Simple way to measure useable battery capacity?

[Kingske]

Is there a simple way to measure or estimate the useable battery capacity? The kWh charged as read from the PMCC charger display is a gross number which does not take any charging losses into account, I suppose? Can we read the net kWh added to the battery somewhere? Or is there a rule-of-thumb for the net/gross ratio?

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[NC_Taycan]

Someone else posted that on the charging display on the lower screen, if you tap the Miles added per Minute value, it will switch to kWh added. I haven't tried but I have seen pictures of this screen displaying kWh added. This should account for charging losses in the PMCC, but perhaps not in the battery electronics. Depends on where the data comes from.

 

[Scandinavian]

Someone else posted that on the charging display on the lower screen, if you tap the Miles added per Minute value, it will switch to kWh added. I haven't tried but I have seen pictures of this screen displaying kWh added. This should account for charging losses in the PMCC, but perhaps not in the battery electronics. Depends on where the data comes from.

Wien you read the kW from the PMCC and compare it to the charging display in the Taycan, there is close to a 9 to 10 % difference. That is a value that seems to correspond to some YouTube reviews as well, not only for the Taycan.

Here are two photos of a recent connection.

 

[evanevery]

Is there a simple way to measure or estimate the useable battery capacity? The kWh charged as read from the PMCC charger display is a gross number which does not take any charging losses into account, I suppose? Can we read the net kWh added to the battery somewhere? Or is there a rule-of-thumb for the net/gross ratio?

I know I must be missing something here...

Can't you simply take the current % SOC (as shown in the car) and multiply it by 83.7 KW (total/max useable capacity)?

 

[PanameraFrank]

I know I must be missing something here...

Can't you simply take the current SOC (%) and multiply it by 83.7 KW (total useable capacity)?

Total usable capacity is not the 83.7 KW Porsche claims. Mine started at 87.4 KW and the capacity will drop over time.

To figure out the actual usable capacity you can use a trip of some distance (I'd recommend at least 50 miles) and track battery % used, kWh/100mi, and miles traveled. So a 33 kWh/100mi over 120 miles with 50% battery used indicates 39.6 KW used for a total usable capacity of 79.2.

Best to do this over a few trips to get the most accurate number but I've done it many times and the numbers it pops out seem legit.

 

[evanevery]

Total usable capacity is not the 83.7 KW Porsche claims. Mine started at 87.4 KW and the capacity will drop over time.

To figure out the actual usable capacity you can use a trip of some distance (I'd recommend at least 50 miles) and track battery % used, kWh/100mi, and miles traveled. So a 33 kWh/100mi over 120 miles with 50% battery used indicates 39.6 KW used for a total capacity of 79.2.

Best to do this over a few trips to get the most accurate number.

I see, so the question is how to calculate/confirm the TOTAL useable capacity of the battery... My mistake...

I think this is going to uncover LOTS of variations in the accuracy of numbers we are given (by the car and the chargers). I wouldn't expect ANY of the numbers to be accurate enough to reach a precise conclusion. They might get you into the ballpark, but none of these are meant to be scientific instruments capable of the precision I think would be needed. EV's certainly need more than an analog fuel gauge, but I'm guessing the digital information we are getting is likely just skipping the digital to analog conversion (not enhanced precision)...

Its certainly an interesting quest, but I would be careful about what conclusions are drawn...

 

[PanameraFrank]

The battery % and trip computer kWh/100 mi seem to be hard numbers tied to the actual battery capacity. They are quite accurate.

 

[arijaycomet]

The battery % and trip computer kWh/100 mi seem to be hard numbers tied to the actual battery capacity. They are quite accurate.

Don't be so sure of that! Friend of mine has a Tesla Model S (2015) and a ribbon cable that ties into a 3rd party device that pulls data directly off the computer "guts" of the Tesla. He has found there is a fair bit of rounding and a lot of "fluff" put into the data you see on the screen. Granted this is Tesla, but I'd anticipate the same data here on a Taycan.

For example, even when your battery is at 0% on the screen (Tesla), there is still a reserve left to ensure you don't truly brick the battery. Same would likely apply to the Porsche, such that 1% of battery life isn't actually 1% of any "known" number. That said, if you had a scan tool (like my friend has) he can see each battery pack (there are what, 8 in the model S? I forget, maybe a dozen?) -- each has own Ah and Voltage and such--- that data provides a more realistic picture of what the car's software is seeing and adjusting for--

In a petrol car, I'd take the displayed MPG with a grain of salt. Matter of fact, plenty of YouTube reviews show that the miles travelled aren't even accurate in many cars, Taycan included. (see just about any Bjorn video). Hence the data on the screen you're seeing, if based off distance travelled that is off/not perfectly calibrated (likely off RPM, not GPS, so its an estimate, at best)-- then the mi/kWh and other data would equally have the same multiplier of error embedded into that calculation. Causing a cascading effect of wrong/bad data.

