For many people who are homeless or members of underserved populations, cell phones are a lifeline; a phone may be their only way to keep in touch with distant friends and relatives, schedule medical appointments and job interviews, and deal with emergencies. Yet there are very few free public charging outlets available, especially outside of traditional business hours—meaning these vulnerable populations could go for long stretches with dead batteries.

That’s why a unique Little Free Library erected in Lisa Link Peace Park, a small gathering space on State Street in Madison, Wisconsin, proved popular. The kiosk, developed by University of Wisconsin-Madison engineering students, was attached to a light post in the popular shopping area and topped with solar panels. It contained a lithium-ion storage battery and multi-tipped charging cables, as well as a nice selection of books, allowing people to top off their phones 24 hours a day.