I decided to use the Hackaday Prize 2017 as motivation to try and finally push this project to production! :)

I've had a unit outdoors in my #SoundBeacon project through the Colorado winter. We didn't have a very severe winter, so it wasn't a great test, but it seemed to work fine. I never saw the battery voltage go below 3.2V.

I've incorporated some changes based on what I've learned from testing the #LiFePO4wered/18650. I've gone to a bigger, higher current Schottky diode, and a newer, higher current MOSFET that I also intend to use in the #LiFePO4wered/Pi+. I hope to streamline my operations by trying to standardize components across projects. I also added a lot of thermal vias near the power components, especially the Schottky diode, to pull heat to the ground plane. With these changes, I hope that the system will be capable of 3A charge and load currents. To accommodate this, I also changed the thermal fuse on the output. The updated schematic is available in the files section.

If I did all my calculations correctly, the thermal protection system should be able to work with 10K NTC thermistors with B value between 3450 and 3950 (this seems to be some sort of "standard" in battery packs... I wish they'd ACTUALLY pick a standard resistance and B value instead). The battery will stop charging and the heater will kick in when the temperature falls below -3.1 to 0.2 degrees C. Charging will resume when the temperature rises above -2.5 to 0.7 degrees C. On the high temperature end, charging will stop when the temperature goes above 58 to 63 degrees C and will resume when the temperature falls below 52 to 57 degrees C.

I decided I need to get many more prototypes into the field to build confidence in the system. So I designed a panel so I can build a good number:

It's a small panel designed to fit within the 10x10 cm limits of Elecrow's Special Offer. So I will be getting 10 panels of 6 PCBs and I intend to use Tinkermill's TM245P pick-and-place machine to build these myself, since there are no difficult, small pitch components.

They are not officially for sale, but I had one customer very helpfully offer to buy some (at reduced "prototype" price) and test them in his devices. Thanks much Will! I want to encourage anyone else who has a need for a solar power supply to do so as well. The more field test data I can collect, the better!