceleration. So the energy graph and the trip meters don't count energy usage while the car is standing.

I don't believe that is correct. If I am driving in slow, stop-and-go traffic with the a/c on, I will definitely show higher Wh/mi numbers than under the same conditions with a/c off, and it is reflected in the energy graph. The same is true when I'm using heat in the winter. If the car is on and standing, it shows (at least on mine) like a very high spike when I start to move, even if I just start moving slowly. I don't get this spike with a/c or heat off.

Where I don't see it recorded, and it never shows up on the graph is if the car is "Off" but the HVAC is or was running. For example, if I turn it on from the mobile app or if I'm sitting in the car with it "Off" (i.e. I haven't tapped the brake pedal and the speedo is not showing in the center display).

In other words, I don't see it recorded if the car is both standing and Off.