I wanted to make a quick battery charger just for giggles, the basic requirements were “allows me to plug in a USB cable to charge my phone” - which is fairly easy to accomplish going by all the crazy devices you can buy on the market currently.
Rather than bother with tight fitting casings and springs or complex battery wiring, I piled the batteries up in an old poster tube and made a contact for each end. A section out of an egg cup friction fit into the tube, and I jammed a server rack cage nut into it with some wire. The cage nut is roughly in the shape of a small letter ‘e' and the bottom of the character is pushed through the cardboard with an interference fit. 🙂
I split some dual-core wire and ran a length to each end, giving me a positive and negative terminal. The other ends of the wires I spliced into a cigarette lighter extension/multiplier that I had lying around.
After I wired it up, I realised that it has a little LED in the extender which would eventually drain the batteries given time. Going with the ghetto theme, I embraced the beauty of my creation by turning the loose fit of one of the egg cups into a switch! Now, no more flat batteries.
Before plugging in my ridiculously expensive iPhone I used a multimeter to test the output voltage, which came up as a healthy 12.6 volts, well within spec for the phone charger I had on hand. All good.
A little more tape, to hold the wires in place and the “negative terminal” from falling out, and it was all done. Easy peasy.
Final(ish) design, for now 🙂 I’m fairly sure the battery life is going to be terrible, but it’s not designed to be good - just simple and serviceable.