The 3.7 V Li-ion battery used by cellphones has a good shelf life, is compact and lightweight, and charges quickly with little degradation. Its voltage is pretty compatible with devices that normally accept two AA alkaline batteries, and so it's a natural substitute.
This instructable describes how to attach a 3.5 mm audio jack to a cellphone battery, allowing it to be connected to a headlamp that has a 3.5 mm receptacle. Such jacks and receptacles can be scavenged from broken audio headphones and radios by desoldering their leads.
A particular detail is the creation of a custom shrink-fit sleeve for the battery using a polyethylene terephthalate (PET) water bottle.
Step 1: Materials and Equipment
Here are the things I used, minus the soldering stuff, cardboard and heat gun. PET plastic will shrink under a heat gun, but not enough with a hair dryer or boiling water. You'll also need pliers, wire cutters/strippers and maybe a knife.
Step 2: Recovering the 3.5 Mm Jack
Remove the plastic covering over the 3.5 mm jack. Use a sharp knife to cut away the plastic. This step will vary depending on your headphones, since some use soft plastic, while others use hard plastic. Some may even be conveniently encased in a threaded cover, saving you a lot of trouble.
The audio cord has four wires, corresponding to Left, Right and Mix, I think. I decided to use my own wire, taken from a 5V power adapter. It may be that you can get away with ignoring the wires going to the middle part of the jack (mixed left and right), and connect to your battery with the Left and Right wires directly. That would save a lot of trouble.
If I try this, I'll add a revision.
Warning: OK, you're reading Instructables. I will figure you know that these things can hurt you.
Step 3: Solder Your Power Cord
I am using insulated wire taken from a broken power supply. I had previously found, using my meter, that my headlamp's receptacle would use the tip and base of the 3.5 mm jack, and ignores the middle contact.
Note: Soldering is not shown here. It needs to be done delicately, because the plastic spacers in the jack will melt if you take too long and heat the jack too much. Solder one wire, then let everything cool before soldering the next.
Add a piece of heat shrink tubing to cover your work after you're done. I let the tubing shrink over the wide collar of the jack, then cut it away flush afterward.
Step 4: Prepare the Shrink-fit Sleeve
Using a PET plastic water bottle, make a sleeve to cover the battery and hold the wire in place.
It is dangerous to solder directly onto the leads of a battery. If your timing is bad, the battery may explode, so the plastic sleeve is used to hold the wires onto the battery contacts by pressure only.
First, cut a uniform cylinder out of the drink bottle. A smooth cut works best, and the cylinder should be wider than the battery.
Next, make a 'dummy' battery, slightly larger than the actual battery, out of cardboard. I used two sheets of cardboard for extra thickness.
Put the dummy into the plastic cylinder, then use a heat gun to shrink the plastic over the cardboard. Use pliers or forceps to hold the cardboard, since it will be too hot to use your hands.
I do not know if toxic fumes are released by heated PET plastic. To be safer, do this in a well-ventilated area or under a fume hood.
After the plastic is cool, remove the cardboard.
Step 5: Prepare Your Wire Contacts
The contacts on the battery are recessed, and it is dangerous to solder directly onto a battery. Instead, the plastic sleeve will hold the wires in place by pressure alone.
Twist and bend the ends of your wire leads to create a pad that will fit into the recessed battery contacts, then add a bit of solder to create a bulge that will touch the battery. Test your contacts to make sure they fit and do not need too much pressure to stay connected.
Remember, I am following a convention of tip = positive
Step 6: Slip the Battery and Wires Into the Sleeve
Trim the rough edges of the plastic sleeve. The edges are more shrunken than the main part of the sleeve, so they must be trimmed to allow the battery to enter. You may want to trim only one side, and leave the other side as a 'stop' for the battery.
The battery and wires can now be slipped into the plastic sleeve. The sleeve is a bit longer than the battery to start, but makes a snug fit when the wires are in place. The pressure holds the wires in contact with the battery.
This step is a bit tricky. I may modify it to use a long sleeve that gives me a loose fit, then tighten everything by introducing a wooden chopstick against the bottom of the battery, forcing it onto the top of the sleeve.
Step 7: Connect to Your Device
My headlamp uses a 3.5 mm receptacle. I have since used audio receptacles scavenged from broken radios, etc as power inputs for these batteries. Remember that the tip is positive in my setup.
Step 8: Final Notes
Recharging the Batteries
I made up a wooden spacer with metal contacts connected to a 3.5 mm receptacle that fits into a battery charger suitable for cellphone batteries. With this, I can charge any of my Li-ion batteries. You may have to come up with something else, though.
Where to use these?
I have used this battery to power digital cameras, a Flip video camera, my headlamp and a radio. These are all devices that normally took two AA batteries. They didn't seem to object to the extra 0.7 V, but you need to be aware that this kind of battery hack may damage sensitive electronic devices. Use it at your own risk.