Zak Kemble blogs about a small USB pass-through which keeps a USB power bank (cell phone/mobile charger) from shutting itself off while powering low current projects, something many of us have run into.

Ever wanted to power a project from a USB power bank, only to have it keep shutting itself off because the current draw was too low? This project aims to fix that with these handy and slightly over-engineered USB modules containing a microcontroller, op-amp, MOSFET and a few other bits to create a pulsed adjustable constant current sink, as well as supporting USB 3 pass-through!

A quick hack to keep power banks alive is to use something like a 150R resistor across the power output to draw an extra 33mA, but some power banks might need as much as 100mA to stay on, requiring a 50R 1W power resistor. Usually, power banks don’t need to have current continuously flowing to stay on, where a 2 second pulse every 15 seconds might be enough to keep it alive. This pulsing technique drastically improves the battery life of the power bank, perfect for powering a small project for a few weeks.

See the blog post on this amazing item with schematic and design/code on GitHub and you can even buy one on Tindie.