For just US$2.29 , buy Dual USB 5V 1A 2.1A Mobile Power Bank 18650 Battery Charger PCB Module Board from the China wholesale webshop.
Motherboard comes with protection function: Charging overcharge protection, over-discharge protection, overcurrent protection.
Intelligent output device after charging full, automatically stop charging, to prevent mobile phones overcharge.
Batteries Type: liquid lithium-ion batteries (18650) 3.7 lithium-ion battery only
The circuit board is connected to the battery negative B-, B + is the positive, do not connecting reversed, connecting reversed will burn the board, the battery is connected in parallel welding, assembling battery. Note that the insulation between the battery and the circuit board, battery lead not to touch the circuit board, so as not to cause a short circuit.