Cutting the cord: making the device stand on its own

For a long stretch, every test session came with a small knot of anxiety: my laptop, tethered to the device, sitting a few feet from a pool full of water. One bad splash and the whole project takes a very expensive detour. I was tired of fearing for its life. The device needed to stand on its own — and that meant a battery.

My first instinct was a LiPo pack. Then I picked up a SparkFun board with a battery port, saw a 7,000 mAh hobby pack, and grabbed it without reading the label. It was 7.2 volts, and I found that out by burning my thumb on it. Lesson one: a battery I wouldn't trust in my own hands is not a battery I'm going to hand a coach. Hobby cells were out.

The charger that kept falling asleep

So I tried a normal portable phone charger instead. It worked for about 30 seconds, then shut off. Every time. It turns out many power banks have a feature where, if they don't sense a minimum current draw, they assume nothing's plugged in and go to sleep to save power — and the device's draw sat just below that threshold. After some digging I learned INIU chargers don't do this, spent $25 to confirm it, and finally had a power source that stayed awake.

A box that fits the battery

Of course, the battery pack was big, so the enclosure needed a redesign. After a lot of iterations I settled on separate compartments — one for the battery, one for the rest of the electronics — which kept the wiring sane and the heat away from things that don't like heat. I also added a status LED so there's a simple visual signal of what the device is doing.

The payoff showed up the very next time I tested with the club swim group: I could leave my laptop back in the dry zone, well away from the splashes, and just let the device do its thing. That alone made the whole detour worth it.