Remarkably Accurate Quartz Clock

I've tuned the clock 3 times since the last update. Before these, the last time had been September 27.

2023-12-27: Battery: 2.973; tune: 40/32 sec late: new error: -7.44
2024-02-14: Battery: 2.924; tune: 40/32 sec late: new error: -7.73
2024-03-27: Battery: 2.881; tune: 48/32 sec late; new error: -8.14

The clock started its life on the final PCB with the current battery on March 16, 2024, over a year ago. I haven't drawn the voltage curve on a chart yet so I'm not read to speculate how long I expect it to go, but so far I'm pretty content - comfortably over a year is fine with me!

After the last tuning on September 27, the watch started to drift faster than before. I tuned again after only 2 weeks, on October 11. It ws 0.5 sec late; the error went from -6.85 to to -7.25 PPM.

In the 2 months since then, the clock has grown late again, but only by less than a second. Not tuning yet.

Battery measurements show a steady, slow slide:

2023-10-25: 2.988 V
2023-11-08: 2.984 V
2023-12-13: 2.970 V

In 5 weeks the clock became 1.5 seconds late. This is an accelerating rate of drift after the many stable months leading up to the previous calibration! The updated error has gone from -6.37 PPM to -6.85 PPM. Extrapolating the currently corrected error, this would be about 15 seconds late per year.

Battery is sinking noticeably too, now below the 3 V line at 2.998 V.

The clock accrued some noticable delay, being almost 2 seconds late. The calibration ended up being nudging the error from -6.19 PPM to -6.37 PPM. This was for 4 months, so it'd extrapolate to being 6 seconds later per year.

Battery is at 3.011 V, but I measured values down to 3.003 in the past weeks. (It was at least 5 C colder then.)

It's been nearly 3 months since the last calibration. Clock is still less than 1 sec late. This would mean under 4 seconds per year?! I want to wait until it's above 1 second before I calibrate again.

Battery is at 3.011V.

It's been 3 weeks since the last calibration. Clock is about 0.2 sec late (a subjective estimate, holding clock side-by-side with time.is). Drift not large enough for recalibration yet.

Battery is at 3.006V.

I last calibrated the clock 34 days ago. Today it was about 0.4 sec late, which would translate into under 5 seconds per year. Battery in this time frame went from 3.092V to 3.009V.