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DucRider

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I guess any basic textbook on batteries would do
I guess my google skills are lacking, as all references I can find are essentially:

"Battery capacity" is a measure (typically in Amp-hr) of the charge stored by the battery, and is determined by the mass of active material contained in the battery. The battery capacity represents the maximum amount of energy that can be extracted from the battery under certain specified conditions. However, the actual energy storage capabilities of the battery can vary significantly from the "nominal" rated capacity, as the battery capacity depends strongly on the age and past history of the battery, the charging or discharging regimes of the battery and the temperature.​
Please point me to somewhere that talks about "charge" capacity and "discharge" capacity as I am anxious to learn
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ohmman

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While your technical discussion of actual battery ratings is informative, don't forget that there is also a marketing component of battery capacity badging. Tesla has mislabeled total and/or usable capacity on a number of their packs.

Jason Hughes (wk057 on Twitter and at TMC), posted this at TMC a while ago:

  • Original 60 - ~61 kWh total capacity, ~58.5 kWh usable.
  • 85/P85/85D/P85D - ~81.5 kWh total capacity, ~77.5 kWh usable
  • 90D/P90D - ~85.8 kWh total capacity, 81.8 kWh usable
  • Original 70 - ~71.2 kWh total capacity, 68.8 kWh usable
  • 75/75D - 75 kWh total capacity, 72.6 kWh usable
  • Software limited 60/60D - 62.4 kWh usable
  • Software limited 70/70D - 65.9 kWh usable
So there is one other factor that plays into the advertised capacity - marketing.
 

ajdelange

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I guess my google skills are lacking,
I don't think it's your Google skills. It's what you find on the internet and your unfamiliarity with electronics that's responsible.

First hit I get on a search for battery capacity (Duc Duc Go, not Google) is to bateryeducation.com. It says, at the top of the page:

"Battery capacity is a reference to the total amount of energy stored within a battery. Battery capacity is rated in Ampere-hours (AH), which is the product of:
AH= Current X Hours to Total Discharge"

OK - that's just plain wrong. One ampere represents a charge transfer of 1 coulomb per second. One ampere hour is thus 3600 coulombs. A coulomb is a unit of electrical charge. A proton carries a charge of +1.602176634×10⁻¹⁹ coulombs and an electron -1.602176634×10⁻¹⁹ coulombs. Energy is expended or released when charge is forced through or falls through and electric potential difference expressed in volts. One coulomb/second (i.e. 1 ampere) transiting 1 volt releases or absorbs energy at a rate of 1 joule/second = 1 Watt.

Wikipedia says "A battery's capacity is the amount of electric charge it can deliver at the rated voltage." At least they recognize that capacity is rated in units of charge but there is clearly a problem in this sentence as a battery is only at it's rated voltage for an instant of time. If I buy a 30 Ah battery for my Tesla it had better give me 30 Ah even as the battery discharges past the rated voltage.

pveducation.org says "The energy stored in a battery, called the battery capacity, is measured in either watt-hours (Wh), kilowatt-hours (kWh), or ampere-hours (Ahr). The most common measure of battery capacity is Ah, defined as the number of hours for which a battery can provide a current equal to the discharge rate at the nominal voltage of the battery. The unit of Ah is commonly used when working with battery systems as the battery voltage will vary throughout the charging or discharging cycle."

batteryuniversity.com says "The tests followed SAE J537 standards by applying a full charge and a 24-hour rest, followed by a regulated 25A discharge to 10.50V (1.75V/cell)" In case it's not obvious the capacity is 25 times the number of hours the battery takes to discharge to 1.75 V/cell.

Here's another one that conflates energy and charge: "The capacity of a battery or accumulator is the amount of energy stored according to specific temperature, charge and discharge current value and time of charge or discharge.
..... so it takes two hours to charge the battery at the rating capacity of 1000 Ah:

So based on my Googling (or Ducing) I guess I can see why someone with no technical background would be confused.
Please point me to somewhere that talks about "charge" capacity and "discharge" capacity as I am anxious to learn
If you really want to learn go read my previous post. If you can't understand it then again all I can do is recommend some basic texts on electronics to get you to the point where you are comfortable with watts, joules, coulombs, volts etc. and then on to a text on batteries.
 

ajdelange

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While your technical discussion of actual battery ratings is informative, don't forget that there is also a marketing component of battery capacity badging. Tesla has mislabeled total and/or usable capacity on a number of their packs.

