When I was a kid, my dad brought home a set of Craftsman battery powered yard tools (weedeater, hedge trimmer, blower). Back then, battery powered tools were just coming out and these used an external battery pack. The pack was a hard case with a handle, a shoulder strap, a charging jack and a cigarette lighter socket, into which you would plug the power tools. They built things to last back then. My dad is passed, but I still have the tools and one of the battery packs. After replacing the internal battery (a standard 12 sealed lead acid battery), I decided that I could use this for other things around my shop. I wanted to be able to easily tap into for uses where there wasn't a cigarette lighter plug, but I didn't want to modify the case.
This adapter plugs into any standard cigarette lighter socket and brings the voltage out to two 1/4" bolts that you can use with bare wire leads, clips or spade/lug connectors.
I've had the plugs in my inventory for years, so I do not know the exact specs, but I think the threads are pretty common to this kind of plug. You should be able to use this with just about any cigarette lighter plug that has a screw on end.
There are two versions. v2 has a red insert on one side to differentiate the polarity of the two taps, while v1 does not. Use which ever suits your needs best.
You will need 2x 1-1/4" bolts; 4x 1/4" nuts; 2x 1/4" washers; 4x 1/4" washers (optional).
Press the bolts through the holes from the bottom, followed by a washer and an nut. If you plan on using spade/lug connectors or bare wires to connect to the taps, I'd suggest placing two more washers on top of the first nut, followed by another nut. If you are only clipping to to taps, just add another nut without the additional washers.
Solder the wires to the lighter end first. Run both wire through the adapter and out one side. Twist the wires as you screw the two halves together to keep the wires from twisting off the connections. Once the pieces are screwed together, you can pull one wire through to the other side and then make your connections to the bolts.
How I Designed This
I am really proud of this design. Not only is this my first real model using FreeCAD - which I am only just now learning - but it is also my very first attempt at designing printed screw threads. True to form, I don't start with something simple - where you just look up standard threads on a chart and plug in values - no... I have to start with something that has non standard threads that I had to measure myself. Having to only make two small changes in the size to get it right, I am extremely happy with the results.