A solar supercap powered Moteino (15Farad charged by BQ25504)

[WhiteHare]

Are you abandoning the large cap version now then? I'm a little confused, I though that was needed to keep it going for a day..

Mark.

Not at all.  I think I want both.  I'm following through on what I posted last week on this same thread:

Eventually, I think I'd like to somehow run the Moteino from TWO supercaps: a small one, and a big one.  The idea being: quickly charge the small one first, so the Moteino can get up and running right away, even if all it does is transmit, "I'm alive" and give a steady stream of wireless reports on what the rising charge voltage is on the bigger supercap.  That would be more satisfying than setting it up only to wait a very long time just to hear that that the voltage on the big supercap finally crossed the 1.8v threshhold, because in the interim you're left wondering just what the heck, if anything, is going on with it.     

Fortunately, right now it's looking like the small cap won't need to be a supercap.  My WAG is that something in the ballpark of 2,000uF will be sufficient.  Maybe even less.

So, there will need to be some kind of additional circuitry to manage this, so that the small cap charges instantly so that it can instantly power-up the Moteino and then stay charged up enough so that the Moteino can give wireless status reports on the big supercap's charging progress.

So here's the roadmap:  As of now, we have the circuitry for charging the big supercap.  We could just stop here and declare victory, but I want to peak around the corner and take it a bit further.  Therefore, I've switched focus from the big supercap (which is now a solved probelm) to the smaller startup cap, in order to get that working.  After that's solved, I'll need to consider the circuitry for how to unify the two caps into one system, so as to get the best of both worlds.  I anticipate that will leverage the work already done on the big supercap, so none of the work done to date will be wasted.

Make sense?

[ChemE]

How small of a solar panel are you using currently?  In a perfect world the panel would be roughly the same size as the mote itself and I could envision stashing these all over and never having to fool with batteries.  Can you provide link to some suitable small panels that I can fool with?

[perky]

I'll need to consider the circuitry for how to unify the two caps into one system, so as to get the best of both worlds.  I anticipate that will leverage the work already done on the big supercap, so none of the work done to date will be wasted.

Got it. I think you might get to use two of those ideal diodes now to produce the highest voltage of the two caps (BTW that opamp is available in a dual version)

Mark.

[WhiteHare]

How small of a solar panel are you using currently?  In a perfect world the panel would be roughly the same size as the mote itself and I could envision stashing these all over and never having to fool with batteries.  Can you provide link to some suitable small panels that I can fool with?

Generally about 60mmx120mm or 60mmx100mm.  That seems to be a sweet spot in the marketplace in terms of bang for buck.  Of course, you can use a smaller panel, and I may eventually switch to something smaller myself.

Aliexpress is a good place to shop.  They have lots of different panels to choose from at generally lower prices than anywhere else that I'm aware of.

A couple panels I've purchased before that seem to perform well:

https://www.aliexpress.com/item/2PCS-X-3-5V-250mA-Mini-monocrystalline-polycrystalline-solar-Panel-small-solar-cell-PV-module-for/32337067151.html?spm=2114.13010608.0.0.HeCGjM

and

https://www.aliexpress.com/item/6V-0-6W-Solar-Power-Panel-Module-DIY-Small-Cell-Charger-For-Light-Battery-Phone-Toy/32723002222.html?spm=2114.13010608.0.0.m3jUdD

[WhiteHare]

By the way, I've had no luck finding truly tiny solar panels. If anyone finds a good source, please do let me know.  To get a genuinely tiny solar panel at a reasonable price, I might have to buy a cheap $1 solar powered calculator and salvage it from that. 

[pjeran]

for a good selection of solar panels I have found that banggood has a good selection.

here is a panel that is similar in size to a mote.

http://www.banggood.com/5V-30MA-0_15w-53-x-30-x-3mm-Polycrystalline-Mini-Solar-Panel-Photovoltaic-Panel-p-1020641.html?rmmds=search

If you go back to a boost system you can get smaller but it is only 1V.

http://www.banggood.com/1V-35mA-Mini-Epoxy-Solar-Panel-Photovoltaic-Panel-p-987801.html?rmmds=search

Paul

[WhiteHare]

Now for the really GREAT NEWS:  With Optiboot 6.2 installed, I was able to power up an ATMEGA328p--and by that I mean, from completely powered off to pin 9 going HIGH--from a mere 1,100uF capacitance charged to 2.0v.  Well, why is that great news?  Because my mini solar panel can charge that 1,100uF of capacitance to that voltage in the blink of an eye.

