If the batteries are the heart of the Li-ion battery, then the PCB is the brain. This
is the one I used. It was 6.50 at Batteryspace. It was easy to solder wires on the PCB. It is designed to stop solder from spilling onto the rest of the board.
PCBs come indifferent varieties depending on number of cells, voltage, and capacity. Here
is a list of all the PCBs you could use with 18650s on batteryspace.
Here are the specs for the one I used and I will explain what everything means...
Overcharge protection voltage for single cell: 4.35V
Over discharge protection voltage for single cell: 2.40V
Over current detection protection: 4-6A
Supply current: Max 30uA
Short circuit protection
Protection circuitry resistance: <=50mohms
The PCB prevents overcharging because the delicate lithium ion chemistry of the battery can be damaged if charged with too high a voltage and the PCB will cut power to the cells if you did so. This should not be a problem if you charge with a smart balance charger. If you charge a cell with 4.2 volts, then the cell voltage will never rise above 4.2 volts, even if you charged the cell for weeks. You still don't want to charge a cell beyond the point at which it is charged. A smart charger will turn off once it has finished charging.
Many batteries can be discharged all the way to zero volts, this is not one of them. If the voltage of a lithium ion battery dropped to zero, or even below 2 volts, it would be damaged, and would never charge back up. Cell phones have this same protection. If you measured the voltage of a "dead" cell phone battery it would probably read 2.5 volts.
Over drain protection is necessary because this is a small PCB with tiny components and can only handle so much current. It shuts down to save itself when drawing between 4 and 6 amps.
Supply current is the current draw from the electronics on the PCB. It is practically nothing and will not drain your battery.
Short circuit protection means the PCB will turn off if it detected a short; if a wire became disconnected or if the wires crossed.
Protection circuitry resistance is the resistance caused by the PCB. All circuitry produces a little resistance. Again the drain is so little you will not notice it.