If connected correctly, yes. But they connected drain to the battery - in a p-channel FET current flows from source to drain. You can also tell from the direction the built in protection diode is facing - it should be pointing to the battery.
Like it was done here:
That’s from a different part of the schematic. (the current flows from 3V3 to 3V3_S)
But I now realize that this is probably not much of a problem in a pure router application - since VCC supplies power only to the 3.3v linear regulator. 3.3v then does go into the RAK4631 module, along with VBAT, but I haven’t figured out yet what it is used for there.
Current can flow from drain to source too as long as the gate is at a sufficiently lower potential than source. It is very common to use a p- channel as they did for reverse polarity protection.
I can attest that my external charger and undervoltage lockout board does work. The RAK4631 reliably comes back on after the battery got disconnected.
But I encountered a problem: The node lost its lora.region setting, which is a bit concerning. Is there a “save to flash” function that I forgot to call?
So, @K1WIZ
It’s been a year since your original post here, but I just now finished my solar repeater with brown out recovery add-on board. It was a satisfying build and I would be very honored if others were to adopt my design. It came out reasonably compact, uses just a 1.5 Ah li-ion pouch cell instead of the 6.4 Ah you would need to work around the HW bug using excess capacity (like user Lun uses successfully). Thanks to @Drum for the idea.
It’s basically a li-ion protection circuit (with a slightly elevated cutoff voltage) that also includes a charger.
With this add-on board a solar charger can recover from badness events, like the panel being under snow for two consecutive weeks. It is also small enough to be very inconspicuous.
I am now testing it some more to get more certainty of it working correctly and then I will place it somewhere where it would be very expensive to perform manual interventions.
I am really interested to see how this works, and I will have to do some research on the “N channel FET” Never heard of it but if ir works I see it in my future!
n-channel FETs are the most common field effect transistor these days. The p-channel variant mentioned above is basically the same but with all the voltages flipped around, so it can be used to switch the positive rail, like you would with an PNP bipolar junction transistor. All the voltage supervisor ICs i found so far are designed to work with the more common n-channel FET, so the switching transistor has to be placed on the ground connection of the attached load.
Waiting for the results. Could this be built with some off the shelf adjustable low voltage cut-off? And it needs a separate charger?
Was thinking could you just use a low voltage cut off in between the battery and board, but not sure if it will allow current to flow both ways so charging could still happen.
You could connect a generic li-ion charger board to the battery and then, if you can find one, an “off the shelf adjustable low voltage cut-off” between the battery and the battery connector of the RAK baseboard. I didn’t even try to find such a board, because I wanted a solution that fit into the existing enclosure.
Good luck finding an off-the-shelf low voltage cutoff that works at a voltage between 2.7v and 3v, I didn’t see anything with a quick google.
Came across a post on Reddit that this whole problem is solved by plugging the solar charger staright to the usb. Apparently this will allow a clean reboot.
@garth is correct, both the solar input (Green Power) and the USB bus power (VBUS) are treated the exact same way: they both go through a schottky diode and end up as VBUS_D, which then goes through another schottky diode and into the charger IC.
This is a great thread, thanks Michimartini.
I have a little request since i am a bit confused on what schematic has been used
may be a summary with the pcb layout would be usefull for many newbies
Thanks
Hi Gilles and welcome to the forum.
The schematic posted above is the one I used - search for “easy to build”. Now that I know that it works, I might put a PDF with the pcb layout and a BOM here so anyone interested could build their own.
Although I could think of two improvements: Using a charger IC with a charge indicator LED output, so you can actually verify that it is charging, and maybe going with a more powerful one, that can charge with up to 2A so you can make better use of short burst of intense sunlight. But those are just nice-to-haves.
Hi michimartini
Thanks for you reply… got your schematic… i believe i can draw the pcb since it is simple…
But sure, you should make both available … may be on a blog or asking mestastic to make reference since it is a must for solar powered lora devices…
I also agree upon a flahing led when battery is loading…should a few 10s microamp only
Again… thanks for your usefull work
Regards
Gilles
Btw, i am looking to package your circuitry and the battery in the solar panel box so ill have place for a big battery .and no change to the rak device and box… it will deliver 3 to 4 volts (or zero) to the rak …like an embeddef battery… in fact, it becomes a solar with a battery protected and can be used for other devices
