A while ago I did some research on recharging devices that charge via a USB connection (i.e. 5V DC) using the power from other batteries. Not having done any electronics since school, a simple approach was required that wouldn’t be too difficult to build. My first effort was based around a linear 5V regulator and a 9V PP3 battery. It worked, sort of… The problem was that it wasn’t very efficient. I would get about half a full charge from one battery, and the effects of reducing 9V to 5V produced a fair bit of heat. Most of these shortcomings stemmed from the limitations of the PP3 battery.
After further research, I found a neat device called the MintyBoost which was designed to boost two AA batteries (~3V) up to 5V. The capacities of AA batteries are much better than PP3 (less internal resistance and much higher mAh rating). At $20 for a DIY kit it looked a good solution to the problem and a bit of fun to build it.
On the left: this is what you get in the packet. The image in the middle is the result of 20 minutes work with a soldering iron, no swearing, burns to self or marks to the dining room table. On the right with all soldering complete, it fits neatly into a tubeless tyre repair kit box and gives me enough room for charging lead too.
So this kit will give me about two charges for my GPS from 2 AA alkaline batteries. It’s perfect for the short touring rides I have planned, or seeing as I can pick up AA’s in most small shops, it would be of use for much longer trips. It only weighs 87g too, so no serious weight penalty either.