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.
Very very cool!
Ever thought of designing a 3D printed case for it?
Hi Brian
Thanks. Yes I’ve considered it but I find 3d printing to be a frustratingly slow process particularly if you don’t have your own printer that you can just run overnight or while you’re at work…
Lukas
Great work! Tons of useful information. I’m impressed. Thank you for sharing.
Lukas, do you have a BOM and the firmware available for those of us that want to try and build your charger?
Hi.
For the BOM I advise you to extract it directly from eagle. Schematic->Edit->Export BOM or something like this. The Eagle files are on github: https://github.com/soldernerd/SolarChargerHardware.
The firmware is on github as well: https://github.com/soldernerd/SolarChargerRevC_Software
You are highly encouraged to try and build your own…
Lukas
Hi, Lukas!
I find your design very interesting – especially the high efficency at maximum load is very nice!
After studiing the schematic in SolarCharger_Rev2_Schematic.pdf
for a while, i noticed one thing i do not understand:
How do you protect the solar panel against reverse feed from the battery when the panel is in darkness? I think the current may take the following path: battery + -> inductor L1 -> bulk Diode of Q1 -> Panel + -> Panel – -> Battery -.
Since there is no limiting element, this backward current may become quite high and may even damage the circuit and the panel. But you said that you did not kill any parts during testing. What do i overlook?
cheers and kudos for this great project! 🙂
Jochen
Hi Jochen
Well noticed. In the Rev 2 design, current can flow from the battery to the pannel when it’s dark. But a 12V pannel doesn’t pull enough current to distroy anything. In my case it’s a few milliamps that the pannel pulls at 12V. Enough to drain the battery in the long term but no danger to the circuit.
All further revisions (current version is Rev F) use a mosfet to disconnect the pannel when it’s not in use.
Lukas
May I ask what is the value of the resistor shunt in The INA213?
Hi Clark
The INA213 doesn’t have a shunt resistor built in, you’ll need to provide one. It has a fixed gain of 50 so that with a voltage reference of 2.048V you want a maximum of about 40mV accross your shunt resistor. That’s how you chose one…
Great work, and thanks for sharing, helps all of us learn.
What are your thoughts on this project from microchip? You think its a good thread to explore?
http://ww1.microchip.com/downloads/en/AppNotes/00002321A.pdf
I would love to know how your system compares to something like this, i think the main advantage is keeping software alot simpler. Again, thanks for sharing your knowledge with us.
Hi Jonathan
Thank you very much for your comment. I was not aware of that app note but it looks very relevant for what I’m doing here. I’ll check it out shortly and let you know.
Lukas
is the software for the charger also in github? Danke schon
Hi Clark
Yes, it is: https://github.com/soldernerd/SolarChargerRevE_Software
Lukas