In this instructable I'll show you a very useful life hack that is used to protect any 18650 cell. You can make it only with an old cell phone battery and with some wires. The Lithium ion batteries that are used in phones have a build in protection circuit, that automatically turns off if the voltage of the cell is smaller than 3 volts. This will save the battery from overdischarging. We can use this protection circuit for any Lithium ion cell, like a 18650. The salvaged (from laptop) and the cheap 18650s haven't a protection circuit.

Now go salvage the parts!

Step 1: Tools and Parts

TOOLS:
*multimeter
*soldering iron
*wire cutter

MATERIALS:
*18650 or any Lithium ion cell that you want to protect
*some wires
*an old phone battery

Step 2: Prepare and Salvage the Circuit

Get your cutting tool then try to open up the battery. Be careful, do not short the circuit, may cause fire! Cut the input wires of the circuit then check which is the positive and the negative input. You'll see there three output pins. One is the signal pin and the others are the negaitve and the positive output. You can see the + and - marks on the battery, these marks show you which are the output pins. If you salvaged the circuit go solder to the 18650.

Step 3: Prepare Wires

Prepare four pieces of copper wire.

Step 4: Heat Up Your Soldering Iron!

Easily solder together the circuit and the battery using some wires. Be careful with the polarity and do not short the battery. Do not leave your soldering iron on the battery for much time.

Step 5: Testing

Get your multimeter and test the output voltage. If it,s Ok you're done. You can use now for any project because the cell is protected. That's all.

Hope you liked, write your opinion or other ideas! Good bye!