A cheap bike light, opened
8 September 2026
The light stopped charging after a wet ride, which is exactly what the box said would not happen. Four small screws under rubber plugs, and the case came apart with a bit of persuasion from a plastic card.
Inside: one 18650 cell, a charging board, an LED on an aluminium star, and a gasket that had been compressed once and never recovered. Water had gone in through the button, not the seam. There was a thin line of corrosion along the board edge, green and already eating the trace to the charge controller.
The cell was labelled 2600 mAh. Measured discharge at 0.5 A gave 1830 mAh before the protection cut in — so either the label is optimistic or the cell is old stock, and for a six-euro light it is probably both.
I cleaned the corrosion with a cotton bud and isopropyl, re-soldered the trace with a bridge of thin wire, and put it back together with a smear of silicone grease on the gasket. It charges again. It will leak again, because the button is still the same button, but now I know where to look.
Most of what I learn from these things is not how the device works. It is how the price was reached.