Forerunner 55 showed excessive battery drain. I fully charged the device in the evening, wore it overnight, and the battery was down by 30-40% the following day. This was with no tracked activities or bluetooth

Thank you for all the suggestions and advice, I really appreciate all the input from everyone. I am going to update the retailer on these forums too, what I have done and that I am not the only one affected, although I seem to be the most affected (it dropped 70% charge in 24 hours yesterday/today). Let's see what response I get. They said they would contact Garmin about this, I am interested to hear what they have to say. 

I got my FR55 5 days ago and it was working well (including the battery) – until last night. The battery level was around 60% when I went to sleep and ~15% when I woke up this morning. To make things worse, the watch did not record any sleep data (GC said sleep was 'unmeasurable') although I was wearing it the same way as before.

I had an intensive workout (bike ride) yesterday evening. Don't know if that changed anything.

Not what I'd expect from a €200 watch.

I would like to update you on my experience with this bug. In the past 10 days, since I charged my watch, I haven't experienced it at all and I am sitting at 29% battery.

The only factor I can think of is that I have reduced my running due to injury and have been using GPS for only ~1h in total. So maybe, GPS sometimes doesn't turn off after running and then it tries to get a location indoors. I don't know...

I think GPS not turning off may be the issue for some. I don't think it is my problem as I am only doing indoor/"Treadmill" runs. I certainly think the main issue is somehow linked to activity and sleep as a combination as I get the worst drops following activity and sleep together. I am still charging pretty much every day other than following my rest day (I run 6 days a week). I charge the device on my rest day, do an activity the next day and only have to charge the following morning, so effectively about 40 hours. Every other day I am charging every 16 to 24 hours (It's actually getting worse).

I am giving this feedback to try and help anyone else and for Garmin to see the extent of my problem,

Since being in touch with the retailer again and them being in touch with Garmin, they are going to send it straight to Garmin when I come in next month. But they don't know how long Garmin will take to get to and assess the problem. They are quoting me up to 2 to 3 weeks with Garmin. I am not being offered another device or a refund at all or even a loner device for the weeks I may be without my Forerunner 55. To say I am furious with this entire situation is an understatement of epic proportions. 

Hello, I have that same problem but only after indoor activities. I known that is strange, but after I've done an indoor swimming the battery drained like crazy and I think that the problem is the GPS, for some reason (SW bug), starts and stays on and that is what is draining the battery.

After I suspected that, whenever I do a indoor training and after saving the activity I do a restart (off no the menu and on).

After doing this and after several indoor trannings including an indoor swimming training and a 45 min outdoor run the battery is at 51% and I've charged it 7 and a half days ago. 

Try this workaround (after indoor training and saving the activity do a off/on) and see if it helps.

Thanks! This certainly sounds promising and similar to my problem.

I may have stumbled on a workaround myself yesterday. I didn't want to post about it until I had a few days good results but I am already about 40 hours since my last full charge and it only dropped 5% (first time ever!). I have had two sleep sessions and one workout so far. This is the only thing that has done anything to stop the battery drop after a workout and sleep session. I wanted to track my activity but didn't want to fiddle with turning the bluetooth on and syncing with my phone yesterday so I quickly plugged the forerunner 55 into my computer to sync. It was plugged in and charging only enough time to sync, so a minute or two, and it did not top up the slight charge drop from my indoor workout. Since that sync it dropped 5% total and I am 27 hours since that sync. I am going to sync with the computer after my workout today etc. and see if the extended battery life continues. I suspect the syncing I did is doing the same thing as the restart you did. I will update again in a few days.

Check your local consumer laws. If a device is defective the retailer should be liable for an immediate refund.

I wanted to post an update. I have not had to do a full charge since I last posted. I am onto my 10th day since I last put the Forerunner on full charge and I have had 9 indoor workouts, my battery is currently at 73% charge. This is the first time I have had reasonable battery life. The only difference between the last 10 days and the disastrous time before is that I immediately sync the Forerunner on my computer following a workout and then remove it once synced.

I am going to test turning settings back on and trying the restart method as well to see what functionality I can get back in the meantime.

Thanks also for the suggestions re consumer protection laws, I am going to investigate what my rights are and how to exercise them in this instance as there is still clearly a significant software/settings fault with the Forerunner 55, I am just relieved to be able to better use the device in the meantime. 

THANKS FOR ALL ! I bought the 55 in Brazil and I am having the same issue. I will try PhysicsRunner's suggestion and then see what happens. I am really pissed off with garmin.