How to run a light from a USB Power Pack
USB Power packs are handy for charging your cell phone, but could you use them to power your bike lights? Nowadays many models come with 2A or even 3A current output, which equals to 10-15 Watts (the equation is P = U*I, in which P is the “power” and U and I are voltage and current). The 5 V output on power packs is limiting, but 10-15 Watts should be enough for low powered lights at least.
Experiment — Can you use a USB power pack to power a cheap Chinese made headlight?
There’s a reason why I decided to switch to USB-connector. The old battery pack broke down, I don’t know what went wrong with it but it just refused to power the light one day. I had an old but functional 5200 mAh USB power pack that didn’t see any use, so I decided to sacrifice it for this little experiment.
Right away there was a problem. The light is designed to be used with 8.4 V battery pack, yet the USB power pack can only provide 5 V. To make it even worse, the pack can only provide 1 A. I have no idea how much current the 600-lumen light is actually drawing, so I decided to try and see if it works. There are many similar looking lights sold with USB connectors online, so that sounded promising at least.
How to make the adaptor cable
For attaching a few wires together you only need a few tools:
- A solder is definitely helpful
- Short strip of tin
- Wire strippers
- Heat shrink (optional, but recommended)
Making the cable
I wanted to run my light from a USB power pack. Therefore I needed to switch from DC connector to USB male-connector. I already had both connectors alongside with electric cords, so I decided to simply attach the two cords together.
Connecting two wires together is easy. First, you need to remove the insulating cover on the wires.
Before soldering the wires together it’s advised to add some heat shrink on the wires to avoid short circuits later on.
Next solder the wires together, slide the heat shrink on the exposed connection and apply some heat to make the heat shrink, erm, shrink.
Well, does it work?
Short answer: yes, but only on half power mode.
Long answer: the power pack can run the light. However, due to the lower supplied voltage, the light has a low battery warning on as soon as you turn it on. It seems the cut off voltage for the light is at just under 5 V, which means the light is going to turn itself off quite quickly when it’s running on full power. I don’t think the power pack can even provide enough current to run the light on full power, but there still is a significant difference between the different power modes. The half power mode is where things get interesting. It might be that 1 A is enough current to produce around 300-lumens, so maybe it works better? Well, here are the run time results:
- Full power: 45 minutes
- Half power: over 5 hours
The results are quite strange indeed! On full power, I think the power pack is struggling to produce enough current. This causes the voltage to “sag” or drop much faster and that’s when the light turns itself off. Maybe a power pack with a higher current output (2A or 3A) would fare better as the voltage wouldn’t sag as badly?
It’s a completely different story on half power mode. The light is still easily bright enough for commuting and head light use thanks to its heavily focused beam. Neither the battery or the light heat up significantly when ran on half power. So as long as you use only the half power mode and don’t turn it off during your rides, the light is completely usable. However, if you run the light for a few hours and then switch to the full power mode, the light, and the power pack will shut down and refuse to turn on afterward. Only charging brings the power pack back to life after that. Not the most practical setup for sure.
Turns out you can use a power pack to power a cheap Chinese made light. However, it only works well when the light is running on half power mode (which is around 300-lumens). Would I recommend using a power pack for this job? No, I wouldn’t, because higher quality and higher voltage batteries are going to give you much better performance and runtime. USB connectors aren’t very well suited for cycling environments anyway. Lots of vibrations and impacts aren’t going to do any good for them in the long run. So you won’t be getting good performance or durability out of a USB power pack.
What I recommend for budget conscious cyclists are Solarstorm lights. The quality might be off and you need to be careful with the batteries, but for the price they can’t be beaten. There’s a link at the bottom of the post.