So there is one other factor that plays into the advertised capacity - marketing.
Absolutely agree! Especially in the current context. And that's why I question whether the capacity of my battery is discharge or charge capacity. i had an X100D. Everyone presumed that car to have a 100kW battery just as they assumed a car labeled 90D had a 90 kW battery. At that time the manufacturers were pushing range. Obviously the bigger the battery the more range you had so if it takes 100 kW to charge in order to get 90 kW to run which do you advertise? Keep in mind that one cannot really specify the capacity in terms of power without specifying a lot of other things. Coulomb charge is much more independent of these, especially in a "coulomb efficient" lithium ion battery.

I now have a X LR+ which presumably also has a 100 kWh battery. I just found out that there is a sticker visible in the passenger wheel well which has a number on it and I'd look at that except the car is 100 miles away at the SC. Anyway, there is an API which reports charges and discharges producing results that look like

Rivian R1T R1S Lucid Motors SUV char


From this data it is apparent that the energy needed to charge this battery to 100% SoC is
17.82/(.9-.72) = 99 kWh. I got similar results with the X100D. That's awfully close to the "advertised" X battery size of 100 kWh.

For drives it produces data like this

Rivian R1T R1S Lucid Motors SUV disch


From which it is clear that the battery will be completely discharged if 32.91/.36 = 91.4167 kWh are withdrawn.

So if Telsa wants to amp up how big the X's battery is, which they definitely did as it justifies the higher price for the cars with the bigger batteries, they would emphasize charge capacity (if their conscience let them) but if they want to amp up efficiency they would use the smaller number i.e. the discharge capacity.


There is definitely the opportunity to play some games with "battery size" and that is really what I am suggesting here when speaking of charging as opposed to discharging capacity,
 

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again AJ uses 6 ways to say the same thing to confuse you and still doesn't answer the basic question of what is "charge/discharge capacity" mean? AJ why can't you just answer he question what is "charge/discharge capacity" not "battery capacity"!
 

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ajdelange

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Charge capacity is the amount of charge you have to put in to fill the battery. Discharge capacity is the amount you can get out.

Think of a 5 gallon gas can. It takes 5 gallons of gas to fill it up. But it has a little leak or there is some evaporation or you spill some (whilst filling and/or emptying) so when you empty it you get less than 5 gallons. The capacity of the can is 5 gallons but it won't deliver that much gas.
 
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ohmman

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Anyway, there is an API which reports charges and discharges producing results that look like
It appears you're looking at TeslaFi, which I have also used. Note that it estimates data and doesn't get real consumption. In fact, there is a tweaking parameter you can use to get it to more accurately reflect what you're seeing in the real world. I had to do this during some of my towing trips because it was way off.

The only way to get more accurate data is through the CAN bus.
 

DucRider

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Charge capacity is the amount of charge you have to put in to fill the battery. Discharge capacity is the amount you can get out.

Think of a 5 gallon gas can. It takes 5 gallons of gas to fill it up. But it has a little leak or there is some evaporation or you spill some (whilst filling and/or emptying) so when you empty it you get less than 5 gallons. The capacity of the can is 5 gallons but it won't deliver that much gas.
EXACTLY!
Rated capacity is the amount you can get out and what you may be referring to as discharge capacity. If your 92(?) kWh figure for your Model X comes from one of your efficiency screens, you are likely looking at the system conversion efficiency and not the rated battery capacity (whether "charge" or "discharge"). The actual amount you can get will vary with load and temperature - capacity is a variable and the rated capacity is for a very specific set of conditions. Cpacity is measured between the manufacturer specified fully charged voltage and fully discharged voltage, usable capacity is almost always at a set of voltages that provides both a top and bottom buffer.

Vampire drain on a battery or a leak in a gas can does not change it's capacity - it still holds that same 5 gallons or 100 kWh. If some "leaks out" before you get a chance to use it, it does not make the vessel smaller.

According to Tesla, a 2020 Model X LR takes 114.8 kWh to charge under laboratory conditions.
Rivian R1T R1S Lucid Motors SUV 1598825741245

https://iaspub.epa.gov/otaqpub/display_file.jsp?docid=48716&flag=1
Does that make it's "charge capacity" 115 kWh? Or is that some other figure not related to the amount of energy it takes to fully charge the battery? AFAIK, Tesla has never stated an actual battery capacity for any of their vehicles (there was an assumption by many that the 60, 70, 85, etc in the model number designated battery capacity, but that claim was never made by Tesla and it turns out it was only loosely correlated).

There are charging losses (charging efficiency is almost always about 90%), and that is what you seem to think should be counted as "charge capacity". Batteries are rated by their manufacturers in terms of available energy, not the amount of energy it takes to charge them.

