Often a rechargable battery is very useful in a prop, not only for battery savings, but space savings as well.
When I was designing my last sonic screwdriver I needed an easy way to make it rechargable without adding too much complexity.
These instructions are the result, an easy, relatively inexpensive way to add a 5v rechargable source to an electronic prop.
Step 1: Tear Down of a Battery Charger
For my power source I used a USB portable battery charger similar to this one. Make sure the reviews are generally positive, nothing about the battery leaking, overheating or not working.
The first step is to figure out how the device comes apart. There may be screws or poppable tabs, or an end may just screw off as in my case. Be careful of the USB port as it may not be strongly secured to the circuit board, no sudden, violent movements.
Step 2: Modifying the Case
Unless you want to power you device by plugging a usb port into the front of the battery (in which case, ignore most of this instructable), you will need a new hole in the case for wires to escape. My aluminum case barely had enough space for the wires to feed past the battery. Before you drill a hole, make sure your wires can get to the new spot.
For my battery I drilled a small, 1/8th inch hole in the back of my empty battery case and used a needle file to remove any sharp edges.
Step 3: Feeding the Wires
Once you have a new hole in the case you might need to feed wires through the case before soldering them to the battery. In my case I had to feed a pink wire through the back of the case, tie it to my yellow(+) and grey(-) power wires, and pull all three back through the case using the pink wire.
Once the wires are through the case make sure they are seperated so you do not short circuit your battery.
Step 4: Final Steps
Looking at the large USB connector from behind and above, the positive pin (yellow wire) is the furthest right, solder your positive wire to that pin carefully.
Similarly the negative pin (grey wire) is to the left, solder your negative wire to that connection.
There are two other solder connections further out on most standard usb connectors. They connect to the case of the USB port and do not carry power.
Make sure you solder to the power out connector, not the power in USB port if you have one.
Once you solder the connections you may add a bit of hot glue over the connections to insulate the connections and help relieve stress. Since the charging circuit can heat up be very sparse with the hot glue. It will trap the heat from the circuit and could cause it to overheat.
Carefully assemble the battery pack and add some hot glue where the wires exit the case to relieve stress and you are done!