If you can't afford to buy a powerbank this can be a solution
Step 1: Materials
The materials needed are:
-old rechargeable flashlight
-usb socket (female)
-cord with a plug (from junk appliances) (OPTIONAL)
-another battery (from old rechargeable flashlight also)
Step 2: Hacking a Rechargeable Flashlight
Hack the circuit of the rechargeable flashlight
Step 3: Desoldering
Desolder the led lights and battery from the circuit
Step 4: Connecting the Batteries
Connect the batteries in series (since the individual voltage output of the batteries does not exceed 5 volts). Then, connect them back to the circuit.
Step 5: Connecting the Usb Socket
Connect the usb socket (female) replacing the place of the led lights in the circuit.
NOTE: Use the 5v regulator (7805) to connect from the source to the usb socket.
Step 6: Changing the Plug of the Powerbank (OPTIONAL)
Change the plug of the powerbank by the recycled cord with plug.
(I just added this to make the charging cord of the powerbank longer)
Step 7: Done!!
You can now charge your gadgets to this powerbank. You can recharge this powerbank everytime it runs out of power.
It's better if you put it on a nice container.