Desulfator for 12V Car Batteries, in an Altoids Tin

Step 2 Build the box and start your layout.

First, you should cut a piece of perf-board (aka breadboard) the same size as the inside bottom of the can, minus a wee bit for wiggle-room, with a coping saw or jig-saw. Use a disk sander, if you have one, or a sanding block if you don't, to clean up the edges. You'll find that the fiberglass board cuts and sands easily. If sized correctly, the circuit will sit nice and snug inside the can with no mounting screws or other hardware needed to hold it in place, yet be removeable for fit tests or repairs if needed.

Next, place the parts loosely in the Altoids can to get an idea of where you'd like to mount them. My layout roughly follows the schematic and limits the number of jumper wires needed to make connections. I'm sure there are better layouts but what you see worked well enough for me so feel free to copy it.

Early on I'd planned on bolting the FET to the lid so the lid could act as a heatsink but this turned out to be unnecessary. There's just enough room on the perf-board for it and a socket, and since it doesn't get hot at all, no additional heatsinking is required.

You'll need to insulate the metal can from the circuitry by cutting some thin cardboard to cover the bottom, lid, and sides. The "wiggle-room" mentioned above is to allow space for the cardboard sides. Later on, you'll mount the cardboard in with double-stick tape but for now leave the cardboard out while you drill the holes in the can.

The output wires will exit a 5/16" hole in the left side, and in this hole you'll fit a 1/4" rubber grommet. Start small and go gradually up in size with the drill bits as the metal is thin and soft and bends quite easily. Remove the flash with a countersink bit, if you have one, or a larger drill bit twisted with your fingers. Use something round and hard, like the shaft end of a large drill bit or the ball end of a small ball-peen hammer, to flatten the holes after drilling. Don't forget to allow room for the lid which overhangs the sides a 1/4" or so.

The vent holes around the right side are 1/8" diam and spaced about 1/2" apart. A center punch helps a lot here but a nail works as well to dimple the metal a bit to better aim the drill. I also drilled a hole in the lid for the LED so I can see it when the lid is closed. You can do the same but you'll have to measure carefully where it will go after you decide where to put the LED in your layout. Mine fitted nicely inside L1. You'll have to also punch a hole in the lid's cardboard liner for the LED to shine through.

Place the cardboard side strip in the can and tack-tape it in place, then use a pen to mark the holes from outside the can. Use a hole punch to punch holes in the cardboard exactly over the marks you made.

If you decide to mount the can to the back of your charger you'll need to drill four more holes in the bottom for whatever fastening hardware you want to use (I used pop-rivets). You can also make the circuit separate from your charger but you'll have to add lead wires and some clips to attach the circuit to your battery. The parts list shows some clip parts that I used but you may prefer larger ones. The lead wires should be made of at least 16 ga flexible wire, thicker if you can get it, and as short as comfortably possible to avoid losses at the battery. Even if you plan to wire your circuit into your charger it's a good idea to make temporary leads with clips so you can debug the circuit before permanently mounting it.

Once the holes are punched you can double-stick tape the cardboard strip and lid pieces in place and fit the grommet. For now, don't tape the bottom piece in, just use it as an insulator as you build and troubleshoot your circuit. This will allow you to pop-rivet the can onto the charger when the time comes and you can then tape the cardboard in permanently over the pop-rivets. If you don't plan to mount your can onto your charger then it's OK to go ahead and tape in the bottom piece.