Kudos to you for trying to find this data-- I think a better way would be access to what the car is reading for amp hours and other data off the packs; not based on some "calculated" misnomers on the dashboard (like @evanevery alluded to)

 

[Kingske]

I tend to agree with @PanameraFrank . The reason I am asking is that I took some notes when charging my Taycan yesterday in my garage (13C/55F) from 57% SoC to 85% SoC. According to the PMCC display, 28.1 kWh were pumped into the car. If the useable battery capacity is indeed 83.7 kWh, 23.4 kWh should have been added to the battery itself. If so, the gross-to-net loss would be 17% which seems too high, particularly in light of the 9-10% quoted by @Scandinavian above. My guess therefore is that true useable battery capacity is higher - possibly closer to 90 kWh - because 1) Porsche starts out with a high-enough extra margin to anticipate for capacity reduction during the lifetime of the battery, or 2) individual capacity variations between batteries (in which case I must be lucky), or 3) Porsche now allows for a smaller buffer between the useable capacity and the total capacity. Any thoughts?

 

[evanevery]

The battery % and trip computer kWh/100 mi seem to be hard numbers tied to the actual battery capacity. They are quite accurate.

I would submit that predicting the capacity of any battery (based on its SOC at a specific point in time), by even the most precise scientific instruments available, would only be an estimate.

You might be able to measure the exact amount of energy which goes in, and the exact amount of energy which ultimately comes out, but any estimate of capacity at any given "SOC" is going to be just that I think...

Additionally, the total amount of energy that you can squeeze out of a battery (even if you DO specifically measure it along the way) is going to be effected by a bunch of parameters external to your equation(s). For example: environmental parameters (battery temp, etc). IOW: There can be a significant difference in the amount of energy you can get out of a warm battery vs a cold battery...

So many variables...

 

[struther]

I would submit that predicting the capacity of any battery (based on its SOC at a specific point in time), by even the most precise scientific instruments available, would only be an estimate.

You might be able to measure the exact amount of energy which goes in, and the exact amount of energy which ultimately comes out, but any estimate of capacity at any given "SOC" is going to be just that I think...

Additionally, the total amount of energy that you can squeeze out of a battery (even if you DO specifically measure it along the way) is going to be effected by external environmental parameters (bettery temp, etc). IOW: There can be a significant difference in the amount of energy you can get out of a warm battery vs a cold battery...

So many variables...

And regen. If you measure the output, you also have to factor in how much regen is adding to the battery.

 

[wmras]

This is not rocket science but does use some fairly simple math:

kWh Charge / ((Stop % SOC) - (Start % SOC)) = Usable Battery kWh

e.g.
Start % SOC = 10%
Stop % SOC = 100%
∆ % SOC = 90%
kWh Charge = 81.0 (use Taycan display)

81/(100%-10%) = 90.0
kWh Usable = 90.0

These are made up numbers, but my estimate for some time is the actual usable is close to 90 kWh.

 

[PanameraFrank]

I would submit that predicting the capacity of any battery (based on its SOC at a specific point in time), by even the most precise scientific instruments available, would only be an estimate.

You might be able to measure the exact amount of energy which goes in, and the exact amount of energy which ultimately comes out, but any estimate of capacity at any given "SOC" is going to be just that I think...

I guess it depends what you're looking at. If you want to know the actual nuts & bolts of the battery, you're going to need to do a lot of out of warranty digging.

But if you want to know what is allotted for your use, the SOC % and kWh/100 mi are accurate. WIth a little math they give the physical amount you have available to you at any given time, plus a likely emergency buffer at 0%.

In practical use, those two numbers are quite accurate enough. At any time if you look at those numbers, plus distance traveled, and compare against detailed charging logs.. it all adds up correctly.

Also regen IS calculated in the kWh/100 mi number. If you start at the top of a mountain and coast down you WILL get a negative kWh/100 mi number, I do it every week.

 

[Doc B]

I tend to agree with @PanameraFrank . The reason I am asking is that I took some notes when charging my Taycan yesterday in my garage (13C/55F) from 57% SoC to 85% SoC. According to the PMCC display, 28.1 kWh were pumped into the car. If the useable battery capacity is indeed 83.7 kWh, 23.4 kWh should have been added to the battery itself. If so, the gross-to-net loss would be 17% which seems too high, particularly in light of the 9-10% quoted by @Scandinavian above. My guess therefore is that true useable battery capacity is higher - possibly closer to 90 kWh - because 1) Porsche starts out with a high-enough extra margin to anticipate for capacity reduction during the lifetime of the battery, or 2) individual capacity variations between batteries (in which case I must be lucky), or 3) Porsche now allows for a smaller buffer between the useable capacity and the total capacity. Any thoughts?

1. means Porsche probably expects my range to get even worse pretty quickly, and 2. means I have a dud, so really, really hoping it's 3...

There was some speculation on this forum ages ago that at some point in time the usable capacity was increased by Porsche (possibly around the time of the EPA figures being released). All just speculation though - no hard facts showing someone recording different SoC charging figures during ownership.

 

[grahamsimmonds]

I think my answer is that it does not matter.

My original concern was that my Turbo S would have less range than the Tesla MSP100D it replaced. Then I went on a 92 mile journey that I do regularly and drove the Taycan the same way as the Tesla. I used exactly the same in both cars - 40%. However, the usable capacity in a P100D is about 95 kW where the Taycan is supposedly 83.7 kW. Therefore the Taycan used 13.5% less energy than the MS to do the same journey. I have not worried about range since.