After 3+ years of using a Garmin Forerunner 55 as my main watch, I began to notice that the battery was lasting shorter and shorter amounts of time, from about a week on a full charge to about two days if I’m lucky. Therefore, I set about attempting to replace the battery, as this would be much cheaper (and better for the environment) than buying a new watch.

The Teardown

First I used a heat gun towards the screen to soften the adhesive between the screen assembly and the main body of the watch, and then used a small plastic tool and a metal pry tool to separate them.

After separating the screen, I used the plastic tool to carefully disconnect the screen from the PCB.

After doing this, I could get a good look at the PCB in the main body.

I think (but I’m not entirely sure) that the small circle in the middle is a piezo buzzer for the sounds that the watch can make.

There is one additional connector for the heartbeat sensor, and three small phillips screws that hold the PCB to the plastic body.

With the sensor connector disconnected, and the three small screws removed (don’t lose them!), the PCB separates easily from the body.

One thing that was interesting to me, is that the USB connector is connected to the PCB by some little spring pins that push onto the contacts exposed through the body.

On the back side of the main PCB, the battery is soldered onto two points around the edge of the PCB, and is double-sided-taped to the PCB. I was able to remove it with a metal tool, but make sure you desolder the battery first so you don’t damage the circuitry!

You can also see the small vibration motor above the battery, and the USB spring contacts beneath it.

(note: this picture is not censored, the battery came with the stricken out part as-is)

The PCB sits on some little metal springs around the side of the body to hold the button switches in the correct places.

Replacing the Battery

I bought a replacement battery for about NZD$15 on AliExpress, and all that was involved was attaching it via double-sided tape, and soldering the wires the same way the original battery was attached.

After putting the new battery in (that’s the one that’s pictured above), I noticed that it seemed a lot flatter than the original, which confirms my theory that the battery was deteriorating.

Re-Assembly

This was very straight-forward, with just assembling the PCB and inserting it back into the body the same way it came out, however the adhesive on the screen was now dead, so I had to find a suitable alternative.

First though, I cleaned the old adhesive off with isopropyl alcohol, and a plastic pick, and this was quite a task as the old adhesive disintegrated.

After this, as an interim measure, I used some small pieces of the same double-sided tape to hold it shut for a test, which showed that it all worked like normal. I think that if you don’t have access to fancier adhesives this may be the best way to go, as it is very similar to what the factory adhesive is like.

However, I wanted to try and protect my watch against moisture as much as possible (as disassembling the watch in the first place breaks the factory water-resistance), so I settled on neutral-cure silicone adhesive, something I had spare from a recent repair of my shower (the plastic panels were not attached to the wall properly). This involved using a toothpick to smear the adhesive in the groove of the screen part, then while being very careful to not get the adhesive everywhere, attaching the screen connector to the PCB and then pressing the screen down. After this, I used paper towels and more IPA to clean off the excess adhesive, while making sure to not touch the IPA to the seam where the adhesive is sitting, as that could cause it to not cure. Finally, I put some small rubber bands around the screen to apply a bit of pressure, so the adhesive takes, and kept it warm so that the curing is on the faster side.

I have yet to have a problem with water ingress, but I have not been exposing it to too much water, in the interest of keeping the watch working.

The final repaired & rejuvenated watch