Adding solar to Feather FONA

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pdp11
 
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Adding solar to Feather FONA

Post by pdp11 »

I'd like to use my Feather FONA in a mobile application. As such, I'd like to add solar to top off the LiPo. Would I need the Adafruit solar charger board? If so, can I still connect a USB cable with power occasionally?

In a nutshell, I'd like to add solar without eliminating the ability to also charge via USB.

-PDP11

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adafruit_support_mike
 
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Re: Adding solar to Feather FONA

Post by adafruit_support_mike »

The Solar Charger can't charge a FONA's battery directly. It would create a conflict with the FONA's built-in LiPo charger.

It would be easier to follow the path of these tutorials, and make a generic 5V USB charger that's solar powered:

https://learn.adafruit.com/solar-chargi ... g/overview
https://learn.adafruit.com/solar-boost-bag/overview

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ewan
 
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Re: Adding solar to Feather FONA

Post by ewan »

Is there any way to power a Feather directly from the 'Load' output on the Solar Charger, so not using the Feather's LiPo charging?

That way the Solar Charger would handle the panel, and the LiPo, and charging and switching between them, with the Feather just seeing steady power and not knowing anything about the battery at all?

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millercommamatt
 
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Re: Adding solar to Feather FONA

Post by millercommamatt »

You would want something like this (https://www.adafruit.com/product/2190) between the solar charger and the feather. The solar charger and put out up to 6 V which is too much for a Feather.

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pdp11
 
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Re: Adding solar to Feather FONA

Post by pdp11 »

I think we're all mostly on the same page. My goal would be to have a board that allows me to connect USB and solar at the same time and then present a USB interface to the feather:

Image

That way I can leave the solar plugged in topping off the LiPo, but I can add a USB connection if necessary.

Is this possible?

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millercommamatt
 
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Re: Adding solar to Feather FONA

Post by millercommamatt »

Passing through the USB data is the real trick there. You might have to design your own board for that.

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pdp11
 
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Re: Adding solar to Feather FONA

Post by pdp11 »

True... what if it was just power? Data would be nice but the primary goal is powering the unit when deployed in the field.

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ewan
 
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Re: Adding solar to Feather FONA

Post by ewan »

I've been reading around about this a bit, and based on that (and precisely zero practical experience with any of these parts) my current impression is that the best way to do it power-only is to connect the solar panel and the LiPo to the Solar Charger, then connect the load output of that to a Verter, with a KA75330 to disable the Verter if the LiPo battery voltage falls too low (to allow it to trickle charge from dead flat) then take the 5V output from the Verter and stick it in the USB input of the Feather, and ignore the Feather's own LiPo handling support entirely.

That should give a feather that runs 'directly' from the solar panel when it's sunny enough, from the LiPo when it's not, and always sees either a clean 5V, or no power (if there's no solar and the LiPo is discharged). You can connect USB power via the Solar Charger if it needs an extra boost or initial charge, but you'd need to disconnect the Feather and plug just the Feather into a PC to program it.

There's possibly more voltage conversion going on there than is ideal - if there was a way to replace the 5V Verter with a 3.3V output equivalent and then feed that straight to the Feather bypassing its 5V->3.3V conversion that would maybe be better, but I'm not seeing a way to do that with completely off-the-shelf bits. At least not off Adafruit's shelves.

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millercommamatt
 
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Re: Adding solar to Feather FONA

Post by millercommamatt »

Then you want:

Solar Panel + Battery connecting to Solar charger. You can add an external power supply view the DC jack or the breakouts. The load connection from the solar charger should connect to the buck/boost board I linked to a few posts up. That board can connect to the feather.

I suppose you could have a battery connected to the feather as well.

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pdp11
 
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Re: Adding solar to Feather FONA

Post by pdp11 »

So is this a possible solution?

Image

The battery needs to be attached to the FONA "below" the DC in because the SIM800 chip has a higher voltage requirement (3.7v) and can spike current demand more than the DC in can provide.

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ewan
 
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Re: Adding solar to Feather FONA

Post by ewan »

Oh quite right - the Fona is special that way.

What I'm not sure about in that layout is whether the Solar Charger is getting you anything much - it's whole thing is to be able to charge a LiPo from the solar panel, and to be able to switch the load between them. With no LiPo it can't do either of those things. We also know that when the solar panel is lit the charger will just pass the 'raw' panel output through, hence the need for the buck-boost. However, if the converter can take the solar panel voltage directly would it make sense to skip the Solar Charger entirely?

So you'd go solar panel->buck-boost->feather and have the LiPo attached as you have it straight to the Feather. In good sunlight that should deliver a stable 5V to the feather's USB input, which would run the reather and charge the LiPo, and in the dark the feather would switch to the battery on its own.

I'm also not certain:

- whether it'll start up cleanly - you might still want to use the KA75330 to stop the feather booting until the battery is at a decent voltage. I'm not sure whether the feather will charge it's LiPo when the enable pin is tied low though.

- whether there's any reason I'm not seeing that the buck-boost would be unhappy with raw solar panel output.

Edit: After some more reading I'm pretty certain that the Feather will indeed charge the attached LiPo even with it's enable pin low, that's basically the exact example given on the KA75330's product page. So that just leaves the question about whether there's any reason not to connect the panel straight into the buck-boost converter.

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