Once again, if you have reference to any site that discusses charge capacity vs discharge capacity, I would be glad to learn why all the other sites you referenced are wrong.

The measurements you are basing your assertions on are taken in much different conditions than the battery manufacturers specify, and you can never get total battery capacity from them anyway, even if you go from 0% to 100% SOC. The BMS does not allow you access to the full capacity of the battery.
 

ajdelange

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It appears you're looking at TeslaFi,
Yes, roger that. Now I certainly do have my questions about TeslaFi (and, indeed about the API from which it gets its data). What it reports to me indicates that it thinks rated Wh/mi = 260 and 1% of
battery is 260*351/100 = 912.6 W so that the battery capacity is 91.26 kW . In order to come up with 260 had to have that 91.26 kW number as an estimate of the battery capacity or it had to have 260 as a valid value of rated Wh/mi from some other source. If the real Wh/mi number is 300 then the discharge copacity of the battery is 0.3*351 equal to 105.3 kWh and not 91.26. With the Wh/mi then can estimate rated consumption by multiplying % SoC consumed by rated range and then by rated consumption e.g. 0.36*351*.260 equal to 32.85 kWh from the example.

Now when it comes to charging we again get starting and ending SoC percents from which the program calculates kWh added to the battery. The example numbers this time show a Wh/mi equivalent of 282 Wh/mi for charge. You have to put in more than a miles worth of energy in to get a miles worth out. While I not sure readers here grasp that the physics says this must be the case. So while I have a pretty good idea of what they are doing with the discharge (deducing kWh from rated consumption and SoC change) and while it appears they are doing the same for charging but with a different rated consumption I am not at all clear on this unless they have, a-priori, some knowledge of what the charge capacity is or, to put it other terms, some estimate of the battery round trip efficiency. Thinking about it for a moment I'll bet that's it.

In any event, accuracy of TeslaFi's estimates of capacities or consumption have nothing to do with the basic premise that what you choose to call the "capacity" of your battery if you specify capacity in energy leaves lots of wiggle room for marketers. That's one of the reasons engineers don't specify battery capacity in terms of energy.
 
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ajdelange

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EXACTLY!
Rated capacity is the amount you can get out and what you may be referring to as discharge capacity.
Sometimes. Since you apparently have trouble with written material lets try pictures.

Rivian R1T R1S Lucid Motors SUV dually-wrapped-paperplastic-wrapper-bagged-sterile


What's the capacity of the top pipet when filled to the 0 mark? Is it 25 mL? Well no it isn't. That's because it is a TD (to deliver) pipet. You must put more than 25 mL into it in order for it to deliver 25 mL. If you can't see the parallel between that and the battery situation then I am afraid we'll just have to accept that this is a concept you are incapable of comprehending.
 
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ohmman

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What it reports to me indicates that it thinks rated Wh/mi = 260 and 1% of
battery is 260*351/100 = 912.6 W so that the battery capacity is 91.26 kW .
And it's wrong on this point, which emphasizes what I was trying to say about the way that Tesla reports consumption in Wh/mi. The way to back it out is by using rated miles and EPA consumption. In the case of your LR+, you appear to have 98.8kWh of usable capacity, and an EPA rated consumption of 282Wh/mi. The 100kWh badging was relatively honest about usable capacity.

Again, I wouldn't use TeslaFi's (or the Tesla trip computer's) data as anything but a general guideline. The only reliable data comes from the CAN bus. If you have more interest in this topic, I definitely recommend heading over to TMC, where it's discussed ad nauseam.

To your broader point, there are charging losses going in and discharge losses coming out. Capacity should be rated in terms of "usable" energy - used for propulsion, temperature control, brakes, and loss to heat, sound or otherwise. It's going to take a lot more than 100kWh to charge a 100kWh pack, that's just the nature of it.
 

DucRider

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Now I certainly do have my questions about TeslaFi (and, indeed about the API from which it gets its data). What it reports to me indicates that it thinks rated Wh/mi = 260 and 1% of
battery is 260*351/100 = 912.6 W so that the battery capacity is 91.26 kW .
At best, even if accurate, this is only measuring usable capacity between the upper and lower cutoff voltages allowed by the BMS. This is different than the rated capacity of the battery
That's because it is a TD (to deliver) pipet.
Once again, you are confusing actual delivery in a real world situation with the rated capacity of a battery in laboratory conditions. There is no measurement you can take on your vehicle that can reveal the spec the battery manufacturer has placed on the battery as to it's rated capacity.
Charging losses and discharge/system losses are irrelevant to the batteries rated capacity, and the BMS lets you "see" only a portion of the battery to protect it from complete discharge and a true 100% charge.

