EDIT: Ok, well I've just been informed by sladek that this has the potential to blow private bits off. So, be EXTREMELY careful as I cannot delete this instructable. I guess it shows that I have no real training in electronics past my college electrical engineering 101 gen ed.

"sladek says:
Cellphone batteries are lithium based, and with lithium batteries it's dangerous to drop them below their minimum voltage. A full charged 3.7V battery is roughly 4.2V, once it disipates down to 3.7V, it's minimum safe voltage, the battery can swell and explode, without the need for ignition."

I would suggest not even trying this with a phone battery. Replace with a different battery.

I've been a fan of this site for a while, but this is my first instructable and it is very simple. Hopefully it will open the door to other ideas using cell phone batteries to power things.

I know the bike light underglow idea has been done before, but when it came to the batteries, that was a problem for me. It had to charge up a LED light, and I wanted something small, lightweight, and rechargeable, since I was using it for my bike. I didn't want to spend money on D cells (don't have a recharger for them) or for big 6v batteries (no recharger and too big), like in maxwell and shammallamaman's instructables, where I got the initial idea.

I had a couple cell phones I didn't use any more, so I decided to do the green thing and make them useful again. Most cell phone batteries are around 3.7 V. The ones I had were 3.6 and 3.7 V, so it worked out perfect.

Step 1: Parts List

Parts:

-wire
-2 cell phone batteries
-12 V LED light kit - I bought it from Walmart auto section, but you can make one easy. Wire up 2 3 mm LEDs and stick them in a clear plastic tube, one on each end.
-solder
-100 ohm resistor
-switch
-wood
-tissue or foam
-rubber bands
-cassette tape case or Altoids case - Altoids case would probably work better. All I had was the cassette case and the solder melted through.
-tape
-zip ties
-hot glue (optional)

Tools:
-Soldering iron
-Scissors
-Wire strippers (optional, can use scissors carefully)
-Hot glue gun (optional)
-Super glue
-Drill

Step 2: Step 1

All of this is straight forward if you have worked with electronics. I put the batteries in series, soldered in the resistor, then the switch, then the LED.

I want to point out that you have to have an entry and exit for the switch and LEDs and arrange everything before you solder. May seem obvious, but it will save you time from having to go back and cut and resolder everything. Trust me, I know :(.

Step 3: Step 2

This is the tricky part. I couldn't solder the batteries onto anything. They could only touch the metal. This is because to recharge, I had to put the batteries back into the cell phone, where the plug in is. That's the reason for the tissue paper and wood.

Put the wood in the cassette case to make the batteries fit in tight so they don't move around. I also used a ton of solder so if it did move around, it would still make contact.

Once the batteries were in place, I put the tissue paper in to keep the batteries from moving around.

Step 4: Fin

Everything is in place now. Super glue the soldered parts (except for the batteries of course). Use the rubber bands to keep it closed and slap it on your bike with the zip ties.

Next bike project...spinners? body kit? flame paint job? NOS? The sky's the limit.