Headlight battery pack replacement

I've had this headlight on my bike for five years now. While the battery pack started out strong, it's gradually degraded to the point that I can't make it more than 15 miles without it dying.

The obvious solution was to build a 8.4V 4 cell 18650 pack just like the original, or even better, an 8 cell or larger pack that would get me some extra life. I had started down this path, but mass produced high capacity USB battery packs are cheap and would not only power my light (with the help of an eBay buck-boost converter), but my cell phone too. I also get to eliminate the 8.4V battery pack charger from my kit, using the ever present USB charger instead.

The basic steps are straightforward. I grabbed an eBay buck-boost converters from my parts pile and soldered the batter pack plug on the source side and the USB power lines on the sink side. I then plugged the USB end into a USB power strip and adjusted the converter until it was putting out ~8.2 volts and tested it with the battery pack.



To weatherproof, I encapsulated the converter in hot glue. I made an attempt to form in some flex reliefs from a hand drawn template on the wax paper (which reminds me of a lesson I've learned at least twice now - hot glue sticks to wax paper very well).

Wax paper well adhered the the hot glue.

After cutting off all the wax paper (possibly the most time consuming part of this project) and encapsulating the backside.

I had originally intended to do some waterproofing of the battery pack and mounting the pack to my bike with a fancy 3D printed carrier, but zip tying the converter to my rack and putting the pack in my pannier worked well enough for me. The zip tying of the converter was also meant to be temporary (I was a bit nervous it would catch fire), but it was convenient to keep it there.





The first battery pack I ordered did not work for this. I tried this one, but as soon as I powered on the headlight the pack would shut off. Based on the advertised current pull of the light (~2 amps) and the current supply of the pack (4.8A or 2.4A?) this should have been fine. Nonetheless, it did not work. I had previously tested it with the Anker and RAVPower packs we had at home, so I returned it and ordered this one.

The new pack worked with a minor quirk. When I first plug in the light, it draws a small current that powers a LED button for night visibility. After ~30 seconds, presumably due to the low current draw, the battery pack will shut down if I do not turn the headlight on and trying to turn on the headlight after the pack has shut off does nothing. There's a button on the pack to kick it back on so it's only a minor annoyance, but potentially another pack would not do this.

Overall this arrangement worked really well and I kept it for a few months. I've since added a dedicated water tight container for the battery pack and routed USB cables to my other lights. This lets me charge the pack, front flasher and rear flasher with a single USB charger and without taking anything off the bike.