A complete LiFePO4 based solar power supply for IoT devices
Revision 2 schematic, improved to do 3A and use 10K thermistor
Adobe Portable Document Format - 22.38 kB - 05/01/2017 at 17:44
Revision 1 schematic, has not been thoroughly tested!
force-download - 22.17 kB - 11/17/2016 at 19:02
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!
I've added a PDF of the schematic! At this point, the main charging part is know to work, but not thoroughly tested over the full voltage and temperature range. The over and under temperature protection has not been tested either, nor has the heater.
But it might be useful to someone to get an idea. :)
Received some perfect purple PCBs from @oshpark, so it was time to build prototypes!
I decided to build up all three boards at once (I guess I'm an optimist), and use a stencil and solder paste, but place the parts by hand instead of using the Pick-and-Place available here at the Tinkermill. I tried to use the laser cutter to make a kapton stencil. It worked horribly, but after many tries I finally had something usable enough to make these three boards. Here are the PCBs with (fairly messy) paste applied:
I figured the messy paste would be OK in this case since no small pitch components are involved. After spending some time placing the parts, this is how things looked:
Only two boards in the picture because the other one is in the reflow oven. :) The end result after all three boards were reflowed:
Nice, they turned out well! Next, I need to find some time for testing.
[this comment has been deleted]