Cheap and Easy PSU for Canon EOS

I think lot of guys here on Instructables know how much satisfaction is obtained building a gadget entirely by ourselves, but... sometimes it could be a clever solution opting for some nice and ready accessory. I think this is one of these cases.
I recently had the need to have a power supply for my Canon DSLR, an EOS 40D, to make some timelapse. For this pourpose I've bought an LM2596S step-down adjustable power supply module from eBay, it's very cheap, efficient, compact, and (I hope) safe... and it fits exactly inside my exhausted lithium battery case. And this instructable could be handy adapted for almost any digital camera battery, the only requirement is that the case contains the power supply module.

Step 1: Intro

The new psu is intended to work with a wall-plug transformer the same as with any DC power source between 9 and 35V, as a car battery or almost any wall-plug psu. This could be very handy in case of timelapses, gigapan shots, or long movies.

Step 2: What's Inside?

I've made the same modifications on two batteries, because both of them were exhausted. They weren't Canon batteries, indeed the original ones are almost ethernal.
You have to cut/crack the plastic casing all around the perimeter, because it's glued verty sturdy. Inside you'll find two 18490 Li-ion batteries, which are very rare to find to replace.

Step 3: Keep the Shells

Separate the battery parts unsoldering the battery contacts, and clead the case edges with a cutter, they should fit each other so we'll glue them at the end.
Maybe you wish to keep the protection circuit as replacement...

Step 4: Drill the Hole

Now take misures of the aperture which you find on your Canon. It's a little rubber door which you can bend to let the wire pass through. I've found that in battery-grip the aperture is in a different position, but if you're powering the camera by an external powersource probably you don't need the battery-grip. So I've opted for the in-camera wire position.
Drill an hole in that position and file it or cut it untill it becomes of the same shape of the wire. I've also added some heat-shrinking tubes to toughen up the cable extremity.

Step 5: Glue the Circuit

Check that the circuit fits in the battery case, I've had to file a corner to make place for the cable.
I've insulated the two unused contacts, and also added a piece of tape to not let the pcb touch them.
Then I've used a double-tape to lock the circuit in place. I've added another layer of tape over the one you see in the pictures.
You can then begin to solder the wires on input and output terminals.

Step 6: Solder It

You can see the glued pcb in place. You can now connect a power source to the cable ends, maybe 12 or 24 volts work good. With a multimeter misure the output voltage and turn the potentiometer until you'll obtain an (almost) exact voltage of 7.4V. 

Step 7: Label It

Now you can add the DC input range on a label behind the battery, together with output voltage and plug's polarity scheme. I've printed some coloured labels so you can cut them and glue into the shape of the previous one.

Step 8: The Socket

As female socket you could buy one of those nice ones which already have a cable connected. I've used a different type, and I had to solder the wires on them. Then I've enclosed them into some heat-shrinking tubes to obtain a better look.
[UPDATE] You can now see the detail of the cable coming out from the rubber cover near the battery door.

Step 9: Done!

Check another time that everything works good, let the power connected for a few time, try different input voltage, bend the cable, flex it, pull it, also chew it if you dare... it's better to get a shock now than to find your three months timelapse failed because of a bad soldering! Obviously I'm fooling with you, always pay attention to high current sources as car batteries, they're very dangerous!
Everything is ready, it's time to glue the shells pair, maybe with two elements glue, and try your new power adapter!