Battery drains too fast after solar charging

TeodorMan said:

Ambient temperature in the house is lower than outside.

The ambient temperature is not that important. What is important is the thermal conductivity of the cooling medium. Water (and similarly the blood) has the conductivity 24 times higher than air, hence if you leave the watch exposed without any liquid cooling, it will heat up anyway, regardless of the ambient air temperature.

TeodorMan said:

Moreover, the battery charge/consumption does not simply drop this severely due to a change in its temperate by a few degrees.

The consumption or the charge indeed do not change much, but the battery voltage is strongly temperature depending, hence when the temperature changes, the voltage (and the battery gauge) changes too.

trux said:

The ambient temperature is not that important. What is important is the thermal conductivity of the cooling medium. Water (and similarly the blood) has the conductivity 24 times higher than air, hence if you leave the watch exposed without any liquid cooling, it will heat up anyway, regardless of the ambient air temperature.

The point I am making is that the watch does not get as hot as when left on direct sunlight outside regardless wether on my wrist or not. 

trux said:

The consumption or the charge indeed do not change much, but the battery voltage is strongly temperature depending, hence when the temperature changes, the voltage (and the battery gauge) changes too.

I see the point you are making here and I agree with you. Now, does that mean that unless you somehow manage to prevent the temperature of your watch from raising while charging it on the sun you will never get a realistic idea of how much charge you actually added, especially when you have no idea how many LUX hours you have added during the charging session ...

The best for the charging is assuring that the internal temperature of the watch (that influences the battery gauge indicator) remains as much constant as possible. You assure it best by wearing the watch on your hand, or by placing the watch into a container with water.

Even if you wear your watch while in the sun its temperature will still fluctuate meaningfully. 

Basically, it is impossible to get an idea of how much charge you have gained on an Instinct Solar since there is not even a way of checking how much solar power has been absorber by the panels for a given period of time. What a gimmick ...

TeodorMan said:

Even if you wear your watch while in the sun its temperature will still fluctuate meaningfully. 

Not really. As explained, the skin and the blood will keep the temperature pretty stable, because of the much higher thermal conductivity. Knowing how many lux the watch gets from the sun would not help you much, since the consumption is not stable at all anyway. The battery level is not calculated as a balance of input and output, but simply by measuring the voltage of the battery.

trux said:

Knowing how many lux the watch gets from the sun would not help you much

There is no doubt that consumption is not stable as you use the watch in different modes(smartwatch, battery saver, activity tracking). However, I would argue that if you know how many lux hours your watch has collected over a period of time then you can have a reasonable idea of battery charge you have gained. Then you can use that information as you wish. 

As written, you would have to measure and calculate both the consumption and the charge, and that's not how it is working, and the accuracy of that method  vs. its complexity is highly questionable, since there are other factors involved. Measuring the battery voltage is simple and straightforward, not requiring practically any CPU processing. However, feel free to suggest your idea to Garmin through the form at https://www.garmin.com/en-US/forms/ideas/ 

Yes, so yesterday I was out in the sun for a good part of the day, my watch started at 18 days remaining and went to an estimated 22 days (after solar charging), but it's down to 17 in less than 24 hours since then.

Is solar charging a fake feature?

Can anyone explain why solar charging doesn't hold the charge like cable charging does? Any benefit to solar charging seems to vanish within 12-24h, so I'm not understanding how this feature works.

For example, the "days remaining" estimate seems quite accurate when I've charged with a cable, and haven't been outdoors for much of the week. But when I'm outside for hours on end and the battery level goes up, it doesn't take long for those gains to disappear.

I use quite a few other solar-powered/solar-charging devices, and I've never noticed this kind of behaviour with solar vs cable charging.