It’s been a while since I posted anything related to my MPPT Solar Charger project. That doesn’t mean that no progress has been made…
I’ve performed many hours of testing, pushing the charger to and even beyond its 75 watts rating. I was able to confirm the very high efficiency n the range of 96 to 98% over a wide range of loads from 1 to 75 watts. And I’m happy to report that during all those tests I haven’t damaged anything. I’m more than pleased how this little charger is performing.
The most obvious progress that I’ve made is the mechanical design that you can see on the various photos here. The case is a pretty standard 115x90x55mm cast aluminum box for which the boards were designed right from the start. So if you’ve ever wondered about the peculiar shape of the display unit’s PCB you now know why. If I had made it rectangular it won’t fit.
I’ve milled the various holes and slots out of that cast case on a manual mill which was somewhat time consuming but a lot of fun. The slots at the back for the in and outputs have turned out particularly well I find. Hence the close-up ;-)
I’ve also given away some boards over the months. A few of them went to an open-source project named MeshPoint where they are integrated into a WiFi hotspot designed for outdoor use in disaster areas, refugee camps and the like. The project is also on hackaday .io where it runs for this year’s hackaday prize.
Together with the bords that I’ve used myself for testing I’m slowly but surely running out of the protopac that I’d ordered from dirtypcbs.com. So I decided to make a few minor changes to the board resulting in Revision D. The changes are really slight and require no or only slight changes in the firmware.
By far the biggest change concerns the flash chip. As I’ve mentioned before, the last one was simply a bad design choice because it can’t do sector ereases. The Atmel AT45DB161E can do that and provides 16Mbit (i.e. 2 MB) of storage which is plenty for our application. This change of course does require firmware changes but since hardly any code for the flash chip has been written so far that’s not really an issue.
I’ve also included the surge suppression diodes that I removed when going from Rev B to Rev C. They are not really required but make the charger far more rugged against any spikes in the (particularly input) voltage. That also makes testing quite a bit safer so I thought at least having the option of including them is well worth while.
The signals from the three temperature sensors now get RC filtered to hopefully solve some issues I’ve experienced with overly noisy measurements. Placing the capacitors close to the microcontroller was a major challenge but I think I did the best I could.
The other changes are really tiny. I’ve fixed a mistake on the silk screen, changed a number of footprints to make the parts easier to solder and added a diode from the USB bus to the output. That allows to power the charger via USB. That’s obviously only useful for testing and debugging but there it’s a nice feature.
If you want to check out the details, they are on github: https://github.com/soldernerd/SolarChargerHardware. Or click here for the next post in this series.