OK so here's the backstory on this one...
I was going to go camping with my kids, and we do some easy geocaching. I usually use my phone, but wanted something that didn't need a data connection. I'm on a tight budget, and I already had a GPS unit, but the battery doesn't hold a charge.
I looked up buying a battery, and they're relatively cheap... so that's the best way to go.
But I was also short on time and couldn't wait for shipping. So I made this... and it was so cool and versatile it was worth keeping... and sharing.
Step 1: Materials
12v vehicle outlet
Wire (2 colors)
Thin metal plates (optional)
Voltmeter/Multimeter (optional, recommended)
Scrap lumber (optional)
Step 2: Wire the Batteries in Series
This is really easy with just 2 6v lantern batteries. Wire positive on one battery to negative on the other battery.
DO NOT WIRE POSITIVE TO NEGATVE ON THE SAME BATTERY!
For 1.5v types just make a battery pack by wiring positive to negative on the next battery until you have no batteries left. With either configuration you should end up with one positive and one negative terminal remaining 'open' or unwired.
To 'wire' the AA batteries I cut up a plate from the back of a computer tower I had lying around.
As you can see the AA's take more time, are more 'complicated' to wire (you may have noticed I used some scraps instead of wire, too), have a slightly different output voltage (but still acceptable), and the advantages are smaller, and lighter. Also be aware that the lantern batteries will give a lot more battery life.
Step 3: Attach the Leads
Ok. Now we just attach the positive and negative leads to the battery and to the 12v outlet... sort of...
First you want to have the outlet where you want it. To do this simply cut a hole in the box you're mounting it in, take the outlet apart, put the negative (outside 'shell' of the lighter assembly) inside the mount and the positive (center of the lighter assembly) through the hole. Then screw the assembly back together It should bite down on the edge of the hole and stay in place.
Hook up the leads, remember positive to center, negative to the outside, and polarity DOES matter.
You may have noticed that I have some of those metal plates in the box... well, in case you were wondering, I just measured and superglued my battery wiring (the plates) to the box so I can change out the battery easier... It's harder. But since it's done now, I won't change it... in this box. I think if the contacts were glued to the box lid it WOULD be easier... maybe another box will have this.
Battery in, and a bit of 2x4 scrap as a spacer so my battery doesn't move (not really needed if you don't glue your contacts in place). And make sure you wrap up all the wire ends with electrical tape to prevent shorts.
And we have portable 12v power.
Testing and using this box I have been able to simultaneously charge 2 mobile phones and power my GPS with no problems at all (using a 3-way 12v power splitter). I recommend using a plastic box, or at least not something flammable, unlike I did.