Failing Polar M430 battery

At the sport club attended by a family member they use extensively Polar M430 GPS fitness trackers. These M430 combine the flexibility to configure their complex training sessions, waterproofness, GPS tracking and connection to heart band. All at an attractive price. With 2+ hours of training, 7 days a week, their batteries do get lot of use over time, but they hold strong. This is the white M430 specimen, that couldn't hold anymore 2 hours of GPS tracking.

One M430 and its main three parts: frame, back cover+battery, main board.

 

As a "hot stand-by" backup I bought a second M430 that I would use during my occasional outdoor activities. Both M430's were second hand. At one point the black M430 screen went blank. I thought the battery had just drained to 0%, but it didn't charge. The smartwatch would not react, no current would flow over the USB/charging cable.

Polar M430 battery details

Suspecting a bad battery, I ordered a replacement cell (not the Polar replacement that is unobtainable on our usual marketplaces). The replace process failed because of a very low melting point plastic adapter (as confirmed on this Reddit).

I was left with a working white M430 and its tired battery and a black M430 without a battery. Since both devices had reached their end of life, I left them alone.

The other day the family member brought home another black M430 that went dark, from a friend, asking me to replace the battery. This time I knew the correct procedure to solder a new cell, but with 3 watches and 2 batteries I could do a more thorough troubleshooting.

The battery is part of the inner shell, that is in turn connected (detachable) to the main board with a tiny flat cable. 

I installed the supposedly dead black battery in the white watch: it charged and powered up correctly. Note that the battery voltage before charging was 3.6V, so not really dead.

Meanwhile I installed the charged white battery in both black Polars: they would not charge or power up. What?

What's inside a Polar M430.

In the process I learned that when you install a charged battery you need to plug the watch to the charger so it powers up. So it is not enough to just screw everything together and press buttons to boot the watch. Keep that in mind if you plan to play with these devices.

At the moment, with two charged batteries and a working white Polar M430, my impression is that there is something wrong with black Polar M430's. I assembled a white-and-black M430 that is now enjoying daily training sessions.

 

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