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Discussion Starter #1
For the first time, I did a nearly full battery charging session. I ran the battery SOC down to ~9% or 17 miles and then charged to 100% on my ChargePoint Flex 40A charger. The Chargepoint app says it added 90.4 kWh which is obviously more than the ~84.? kWh effective buffered battery capacity at 100% SOH/fade.
I assume there is some inefficiency that creates some level of power transmission loss and recall reading somewhere in other posts. Does someone have a good formula figuring out what the likely effective battery capacity is and this State of Health? Would appreciate smarter views. Thanks
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Unfortunately, the information provided by the car and the official apps is not enough to precisely figure out what you want to know. The watcat android app can tell you how much energy the BMS thinks is in the battery. That number at 100% SoC is the best guess available as to the usable capacity of your battery. As ScienceGeek and others have found out, this number may be total nonsense if your battery is unhealthy.

I don't know of a reliable way to compute capacity based upon a charging session. There is too much loss due to heat, cell balancing, and other factors to get a reliable estimate of how much energy actually went into the battery.

You can, however, compute an estimate based upon what the car reports as average power consumed, the distance traveled, and the SoC delta by driving. This is much simpler and more accurate if you do one long trip driving it from 100% down to some low SoC (perhaps <= 10%) and start with a reset trip meter. For example, if the car reports it consumed an average of 300 Wh/mi over 200 miles using 90% SoC, then 60 kWh (300 * 200) would represent 90% of the actual capacity. Dividing by the SoC delta (60 / .9) then says the battery has 66.67 kWh actual capacity. This scenario is totally made up and you should expect the actual number to be closer to 84 kWh depending on normal degradation assuming a healthy battery.

Once you have the above estimate, you can use that to determine the degradation of your battery by comparing that number to the rated/advertised capacity of the battery.
 

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What juice said.

To make matters more confusing, the API kWh value you get via WattCat is a massaged number that (in my experience with two cars) inflates the OBD value by about 6%. When WattCat reports a full charge at e.g. ca. 87kWh, the OBD value is ca. 82kWh. However, WattCat (the API) will report 0% (and a kWh value at or close to zero) while the OBD still shows some small single-digit percentage. So it's sort of a wash.

To make matters even more confusing (I kid you not), the Trip meter consumption value that juice is talking about is actually consistent with the OBD values, not the massaged API values, when the battery is in good shape.

Bottom line, if you use WattCat to see the API number and it's somewhere around 87kWh at fully charged your battery is just fine. If it repeatedly shows much less than that, like 82 or even <80kWh then you've had some degradation.
 

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P.s.: Level 2 charging in my experience incurs about 13% loss, so if your ChargePoint reports 90.4 kWh the battery got about 79 kWh; if that corresponded to about 91% (with the above caveats) you're approximately in the right ballpark with your capacity, at ca. 86ish API kWh.
 

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Discussion Starter #5
Many thanks for the assessment and I understand the caveats and constraints. The battery tested at 95 percent SoH when purchased in May but it had been sitting at the dealer for nearly a year and had less than 100 miles so never had a full charge cycle for the battery controller to sort out . Seems the cat’s battery is in good shape

Love this car so far !
 

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Unfortunately, the information provided by the car and the official apps is not enough to precisely figure out what you want to know. The watcat android app can tell you how much energy the BMS thinks is in the battery. That number at 100% SoC is the best guess available as to the usable capacity of your battery. As ScienceGeek and others have found out, this number may be total nonsense if your battery is unhealthy.

I don't know of a reliable way to compute capacity based upon a charging session. There is too much loss due to heat, cell balancing, and other factors to get a reliable estimate of how much energy actually went into the battery.

You can, however, compute an estimate based upon what the car reports as average power consumed, the distance traveled, and the SoC delta by driving. This is much simpler and more accurate if you do one long trip driving it from 100% down to some low SoC (perhaps <= 10%) and start with a reset trip meter. For example, if the car reports it consumed an average of 300 Wh/mi over 200 miles using 90% SoC, then 60 kWh (300 * 200) would represent 90% of the actual capacity. Dividing by the SoC delta (60 / .9) then says the battery has 66.67 kWh actual capacity. This scenario is totally made up and you should expect the actual number to be closer to 84 kWh depending on normal degradation assuming a healthy battery.

Once you have the above estimate, you can use that to determine the degradation of your battery by comparing that number to the rated/advertised capacity of the battery.
I've been using this method (efficiency, ∆SOC and distance travelled) to estimate the battery off and on for the last year (dozens of measurements) and unfortunately, the number varies by quite a lot (74-86 kWh), so that does not seem to be reliable. I suspect the battery efficiency is tied in there somehow, so it is not a measure of the battery's baseline capacity, but the capacity given other variables that we are not accounting for.
 

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Excellent note that you noticed it took more to charge your car than what appears to be in the battery after charging it. All those wires, diodes and battery create heat when charging the battery. Lost heat. Gasoline cars loose a lot more heat out the tailpipe and radiator than you do charging and driving your electric car.
 
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