Author Topic: MailboxNotifier with Solar Panel and Supercap - analog voltage reading question.  (Read 545 times)

Digitalsawdust

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I'm setting up the code for a MailboxNotifier using an R3 MotionMote board with an R6 Moetino that will run from voltage produced by a solar panel through a schottky to a supercap, plugged into the battery connector.  I note from the R3 schematic that the voltage divider for battery sense (1Meg between A7 and battery, 2Meg between A7 and ground) gives a battery sense voltage of 2/3 of the battery voltage.  Using 2/3 gives a maximum battery voltage of 4.95 for an A7 voltage of 3.3.  However the solar panel can charge the supercap much higher than that, which puts the sense voltage over 3.3 volts. 

When I put the solar panel in direct 4pm sun with everything connected and on, the battery voltage is 5.775 but the voltage shown on the receiver display is 4.81 volts.  2/3 of 5.775 is 3.85 volts, well over 3.3 volts, so the analog reading is skewed.   I don't yet know what the maximum battery voltage will be in direct noon sun, so I'm not sure if I'm in danger of exceeding the rated maximum voltage on A7.   So far, it's been slightly over 6 volts unloaded around noon with the proper angle.  Installed, it won't have full sun very often.

Does anyone have any field experience with the solar-supercap setup?  Is there a  way of correcting with the analog reading in software when the battery voltage is well over 5 volts to give an accurate battery voltage reading, or would it be better to just keep the solar panel in light shade to reduce the maximum voltage it puts out, which seems to be around 4.2 volts give or take? 

LukaQ

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Hello,

well if you have 5v solar panel, that should probably give max of 5.5v or so and with normal diode you could drop that down into range of adc. You could also change the ratio.
Yes, at least one user I know has  solar-supercap setup, and he probably reads 5v all the time (didn't show how it acts at nights)

If you would need to sense higher voltage, change the resistor divider and in software. That sketch was made for lipo's, that would stop at 4.2v. It wouldn't be better to use it in shade, because of that days when you won't have much light or sun, you still would want max charging. I would change resistors to different ratio and make correction in software

HeneryH

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Just as a data point, I'm using a 5v 7.5 F supercap and get this graph in direct sunlight. The graph is generated by Felix's Gateway app.
This post shows the parts I used.



« Last Edit: April 27, 2018, 11:34:31 AM by Felix »

Kilo95

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Yes, at least one user I know has  solar-supercap setup, and he probably reads 5v all the time (didn't show how it acts at nights)

My Weathershield/supercap/solar panel (outside for a PWS) reads 5v all of the time ( i assume you were referring to me). I have the diode in place but a DVM shows about 5.5v so my graph shows 5v all of the time including at night. I've only seen once in the last several months where it came down to 4.98v.

As a side note, I noticed some instability in my RSSI from my PWS. anywhere from -75 or so to -100. it was worse during periods of rain or snow. I finally realized that the precipitation was causing somewhat of a short between the ground and the "hot" at the antenna (I am using the 915 MHz PCB dipole on this website). It's better during the dry spells. A few days ago, I took some 3M Mastic tape length wise and wrapped the PCB dipole length wise with the tape (folded over the top). I included the SMA connection. Haven't had a problem since. RSSI stable -70 to -75 now for days.

HeneryH

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I have another MotionMot that is constantly reading 4.91v 20x7 but I thought there must have been something wrong with the sketch since I know the PIR design does burn through some power.

Digitalsawdust

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My takeaways from the discussion so far are that the higher solar panel voltage isn't a problem, and that I'd have to change the resistor divider on the MotionMote PCB to get an accurate analog reading.  I'm not inclined to mess with the PCB, and it isn't absolutely necessary that I have an accurate reading, just accurate enough to tell me if something's failing.  That being so, as soon as I get the parts for a water-tight case (PVC pipe), I'll switch over to solar.

The small case I'm using for the solar panel is watertight.  I have a cable coming out one corner at the bottom, sealed with telco pliable goop, and terminating and a micro phone plug.  The case fits both my 2 5v panels and the 6v panel Felix sells just perfectly.  I'll have to see how well it lasts.

https://www.amazon.com/gp/product/B075S11RF5/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1

Thanks for the help!

wile1411

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I didn't want to start another solar panel / superCap / weathermote thread and this thread seemed quite close to what I wanted to ask...

I'm using the Blog solarpanel/superCap setup available in the LPL store (1W panel, 7.5F 5.4V superCap, Shottky Diode)

I had the same issue as this thread where the Voltage was capping out at 4.8V, but I managed to utilize a 1Meg/1Meg voltage divider to read the actual voltage of the SuperCap power supply for my moteino/weathernode.

The attached graph shows when I move the moteino into a west facing room 4 days ago. The voltage shoots up to 6.4V after about 3pm each day once it gets direct sunlight. (blue line is 5.4V indicator). I also added a zoomed in view on just todays voltage chart.

It take an hour to drop from 6.4v down to 5.8V and another 2hrs to get under the 5.4V limit of the SuperCap after the sun has dipped below nextdoors roofline.

Should I be overly worried with a 5.4V SuperCap regularly charging to 6.4V?

I saw in an older thread there was a charger made of SMD parts (I think Perky or WhiteHare made it), but I hoped there might be a through-hole option I can build to prevent the Cap going above 5.4V?
« Last Edit: December 18, 2018, 07:05:35 AM by wile1411 »