Sudden Battery Drains

My Mk1 hasn't done anything like this.  I get really good battery life. 

I tend to charge for an hour on each of Monday and Friday.  I usually do 2 x 20 minute walks each weekday and 1-3 dives on the weekend. My dives are long, too - 90-150 minutes.  I rarely get below 40%.  I've never see the low battery alarm.

Do you have any watch faces or apps that could be draining the battery?

No, no watch faces.

I have OTP widget and 2 3rd party apps, but I doubt they are the issue as those are seldom used.

Battery is working great in most cases, just like yours.

There is a typical pattern when problem occurs though. I rarely charge the battery. When I drain the battery (normal pace) to single digits, I put the watch in the charger to have sufficient power for the activities I'm planning. It typically charges to 45-85% depending on how much time I have, Following this, battery discharges unbelievably fast, last time it took 1x 50 mins dive and roughly 3 hours of surface since it was at 45% to drain completely.After the shutdown and recharge watch operates normally for days.

Garmin staff, any suggestions?

profunditas How are you typically charging your Descent? Charging from a USB port on a computer is the recommended method. If you are using a car charger it could cause the watch to show an inaccurate charge percentage. Try power cycling the watch by holding the top left button until the watch shuts down and then power it back on with the same button. Then charge your watch to 100% using a computer and see if the battery drains at a proper rate.

I charge it from a wall adapter which charges all my other devices without a problem. Saying charging from computer does not mean much, I'd rather know how many Ampers does watch draw when charging? I'm sure 2A which wall adapters have is sufficient. 

It is true that watches does not exhibit battery drain after restart. I believe this is due to something running in the background, will keep monitoring. 

To reply to myself, problem reoccured and it is always the same pattern. I discharge the watch through regular use to single digits (this time it lasted for days), put it back into wall charger, charged it for 2+ hours, went to bed, woke up with watch being shuttdown. 

And did you try using the USB port as suggested? Wall chargers may have slightly higher or less stable voltage, and/or there may be a significant unfiltered AC signal - all of that could play a role. If Garmin tells you should use an USB port, they probably already have the experience that at least some wall chargers cause problems, so insisting on using them is perhaps not the best way to do.

To answer your question, same occurrence happened charged from the wall, PC or TV set. And they charge all my devices, other Garmins included, without any problem.

Garmin support was referring to car chargers and there is a good reason for that; voltage in those varies between 9 and 14.5V and it is hard to have stabilized 5V on a lower end. USB chargers plugged to the wall have to confirm to certain USB standards unless you're posting this from North Korea. 

You don't have to be the sharpest tool in a shed to understand charging your outdoor watch in a PC might not be the most practical thing, do you?

5V/1W - slow and true charging is what our watches are designed around and it is what we highly recommend. The more you are able to stay within that range without using larger voltage/wattage fast charging options, the better it will be for your Lithium-Ion battery lifespan.