I bougth some Blackburn LED Bike lamps some years ago. They can be charged over USB, so i thought they might be a cool thing. But after some time one of the light stopped working and charging was impossible. If you put the lamp on an USB charger, it would just do nothing. After some searching for the failure i read some forum posts, where people said this is caused by a bad battery... Okay so far so good.

Today I took a last chance to revive the lamp and first of all i took it apart. Inside the lamp is no battery management chip, they are just using a PIC 12F510 to manage everything. A quick look on the PCB reveals, that they seems to check for a voltage on the charge pins and enable charging if there is any. But this check obviously only works of the microcontroller is powered! But because the Battery had 0.0V, the micro was not powered at all, so it could not check for charging voltage...

Okay, the microcontroller needs some kickstarting... I put 3.3V on the battery pins to confirm that the micro is still up and running. Et voila: the lamp powered up! The next step was to connect the lamp to the charger, while the 3.3V are still connected. And easy as that: the lamp begun charging!

After having charged for a while, nothing happend. I think the cell is dead - so charging will not help at all...

So what do we learn from it? Better use a depth discharge protection circuit, to prevent this from happening. Or better buy only lamps that use normal AA/AAA batteries. Recently I found a lamp that uses four AAA NiMh cells and is chargeable over USB.