Once again, do you have any references to support your claim of "charge capacity" and "discharge capacity"? The only sources you seem to be able to locate you contradict your claims.
 

ajdelange

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And it's wrong on this point, which emphasizes what I was trying to say about the way that Tesla reports consumption in Wh/mi.
I'm starting to get confused. You are saying that TeslaFi is wrong and that is related to to the way Telsa reports consumption in Wh/mi? Are you saying that Wh/mi isn't a good way to report consumption or that Tesla isn't measurng Wh and miles correctly?

The way to back it out is by using rated miles and EPA consumption.
Where does one get rated miles and EPA consumption information? I just drove my car 102.4 miles and the consumption was 274 Wh/mi. 30% of the battery was used during this trip. Thus it appears that my discharge capacity is (102.4*274/.3) = 93.52 kWh. But those of us who do this for a living (or did so before retiring at any rate) recognize that the Wh/mi, starting battery and ending battery charges are quantized to whole numbers. Thus there is uncertrainty of 1/sqrt(12) = 0.288 Wh/mi in the reported 274 and 0.288% in each of the percentages (start of drive and end of drive) which we differenced to get 30%. Putting those into the formula we find estimated battery size to be 93.52 ± 1.04 kW. Note that TeslaFi was not involved here. The numbers came directly from the car.

Thus I am most curious as to how you came to this conclusion:

In the case of your LR+, you appear to have 98.8kWh of usable capacity, and an EPA rated consumption of 282Wh/mi.

The 100kWh badging was relatively honest about usable capacity.
Note that there is no badging on this car. The number 100 does not appear in its description. There is a label on the front edge of the battery in the passenger side wheel well. It used to say 100kW 400 V (or 90 kW or whatever). That label now says only 400V.



To your broader point, there are charging losses going in and discharge losses coming out. Capacity should be rated in terms of "usable" energy
That's kind of tough to do as the losses depend on the discharge rate. I suppose it could be specified at a standard discharge rate. Of course internally among the battery engineers it is stated in Ampere hours (at a specified C rate).


It's going to take a lot more than 100kWh to charge a 100kWh pack, that's just the nature of it.
Some people think I am talking about the energy going into the charger. I am not. I am talking about the energy going into and coming out of the battery terminals. If the charge and discharge rates are both small then there are few I^2R losses in the battery internal impedance in either direction. Every doubling of the rate increses the spread by a factor of 4.
 
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ohmman

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I'm starting to get confused. You are saying that TeslaFi is wrong and that is related to to the way Telsa reports consumption in Wh/mi? Are you saying that Wh/mi isn't a good way to report consumption or that Tesla isn't measurng Wh and miles correctly?
No. I'm saying that what Tesla reports to the IC trip meter is not entirely accurate or reflective of the actual energy consumption. Again, this has been dissected already, and the best place to look is TMC. I don't want to provide links away from the Rivian Forums out of respect for this forum, and especially since I'm a moderator over at TMC, but there's a search feature there.

Where does one get rated miles and EPA consumption information? I just drove my car 102.4 miles and the consumption was 274 Wh/mi. 30% of the battery was used during this trip. Thus it appears that my discharge capacity is (102.4*274/.3) = 93.52 kWh. But those of us who do this for a living (or did so before retiring at any rate) recognize that the Wh/mi, starting battery and ending battery charges are quantized to whole numbers. Thus there is uncertrainty of 1/sqrt(12) = 0.288 Wh/mi in the reported 274 and 0.288% in each of the percentages (start of drive and end of drive) which we differenced to get 30%. Putting those into the formula we find estimated battery size to be 93.52 ± 1.04 kW. Note that TeslaFi was not involved here. The numbers came directly from the car.
Again, the numbers "directly from the car" are only valid if you're getting them from the CAN bus. Rated miles on your X are 351. Rated EPA consumption shows up on the trip chart as a dotted line and is published elsewhere.
Note that there is no badging on this car. The number 100 does not appear in its description. There is a label on the front edge of the battery in the passenger side wheel well. It used to say 100kW 400 V (or 90 kW or whatever). That label now says only 400V.
Yes, I'm aware they've changed that, but it seems that the pack is the same as the 100D. The Ravens just have a more efficient drive.

It strikes me that you're trying to calculate something that's already been determined, but through multiple layers of abstraction instead. No need to reinvent the wheel - check out the quite complete work done by others. Jason Hughes (@wk057) was probably the first to take apart a Tesla pack, and was the first to find references to 100kWh packs in the software (before they released them), etc. You'll find your questions have already been answered, I believe.
 
 




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