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Fenix 8 MIP Solar 51mm - Solar charging

akiakic
6 months ago

Weirdest thing just happened. Charge it to 100% at midnight and went to bed. Woke up to 98% and few hours after went down to 97%. Approximately 13 hours since full charge and at 97% went outside and got around 46k Lux and now watch shows 100% charge. It is a great thing but little suspicious. A week ago I held the watch for many hours in sun to test it and it barely charge 1% after 200k lux. What is going on here?

  • 0 6 months ago

    Here are some of my ideas for solving the mysteries. They might be completely wrong.

    I don't know how the Garmin watches determines state of charge, but probably not in the very accurate method of Coulomb counting where they keep track of how much actual energy flows in and out of the battery. So most probably they are trying to correlate the battery voltage to state of charge.

    In li-ion batteries when they are close to full at 4.2V, a little discharge can drop the voltage very fast, and a little charge can raise it equally fast. So around 95-100% the readings can be a bit weird and volatile. I would assume that they don't have this perfectly calibrated, especially that these watches can charge with solar which pulls the voltage around than a near constant load would.

    How much solar charge you need to charge the watch by 1 percent we could calculate it based on Garmin's claimed smart watch battery life with and without solar. Garmin claims this about the 51mm solar on the product specs:

    Smartwatch: Up to 30 days/48 days with solar (Solar charging, assuming all-day wear with 3 hours per day outside in 50,000 lux conditions)

    So 150k lux/day adds 18 extra days. 30 days = 100%, 48 days would mean 160%. So averaging 150k lux every day for 48 days charges the watch an extra 60 percent. 150*48 = 7200k lux, 7200k lux solar equals 60 percent extra charge. Which is 120k lux for every 1 percent charge. (7200/60=120)

    In your case let's assume the watch was showing 50%. Internally it might have been close to dropping down to 49 percent, so let's call that 50.001. And then it might have charged to almost, but not quite hitting 52 (so it can be 51.999). So you saw 1 percent increase, but it might have been anything between 0.001 and almost 2. Which lines up with 1 requiring between 1-240k lux solar charging (but on average 120k).

    This also assumes they have a perfectly calibrated battery voltage to percentage calibration, which they don't, and that you had minimal drain during this time (no activity or backlight on, etc.) which would require more.

    Let me know if you see any errors in my calculations, I just did them here for fun. The Enduro 3 comes at at 90k lux/1 percent with the same method, which makes sense with its wider solar ring.