The periodic calibration of the Powerwall 3 is a rather disruptive, yet quite poorly implemented and document process. Here are some notes, including the one and only thing you really need to know about it as the end user, which, unfortunately, Tesla forgot to mention.

(As a side note, the reason for the calibration is that lithium iron phosphate (LFP) cells have a rather flat discharge curve, making it hard to accurately assess the actual charge of the battery; a periodic calibration ensures the battery is being used optimally.)

As a new Powerwall owner, the first you are likely to hear about ‘calibration’ is when the ‘Battery is Calibrating’ banner appears in the Tesla app. Clicking on it gives some basic information about it, notably informing you it can take up to 72 hours and that you will know when the calibration is completed by the banner disappearing (this is a bad UX design, on that later). There is also a link to a bit longer spiel on the Tesla website, the gist of which is:

To complete calibration, Powerwall must charge to 100% state of charge and occasionally discharge below 20% … The time to complete calibration varies from system to system … Once your Powerwall is in calibration for 72 hours, it will exit the process and re-attempt calibration in the future.

Unfortunately, this information is misleading; I am not sure whether it’s based on some older firmware, but that is not how the calibration appears to work at present.

According to the Tesla website the calibration happens every 30-90 days; our kicked in 60days to the second from the Powerwall being first turned on, so it’s probably currently driven by a simple 60 day timer. It went like this:

  • Tuesday mid morning, 60 days after commissioning, the banner appeared in the app, and the battery charged to 100% from solar, then started discharging; solar was still being exported, but the battery didn’t charge from the grid during the off-peak period as normal,
  • Wednesday morning the banner has gone; the battery dropped down over night to 16% and started recharging from solar; that was painless, you think …,
  • Wednesday mid day, the banner is back, following exactly the same pattern … they did say ‘up to 72h’, so I don’t give it much thought,
  • Thursday morning the banner is gone, yay! … Except, but midday it’s back, all over again; this is becoming a nuisance, but they did say up to 72h,
  • Friday morning the banner is gone … guess what? By midday it’s back. We are now over the 72h period now, and have lost three days of electricity exports.

At this point, I come to suspect I have become a victim of bad documentation combined with a UX design fail — I am pretty sure now that we have not been stuck in a 72h long calibration, but rather are in an endless cycle of calibration failures, with the lack of differentiation between the calibration completing successfully and failing in the app masking that.

So, as you do these days, I go looking for an answer on the internet. Beyond quite a lot of frustration on the Tesla forums there is not much to be found, but then in one post I stumble on the one piece of information about this process that actually matters, and that Tesla omitted:

For the calibration to succeed the battery much fully discharge (i.e., down to 0%).

This makes sense once you stop to think about that — if the point is to ascertain the charge curve, then 100% and 0% are the only accurately known data points to work with.

There is, obviously, some internal time limit on how long this can take, which, based on extrapolating from our own experience, is under 24h. Now, for the discharge to happen, there has to be sufficient load; our basic consumption is around 5-7kWh/day, which means that with the battery capacity of 13.5kWh, we could never successfully discharge the battery in 24h! (Given the system has seen this pattern of use for 60 days now, you would have thought it would draw it to your attention, right?)

So, that evening when Linda got back from work with the EV, I dug out the granny charger (the thing that plugs into a normal house socket; our main charger bypasses the Powerwall), and drained the battery into the car, down to about 4%, letting the house load do the rest. And this did the trick, the calibration completing sometime late in that day.

So, when the ‘Battery is Calibrating’ notice appears in the Tesla app, there are two things, you need to / may want to do:

  1. Ensure there is a sufficient load to drain the battery to zero in a matter of hours; if you don’t you might end up in an endless calibration cycle.
  2. Additionally, you might want to go to the battery settings and reduce the backup level to 0%; when the calibration completes, the battery will immediately charge up to the backup level, and you might not want that to happen at peak prices.