Author Topic: A solar supercap powered Moteino (15Farad charged by BQ25504)  (Read 71213 times)

WhiteHare

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #300 on: April 29, 2017, 07:56:55 AM »
In preliminary testing I've run into a bit of an issue with small SMD caps that are preferably meant to hold their charge for a long time: they seem to self discharge a lot faster than the datasheet would suggest.  I first noticed it with the 7500uF part that ChemE had found, which is supposed to leak at just 10na, or thereabouts.  The question is why?  I think flux is creeping underneath them during soldering, creating a discharge path.  But, being surface mounted, that means there's no apparent way to easily clean it off if that's what's happening.

I've only just identified the issue, so first I'll try being less liberal with my use of solder.  However, it may also justify using longer SMD caps, such as 1210 rather than smaller to further avoid the problem. 

Anyone found any other ways of dealing with the issue?  For instance, are there any non-conductive fluxes that still work well?

WhiteHare

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #301 on: April 30, 2017, 04:31:16 PM »
@ChemE
Here's a data-point that may interest you:   it took about 90 seconds for the Solar Bit (above) to charge up your tiny 7500uF capacitor to 2.7v using just indirect sunlight from my window sill. 

Also, prior to that, I noticed that its rate of self discharge had dropped a lot by the time it hit the 2.2v neighborhood.  So, maybe its rate of self discharge is voltage related.  Or maybe the longer it sits with any voltage, the more it self repairs.  Who knows?  It deserves a more careful look than the quick look I gave it earlier.

ChemE

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #302 on: April 30, 2017, 06:17:41 PM »
Very astute data point indeed.  A lot of my motes broadcast every 5 minutes and even that is more frequent then I truly need.  90 seconds to recharge the teensy cap which itself can handle a broadcast is quite interesting.  Didn't I read somewhere that caps leak a lot the first 30 minutes or so at voltage and then they begin to improve and eventually reach their rated leakage?

WhiteHare

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #303 on: April 30, 2017, 07:28:33 PM »
Didn't I read somewhere that caps leak a lot the first 30 minutes or so at voltage and then they begin to improve and eventually reach their rated leakage?
Believe it or not: 72 hours!
http://www.maxwell.com/images/documents/1007239-EN_test_procedures_technote.pdf
I would think that makes it rather hard to extrapolate to most real world scenarios....


ChemE

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #304 on: April 30, 2017, 08:55:20 PM »
Hmmm.  So if a supercap is held at its target voltage for 72 hours to age it, and then you start using it as normal, does the leakage remain at the low quoted value or rise?
« Last Edit: April 30, 2017, 09:52:44 PM by ChemE »

WhiteHare

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #305 on: May 03, 2017, 09:51:49 AM »
I went ahead and designed a Solar Bit and extra circuitry directly onto the main PCB of one of my experimental nodes.  I managed to squeeze it all on without actually enlarging the PCB.  Sent it to the fab this morning, so we'll see how that goes after it arrives.  I made it optional, with an extra port for attaching an external solar cell (or other power source) if preferred. 

Since plants need light too, it may make sense as a plant monitor.  The small cheap Chinese solar cells I linked to earlier make more sense economically, especially for a plant monitor (my wife has lots of plants), but for its higher price a Solar Bit could offer less conspicuousness.  Meh, it's good to have options. 

At this point I think just about any solar cell can be made to work, so the art is just finding the right trade-offs.  The Chinese solar cells are cheaper than tabbed lithium batteries, so I think they may win on material cost, maybe even after adding in the cost of the related parts that a battery wouldn't need.
« Last Edit: May 03, 2017, 11:19:41 AM by WhiteHare »

WhiteHare

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #306 on: May 05, 2017, 11:51:19 AM »
Soldering the solar cell directly to the PCB would seem to encompass at least three trade-offs.  If it were instead attached with relatively stiff solid core wire, then you can aim and tilt it toward the light source of choice without having to aim or tilt the entire PCB.  On the other hand, there may be RF reasons for a directly soldered-to-PCB connection:  I noticed with some of my previous bodge wire connections to DIO3 and DIO4 that they had a negative effect on RSSI.  When I converted them to traces on a PCB, RSSI improved tremendously.  I don't know enough to predict if the same may hold true for power connections or not.  Maybe?

