Jupiter Discovery X5 E-bike - Adding Onboard Power to Tail Light

Something a little odd with this bike is that the rear tail light runs off a 3 volt button cell rather than from the main bike battery like the headlight does. I wanted to rectify this situation so I bought what's called a DC-DC Buck Converter, or Step-Down Converter. It will take 40 volt bike battery voltage and step it down to pretty much anything you'd like. In this case it will step down to 3 volts (or 12 volts as you will notice further down the blog). Here's a photo of the converter. The small rectangular blue thing is an adjustable potentiometer which is used to adjust the output voltage.

One step in the process is to find the power line for the front headlight. In this case it is a green and a black wire. I will tap into these wires to power the voltage converter.

This next photo below shows the tail light LEDs and circuit board with a pair of white wires soldered to the battery terminals. I was doing this work in the basement and using a 30 volt power supply as the input so I could adjust the output to the necessary 3 volts. Well...... something went wrong and I put too much voltage to the tail light circuit board and fried the LED lights and chip. It wasn't totally my fault (or so I tell myself) because I was measuring the output voltage at around 1.7 volts with an input of maybe 12 volts. I erroneously figured that the output would stay fairly constant while cranking up the input voltage to 30. It didn't, and things went South pretty quickly, with the requisite little puff of smoke emanating from the circuitry. Hmm, now what do I do?

I then remembered that I had some left-over LED light tape from another project that I could possibly use for this purpose. It runs on 12 volts so I could just adjust the voltage converter for this new voltage. I cut off a section of light tape that had six LEDs on it. They were spaced too far apart so I folded the tape over onto itself between the lights to get the LEDs closer together and make the whole mess short enough to fit into the red tail light lens. I then adjusted the voltage converter to put out the necessary 12 volts.

Folding the light strip did seem to work and so I hot glued the LEDs into the lens.

This next photo shows the folded light strip a little better.

It looks promising.

Here's the dead board. I couldn't see anything out of the ordinary but something is obviously dead.

I don't know if I've shown this snake pit of a mess before. This pile of wires and controller fit into a small cavity in the bike frame but now I also have to fit in the small step-down converter and extra wiring. I did manage to get it all stuffed back in after fighting things for ten minutes.

The light and bike are all back together and the light is actually quite a bit brighter than the original light on the right (the wife's bike), which I used to think was very bright. The nice thing is, when I turn on the headlight, the rear tail light now comes on too. With the old battery powered light, I'd have to turn it on separately and it is easy to forget. It's also easy to forget to turn the tail light off again after a ride.