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Battery capacity

Is there a way to determine battery pack capacity on the Tesla Powerwall 2? I can access all the relevant details on the api.... but there is nothing about the actual battery pack capacity as it stands compared to when it was new. I know that Tesla warrants 70% capacity at 10 years with daily cycling.

I would like to know what the battery pack is at now as it is over 3 years old. Batteries do degrade over time. So, if I had a theoretical battery capacity of 13.5kWh, what would it be now on a full charge at 100%.

Percentages are meaningless on the app. Say, it says 75% charged... what is 75%? It would be better to display the approximate kWh capacity available. That way, one could estimate how long the battery will run given a load.

The app should have something like the fuel range on a car, you know, how many km/miles left based on the fuel tank level before one runs out of fuel. Surely it can't be hard for the app to work out that it has X amount of kWH and a load of Y amount of kW. Time left would be X over Y.


  • Actually, it's a really difficult problem. Even the car's range indicator is more a guess-o-meter. You're trying to measure the chemical changes over time via its electrical conditions.

    The best way to validate battery degradation is to charge it to 100% and discharge it to 0% at 70F ambient and measure the amount of power drawn down and power used internally. The problem is the act of doing this adds wear to the battery, so it's not desirable to do this often. Also when you get to 0% SOC, the PW 12v to the gateway powers off, and you can't charge the Powerwall. You need to supply external 12v power to start the charging process.
  • Without drawing down to 0%, it might also be useful to start at 100% and go thru the discharge process, noting the energy usage as you draw down to multiple points...75%, 50%, 25% or 90,80,70,60...Deducing the zero point from that data should yield a rational estimate of the total capacity. Not perfect and certainly not exactly in line with a better actual measurement...but...a rational estimate.
  • Thanks and Jones_lab_guy

    I posted a reply but it appears it didn't get there.

    Anyway, what I did last night was record the Percentage % from the http://powerwall-IP/system/status/soe and captured the load imported and battery exported figures at the same time from http://powerwall-IP/api/meters/aggregates. I did that at a couple of intervals. So, extrapolating the figures to 100%, I was able to get a figure of 13,000Wh to 13,300Wh estimate. Assuming, that that battery was at 13,500Wh brand new. It hasn't degraded too much then. My average battery utilisation is about 47% but it does go high in Winter.

    Tesla warrants 70% at 10 years.

    I could do a full load discharge but I will wait till Winter when I run some heaters to make practical use of it during the test. That way I'll know how much was discharged from 100 to 0% (but it'll actually be 95%, as there is 5% reserved). That's only about 2 months away (I'm in Australia, so it's neither hot or cold at the moment).

    I manage a fleet of APC UPS at work, they can show runtimes based on battery capacity. Which is why I was asking why can't Tesla do the same thing on their app. It doesn't have to be 100% accurate but an approximation would be good.

    So, really two features that could be added to the app - battery capacity measured in kWh, not % and runtime availability based on current loading.
  • hi grahamk,

    you can try the python Tesla-api.

    using the get_energy_site_live_status_total_pack_energy function to determine battery pack capacity. However, I don't know how accurate it is.
  • Thanks teslapowerwallPR.

    I don't have the time nor am I interested in programming.

    Is that get_energy_site_live_status_total_pack_energy function accessible from the the Powerwall itself like the api/meters/aggregates?

    Just a simple URL for quick look up. This is why Tesla should add these functions to the app. I'm sure that there would be hundreds of users who are not programmers but would want to be interested some extra numbers about the Powerwall.
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