USB charger for Pentax D-Li90 batteries. I've been using previous versions of these to recharge my phone (which I use as a GPS) on backpacking trips, but the older versions aren't working as reliably with newer phones. This circuit seems to work better and is much smaller. I usually top off my phone with the batteries when they don't have enough charge for my camera, but I haven't checked to see how well that works with this converter.
Uses a step down circuit with USB Port*. A USB power meter* can be helpful for testing.
Ruby Red and Sapphire Blue Translucent PETG from Atomic Filament