DIY 3 Amp USB Power Bank and Lithium Battery Charger

Based on a TI BQ25896 supporting 18650, 20700, and 21700 batteries.

James Lewis
2 years ago • Productivity

Practically everyone has at least one USB power bank. But only some have one that they built! Ratti3 is an IT engineer who dabbles in electronics and has made a DIY lithium battery charger/power bank. This design uses a lesser-known charge controller that provides dual functions.

The module has a 0.96" 128x64 OLED display, a rotary encoder, and a battery cell holder. The display and encoder form the main user interface. And the cell holder accommodates 18650, 20700, and 21700 lithium battery cells.

A microcontroller and a charge controller are the two primary ICs in this design. The Microchip ATmega32U4 controls the OLED and interfaces to the charger IC, a Texas Instruments BQ25896. Ratti3 notes that most lithium battery chargers use a TP4056 IC, which only has a charge capability.

One unique advantage of using the BQ25896 is that it acts as a battery charger, a boost converter, and a USB power bank. It can charge a Lithium cell with a charge current of up to 2 amps. When operating a USB power bank, the IC can deliver up to 5 amps to a USB-C or USB-A host port.

In addition to the active ICs, Ratti3 added NTC thermistors to individually monitor both the battery's and charger IC's temperature. Even with a significant heat sink, during the 3 amp power bank mode, the BQ25896 can get quite warm.

Ratti3 mentions one downside to using the ATmega32U4 is limited flash space. With all of the Arduino libraries needed to support the charger/power bank's features, little space was left over. Sadly, Ratti3 didn't have enough code space to implement a visual feature like a charge gauge.

Demo and explanation video (šŸ“·: Ratti3)

While you cannot purchase this BQ25896-based charger/power bank, you can build one. Ratti3 provides this GitHub repo with the Arduino code, PCB Gerbers, and links to the bill of materials. In addition, you can visit Ratti3's blog for a post with more information.

James Lewis
Electronics enthusiast, Bald Engineer, AddOhms on YouTube and KN6FGY.
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