Good point! The CN3791 unfortunately has no termination voltage adjustment option. Is there a way to fool it into terminating earlier? Would a low value resistor between battery and charger be super silly?
At night the battery would lose at least 160mah if not transmitting. Maybe a smaller solar panel will do…
Routing and board optimizations are done. A repo exists now. Also made a proper BOM with Ali purchase links to the special components. Who takes the callenge to test this thing? @einzeln00? 
Looking at design files, I would love to take the challenge, but I need a day or two to make the schedule. Still working on the last version.
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I ordered a set from JLCPCB plus stainless steel stencil. With a stencil for the complete PCB the assembly should be quite easy. Hopefully there are no serious bugs in the design.
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If the CN3791 is tricked into early termination, the charging will not restart until power is cycled or when battery drops below a certain %. If you can measure the battery with the NRF52 and then enable/disable the charge path (‘reset’ the CN3791O it will be ideal way stop at 4.0V or whatever set point.
Also, I am a bit terrified about batteries in parallel via contacts – if one battery disconnects due to thermal/mechanical conditions and then joins again… Trouble!
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Ok. Maybe I could add a Mosfet in the next rev. Since the Cn3791 does not have a chip enable pin. On the other hand one could connect a different charger to the board. One with a more suitable termination voltage. That’s the advantage of having a choice. For example the CN3722 is a switching charger with such an option…
I also thought about adding a 2A fuse to each cell before they connect in parallel. This will be corrected soon…
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I was thinking about charge termination during daytime… Maybe best topology is not MPPT, maybe best is MPPT + NVDC or simply NVDC charger. This advantage of NDVC is that the daytime power consumption is not from battery, it is from the NVDC path. So if battery gets to 100% in the first few hours of sun, it will stay 100% thoughout the day and battery will be used only at night. If MPPT-classic, then you have a situation where you can end up with battery discharged 10-20% at the end of the day, because the Cn3791 will stop charging and restart only on hysteresis. This is very cheap, but not ‘perfect’ since it doens’t have MPPT. https://www.findchips.com/search/sy207 Also, MP2731.
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Would it be possible to share the schematic in PDF format, I am having trouble converting your file. Thanks!
Never mind, found the .pngs
Came across the SPV1040 by ST. It’s a synchronous boost MPPT charger with many config options but still simple and cheap enough…I think I will create a PCB for it.
Maybe we should open up a solar charging thread
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MPPT is usually only cost effective in rather large solar setups, or when the panels are located far away from the batteries.
Adding a second solar cell could be a better choice if space isn’t an issue.
Also, if you are using single batteries you will likely need a buck converter as common solar cells output is 5 to 6 volts and most battery chemistry top out around 4.2v per cell.
Some MPPT are boost / buck and usually are not very efficient
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Adafruit has a fairly low cost solar charger implementation, it might be worth taking a look at their design. I don’t think a true MPPT implementation is worth it for this use case.
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Yep, MPPT seems to pay off in larger setups or in ultra low power cases as boost MPPT, LORA+BLE is neither. I find the bq24210 very appealing, even if linear, it has a lot of smart features than can prolongue and enhance battery life and might harvest light better than the CN3791 in low light situations.
please read this thread, it talks about DanieCML’s Microchip charger, the BQ24210 and one with powerpath, BQ2407x, which. can be used as a solar charger as well.
https://e2e.ti.com/support/power-management/f/196/p/795048/2950742#2950742?jktype=e2e
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I own some boards based on the AEM10941 which is a boost mppt solar energy harvester for a single cell. These work very well but are hard to get and somewhat expensive. They also contain 2 LDOs and are therefore overkill. I habe a BLE solar sensor node running on this setup reliably for some time now. Works indoors as well. Due to this positive experience with solar energy harvesting I found the SPV1040 appealing. It’s been around for a few years now https://youtu.be/UkFLnVSquu4
But in general I think that linear chargers such as the bq24210 or the adafruit board would do very well in this use case. If you have the right solar panel (MPP around the charging range of a lithium cell, say 3-4V) and and a charger chip that doesnt get confused by the varying output conditions, you don’t need switching topologies and the whole MPPT voodoo. Also, with small panels connected to 18650s current limiting is inherent to the panel itself.The most important thing is actually termination voltage, as discussed earlier
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So if we combined this:
Adafruit Feather nRF52840 Express
$25
With this:
Adafruit Radio FeatherWing - RFM69HCW 900MHz - RadioFruit
SX1231 based module with SPI interface
Encrypted packet engine with AES-128
$10
or this:
Adafruit LoRa Radio FeatherWing - RFM95W 900 MHz - RadioFruit
(SX127x LoRa® based module with SPI interface)
$20
That’s pretty close.
It may be that the sx1262 SPI module could get you some advantages, and I would have to add a GPS receiver feather https://www.adafruit.com/product/3133 ($40) and an antenna https://www.adafruit.com/product/960 ($15) to get all the way there.
I can see where putting all of this on one board with the bootloader could be very attractive as a package.
I’d definitely say bring some extra IO out to a header… and I would expect a keyboard like this:
Could make this standalone with a small display and not need a phone.
I originally thought that I could assemble these pieces and hit a package that allowed all of this under the $50 mark… but clearly that is not true at all.
But this does give a nice reference point for the “legos” that exist out there now.
The idea of building a purpose-built board to efficiently perform these functions make a lot more sense to me after working through this perspective…
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A combo device of these components on one PCB is what started this thread.
I see a clear disadvantage of using a handheld keyboard-based device: you cannot put it up on an elevated position for good rf coverage and still be able to use it at the same time.
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That specific keyboard is Bluetooth. So you would have the same basic range of using the keyboard as you would in communicating to it from a phone… the HID support would have to be there for a keyboard in the BLE stack, however.
Clear line if sight for BLE 5.0 would give you quite a bit of support for an elevated position of the device for good RF coverage.
Your point is very valid though… while I would like a tethered keyboard option as well for when it makes sense… the option to perch the radio at a high spot is a very big design advantage. At these frequencies, even 10 feet up can make a huge difference.
Update: Now that I think about it more… having a detached keyboard like that and no display would make getting messages not possible… because you won’t be able to see the screen if it is 10 feet up! That’s kindof funny to think about…
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Just make sure everyone sends messages in all caps.
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V2 prototype assembled. Turned out quite large. For now I’ve just installed the USB bootloader and tested the 3.3v rail. The test firmware has to be adapted now…
Looks like a vintage phone 
Now that I see this: why not add a small i2c keyboard at the bottom 
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