I carry a spare battery for my PDA, but changing the battery in the middle of a muddy field isn't too lovely, so I'd like to be able to power the PDA from something more substantial.
I ran an ethernet cable from the handlebars to the back of the bike, and four of the conductors out of the eight, are for the ammeter/voltmeter (which I used for the first time on Friday, and I love it. Two more of those wires lead to a female USB port on the handlebars, and I have an adaptor so that I can plug the PDA into that.
Down at the back end of the bike, I had a "power pack", which is a battery that has a USB port. But I got to thinking today, and I've made something better.
I've got a step-down board, that will take any input voltage and step it down to whatever voltage I want; in this case, 5.3 volts. I connected the input of that to an EC5 male (my batteries terminate in EC5 females), so I can plug it in to a bike battery and step down the 14-odd volts to 5.3. At the other end of the step-down unit, I've connected a female USB socket, and put the whole thing in a nice little plastic box. So now I can power the PDA from the main bike batteries; I always have more than one set when I go out for a long circuit, and this takes such a tiny amount of power (I measured it, it's 0.5 watt) that it'll have negligible impact on power usage.