Zak Kemble has come up with a novel solution for running low-power projects from USB power banks that would otherwise go back to sleep: a "handy and slightly over-engineered USB module" design that acts as an adjustable pulsed constant current sink.
USB power banks are a fantastic way to run portable electronics projects, offering high capacity batteries in a rugged housing and a regulated DC output on the ubiquitous USB connector. Unfortunately, all too many of them are "smart" — switching themselves off automatically if they don't detect a certain minimum power draw, often set above the level low-power projects would require.
"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," Kemble explains. "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."
That's what Kemble's Smart Power Bank Keep-Alive board is designed to do. "This handy USB module uses a pulsed constant current sink to keep the power bank alive, and supports USB 3 pass-through," he notes. "The smarts of this device is an ATtiny10 which controls the on and off cycling of the constant current sink. The current is adjustable via a small trim potentiometer from 0mA to 140mA and the supply voltage can be anywhere from 1.8V to 5.5V."
By default, the pass-through board pulses for three seconds every six seconds, but by flashing a modified firmware to the ATtiny10 module it's possible to configure this alongside the current and voltage — handy given that of the three unbranded power banks Kemble tested during development, all three had different requirements.