WhiteHare

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #307 on: May 06, 2017, 01:42:05 PM »
In exploring the low voltage front,  I have a circuit which can boost from 0.7v up to 2.67v.  I have another circuit which can boost from 0.25v up to 1v.  So, that gives me a path for getting from 0.25v up to 2.67v.  The trouble is, booth need to first charge a capacitor to at least that voltage, or else the converter just spins its wheels and does nothing because of insufficient current.  I can probably manage that with the first one, but I don't see a way to detect the state of capacitor charge at 0.25v or  switch on the 0.25v converter at such a low voltage in a cold start scenario.  So, if anyone has suggestions on how to approach that....  If 0.25v is too low to manage, how low can voltages go and still be manageable?

[Edit: Anyhow, even without a boost circuit but just hooking up a number of mini solar panels in series, I bet I could run a Moteino off of mere moonlight, which would be pretty cool.  In the end, that may be a great application for Solar Bits, since they're so compact that the series of panels wouldn't be awkwardly large.  Because of their relatively cost, though, having a viable boost circuit would be cheaper than stringing a bunch of them together.  ]
« Last Edit: May 06, 2017, 03:11:00 PM by WhiteHare »

WhiteHare

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #308 on: May 06, 2017, 11:23:23 PM »
I just now tried four 6v panels powered in series by moonlight just to see what would happen, and it produced only 0.06v.   :(  So, scratch that idea.

ChemE

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #309 on: May 07, 2017, 09:56:33 AM »
Now that lunar power is out, any sense how large a primary cap you'll need to see you through the night and some short overcast winter days?

WhiteHare

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #310 on: May 07, 2017, 11:43:16 AM »
Now that lunar power is out, any sense how large a primary cap you'll need to see you through the night and some short overcast winter days?

Yup.  5F can easily carry my fairly high drain remote control receiver node (which wakes up every 100ms and listens) for 24 hours with no external power.  Given that the 10F supercaps are actually cheaper, at just $2, I'll probably use 10F supercaps.

Probably almost any node you're likely to dream up will use less power than that, so 10F is probably a good number generally.

WhiteHare

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #311 on: May 07, 2017, 07:03:52 PM »
Another feature I'm thinking of adding is the ability for the Moteino to read the raw open circuit voltage of the mini solar panel so as to roughly measure the amount of illumination it's encountering.  Not absolutely essential,  but, well, more data is always better, right?   ;)  An alternative would be to use a specialized sensor meant for just that purpose.  I can see how this feature might eventually get eliminated, especially in more utilitarian nodes, but at least initially, it might be nice to catch a glimpse as to what's actually going on in the environment.

Thoughts?
« Last Edit: May 07, 2017, 07:31:02 PM by WhiteHare »

ChemE

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #312 on: May 08, 2017, 07:36:32 AM »
1) More data is always better
2) I would think for purposes of deployment and orientation that being able to know the open-circuit voltage in real time would be very handy

WhiteHare

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #313 on: May 11, 2017, 05:18:48 PM »
Answering my own question, it turns out there's a good reason to measure the open circuit voltage of the solar panel: the voltage measurement provides clear guidance on whether enabling a boost converter is more advantageous than sending electrons from the the mini solar panel directly to the capacitor.  If the later choice is possible, then it wins hands down on efficiency.

Lately we've had some overcast days here, and for some interior locations far from windows that has meant no charging (well, at least with only a single 6v mini solar panel that is).  I've been playing around with the MCP1640 series of boost converter, and it makes a difference on those days.  Also, unlike the BQ25504, it's cheap enough that it's more cost effective (and more compact)  than bulking up on supercap Farads.

Anyhow, this thread has run dry, so this will be my final post.

HeneryH

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Re: A solar supercap powered Moteino (15Farad charged by BQ25504)
« Reply #314 on: October 09, 2017, 09:38:58 PM »
Could you kind folks tell me if this design has matured to a consensus yet?   It was exhausting reading through it.

I need a handful of remote motes around a yard and would like a solar powered mot that is OK in winter months where temp is 10 - 30 F.

Some of the Mots will be MotionMotes which I think are on the heavier side of power hunger.
« Last Edit: October 09, 2017, 10:30:37 PM by HeneryH »