This is the mobile power module Accessories, 18650 Circuit board shell.
Meet the individual needs of your DIY, but you must understand basic electronic assembly.

Features:
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.
Output voltage and current stable, will not harm user devices.

• Specifications:
  Micro USB input: 5V 1A
  USB Output: 5V 2.1A / 5V 1A(Dual USB interface)
  Charging display: intelligent digital display
  Batteries Type: liquid lithium-ion batteries (18650)
  Product Size: 57mm x 28mm
  Screen size:27mm x 17mm
• Installation instructions:
  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.
• Package Include:
  1x Mobile Power Bank 18650 Battery Charger PCB