More good news: I did some more careful measurements, and I found that with Optiboot 6.2 I could complete the ATMEGA328p startup (i.e. from total powerdown to HIGH on pin 9) by charging a mere 0.47uF ceramic cap to 3.6v.  And I think it could go lower even than that, but that's where I stopped measuring for today.

[ChemE]

More good news: I did some more careful measurements, and I found that with Optiboot 6.2 I could complete the ATMEGA328p startup (i.e. from total powerdown to HIGH on pin 9) by charging a mere 0.47uF ceramic cap to 3.6v.  And I think it could go lower even than that, but that's where I stopped measuring for today.

So this has enough charge to power up a 328p and handover to the supercap?

http://www.mouser.com/ProductDetail/Murata-Electronics/GRM155R71A474KE01D/?qs=sGAEpiMZZMs0AnBnWHyRQHZRmD76Lk0UBjpAiZ7AP75nYgyUImaBjA%3d%3d

[WhiteHare]

Well, I don't yet know about the handover part, but if charged to 3.6v, it's enough to powerup the ATMEGA328p  to fully ON from a fully OFF state.  This is barebones stuff, with no RFM69 yet attached.  I'm waiting for an RFM69W to arrive before I do that measurement.

Really large supercaps can take a long time to charge, especially under overcast conditions.  Presently I have a 400F supercap that's on its second day of charging, and it still hasn't yet reached the 2.7v cutoff point (though it's slowly approaching it).  Even  getting it from 0v to 1.8v can take a very long time under less than ideal light conditions.  So, it will be nice to have interim status reports about the changing state of things before a big supercap reaches the 1.8v minimum useable threshold.

[WhiteHare]

Here's a photo of an assembled breakout board that prevents powering on until the voltage hits 1.87v.  It powers off when the voltage drops below 1.8v.  You can program a different upper "turn-on" threshold by selecting different resistors (see the NCP301 datasheet), but the lower "turn-off" voltage threshold is fixed at 1.8v (unless you were to pick a different NCP301 chip).

[WhiteHare]

Attached is a photo of the assembled version 3 Perky Smart Diode breakout board.

[Edit: I played around with it a bit.  It seems to do its job as a diode, in that it blocks reverse flow of current.  At low voltages on the supercap, it drops about 0.5v.  At higher voltages, it drops about 0.05v.

In contrast, the Schottky diode drops about 0.3v, regardless of what the supercap voltage is.

So, maybe one could get the best of both worlds by wiring the Schottky diode in parallel with the Perky Smart Diode?  I haven't yet tried it, but I'm thinking that at the lower voltages it might drop 0.3v, and then at the higher voltages it would drop 0.05v.

Well, more characerization could be done, but that was just a quick look.]

[perky]

Interesting! Nice to see it in action. The FET in my diode circuit is wired 'in reverse' in that the parasitic diode is pointing towards the supercap, so my guess is that the opamp is not fully functioning until it reaches it's working 1.5V, during this time the parasitic diode conducts. Above 1.5V then the FET is turned on by the opamp. Putting a schottky across that would definitely help I think

Mark.

[WhiteHare]

I just now attached an RFM69W module.  Doing just that and no other changes (same sketch as before to simply drive pin 9 HIGH), it now takes more like 47uF charged to 3.6v.  Neither 0.47uF nor 4.7uF appear to be sufficient. 

Mind you, this is all very crude ballpark testing, but it does give a feel for what factors are in play.

[WhiteHare]

It turns out that 47uF is insufficient to power up and send a packet.  However, 470uF charged to 2.5v is sufficient to powerup the ATMEGA328p and RFM69W and send one packet (but not two).

[joelucid]

Doing just that and no other changes (same sketch as before to simply drive pin 9 HIGH), it now takes more like 47uF charged to 3.6v.  Neither 0.47uF nor 4.7uF appear to be sufficient. 

Instead of using the RFM69 init code try directly using SPI to sleep the radio first thing after boot. Do this by writing mode sleep into the mode register and read back until it sticks. That way starting up uses way less power.

Joe