To make yourself a portable power bank you need two things:

  • Charger case — lots of cases are available from sources like Ebay and AliExpress
  • Battery — typically an 18650 lithium ion battery

Building the charger is simple

All you need to do it open the charger case and insert a lithium ion battery.

Where to Get Charger Cases

Ebay has hundreds of charger cases pretty cheap.  Also, AlieExpress has some good charger cases.

5V 1A Power Bank from AlieExpress

https://www.aliexpress.com/item/Mini-USB-Battery-Storage-Case-5V-1A-POWER-BANK-Suite-18650-BATTERY-External-DIY-Kit-Case/32785262152.html

Where to Get Batteries

18650 batteries are cheap at WalMart.

8 Batteries for $11.28 from WalMart

https://www.walmart.com/ip/8-pcs-Universal-18650-3-7V-9800-mAh-Rechargeable-Lithium-Batteries-Tip-Main-Batery-Cell-For-Flashlight-Torch-Camera/182785464

10  Batteries for $11.29 from WalMart

https://www.walmart.com/ip/Universal-10PCS-18650-6000mah-3-7v-Li-ion-Rechargeable-Battery-Black/954428408

10  Batteries for $9.25 on EBAY

http://www.ebay.com/itm/10pcs-Skywolfeye-18650-5800m-mAH-3-7V-Li-ion-Lithium-Rechargeable-Batteries-/262921558596

The above are some samples of what is available.  It’s best to do your own research.  You may find a better deal than I did.

Integrated Circuits for Charging Lithium Ion Batteries

If you are feeling adventurous or you want to experiment, there are a number of integrated circuits for charging Lithium Ion batteries.  These Include:

  • bq24100 series – Switched-Mode, Li-Ion and Li-Polymer Charge-Management IC With Integrated Power FETs
  • RT9532 –  Linear Single Cell Li-Ion Battery Charger IC for Portable Application

Here is a nice article about the RT9532 by Richtek and here are is a data sheet with some application notes from Texas Instruments on the bq24100 series.

If you want to see some other integrated circuits for charging, to to AliExpress.com and type “charger ic batt” in the search box.

Finally, here is an article from  Digi-Key Ekectronics titled A Designer’s Guide to Lithium (Li-ion) Battery Charging and one from Maxim Corporation The Basics of USB Battery Charging: A Survival Guide.

Battery University: Charging Lithium Batteries