This is a simple power supply. I had some 12V batteries from an old alarm system and telephone backup system. I simply wired them in parallel and wired in a power outlet and alligator clips. I currently use the alligator clips to attach the batteries to a trickle charger. When not charging I put two ziplock minibags on the end to prevent their shorting out. The power outlet is simply drilled into the top of the ammo can.
You can see a picture of the unit powering a lamp that I retrofitted with a 12V LED car headlight. I've testing the duration of this and stopped after 36 hours. I would estimate it would last at least twice as long.
I also use this to power my bicycle compressor.
I'm not sure if it has the amps to jumpstart a car. Combined the batteries put out 7.5 amp hours.
I plan to get a power plug so that I can charge the unit with the lid closed but I will keep the alligator clips because you never know when you need to directly wire something. I will also probably get a rubber stopper to put into the top of the outlet when not in use to keep out dust or moisture.
The total cost of making this was $0. I had all the materials lying around. The materials included:
-two old 12V batteries
-two alligator clips
-two wire nuts
18 SEP 2013
- I modified the battery input system. (Thanks Klee27x) Rather than add posts to the box itself I built a charging station so that I can keep the profile of the ammo can intact. Also, I worried a bit about shorting the posts if it was integrated into the box itself. The charger is made completely of reused components including a piece of 3/4 inch wood that is cut into a T-shape so that the alligator clips from the charger can't short. It has two bolts with nuts fastening them. There are two washers underneath and one on top. Wire is wound between the two lower washers and the nut is tightened to keep it all in place. I've identified which side is red and which is black.
28 SEP 2013
- I realized I can use the battery input system as an output system too. In the last picture you will see that I am powering a Jeep VIC from the posts. I hope to put the VIC into a clock as per http://www.instructables.com/id/Custom-digital-clock-from-wood-and-Jeep-VIC/
28 SEP 2013 - I found this cap in my junk box. It was from an old automobile lighter outlet. This will keep out dust and moisture in the field. See the picture