Supercap Solar Powered Mysensors nodes as cheap as possible
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I maybe up to something: I used a MCP1700 ldo and on Vout a diode + a schottky in series to drop the voltage ( could have used just the diode, but the Vout was 2.7x volts so for supercap longevity I added the schottky to have a safe margin) and it is charging even with panel pointed away from the sun. I'll see how it goes tomorrow, but I am pretty happy with the results so far. If it works I think i can't go cheaper then a LDO + 2 diodes
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It started charging at around 7:30am: it is placed on an eastern wall in vertical position with practically no direct sunlight. It takes a while to stabilize around 2.55V but overall it looks reasonable for the cost of all the power circuit.
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What is the cost? Also, it's not clear to me how much usable charge you're getting.
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1.5€ for solar panel, 1.7€ for 100F supercap, 1€ for booster and some cents for MCP1700, diodes and a small PCB. A fully charged supercap gives 3 days of working time, with send every 10 minutes of temp. hum, TX rssi, RX RSSI, battery voltage and battery percentage
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Sounds sorta roughly like the kind of circuit used in solar garden lamps, except you've replaced the typical rechargeable battery with a supercap and are boosting to a higher voltage than they typically do. I wonder whether any of them do boost to the same voltage that you're targeting? Perhaps the brighter ones, for example? Those solar garden lights typically have a single, inexpensive integrated circuit that does it all.
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the booster is plugged in the Easy PCB and it is used to power arduino and sensor
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@gohan said in Supercap Solar Powered Mysensors nodes as cheap as possible:
the booster is plugged in the Easy PCB and it is used to power arduino and sensor
By "plugged in," do you mean like in your earlier photo?
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nope, that was the 5.5v supercap version. I mean plugged in in the socket next to the green screw terminal. Of course the buck boost module now it is not used. I have to put the new photo I know.
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Since you're not posting schematics, maybe it's time for a new photo....
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I need to learn how to make schematics
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Does it make sense? The output terminals go to the Easy Pcb green screw terminals .
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@gohan said in Supercap Solar Powered Mysensors nodes as cheap as possible:
Does it make sense?
I suppose. Why the two diodes again instead of just the one?
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Because only one was not dropping enough voltage to have a safe margin, the schottky is giving that extra 0.2v
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You'll have a very long charge tail with the diode(s) in that position.
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Do you mean it will take long time to fully charge?
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@gohan Yes.
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It took about 5 hours to charge and it wasn't in direct sunlight, I'm still happy with the results given I'm using general purpose parts I already had around and no sophisticated charge controllers (and no particular knowledge about electronics
)
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Today was overcast all day so it took about 3 hours longer (until 3pm) to fully charge but overall it looks like a nice graph for the last 3 days.
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2 days of rain and it can't charge, luckily it has more than 3 days of running time without charge. Being not in direct sunlight (vertically positioned on a wall just below roof) it has some drawbacks, but today it is catching up
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The overcast days are where the two diodes are burning you.
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if it is overcast it is not that dramatic as it still charges, it is the rainy day that is bad. I know that a more efficient and specific charging circuit would work better even at lower light/voltages, but still this is working fine as it recovered to 2.4V within 3 hours and 3 more to reach 2.5V. Unless I get 4 days or rain I should be fine but in the event of 4 rainy days I think I would be more worried about other things
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That is looking great so far (the spike was a reboot after I installed the SMA antenna)
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Since your goal was "as cheap as possible," what was your final tally on how much it cost? It would be good to know just how cheap is possible.
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@gohan said in Supercap Solar Powered Mysensors nodes as cheap as possible:
1.5€ for solar panel, 1.7€ for 100F supercap, 1€ for booster and some cents for MCP1700, diodes and a small PCB. A fully charged supercap gives 3 days of working time, with send every 10 minutes of temp. hum, TX rssi, RX RSSI, battery voltage and battery percentage
I already wrote it 3 weeks ago.
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So, something like $4 then I guess at today's exchange rate.
@gohan said in Supercap Solar Powered Mysensors nodes as cheap as possible:
.5€ for solar panel, 1.7€ for 100F supercap, 1€ for booster and some cents for MCP1700, diodes and a small PCB.
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yes 5$ range, of course you need to add the other costs like box, the node itself and everything you need to have the node working outdoor
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I wonder if you might do better with the cheaper 10F chinese supercaps. Obviously they hold less initially, but the question boils down to whether they end up holding more in the end if only because they're losing charge at a much slower rate. I don't know what the answer will be. It's really an empirical question.
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Of course a better quality one would hold the charge better, but at what cost? Would it be really beneficial also from a cost perspective? Maybe, I don't know... It could be that a couple of good quality 10F caps are fine and that you could get away also with a smaller/cheaper solar panel, it's all a matter of trying
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I think I found a major flaw in the design: if the supercap voltage drops below 0.8V the booster is not able to work properly and instead of shutting off it starts drawing current until the supercap is empty. I did some startup tests at 0.8V and I saw current up to 140mA, much more than the solar panel can provide. I would need to reach around 1V on the supercap in order to have a safe margin to have a complete startup cycle. At this time of the year the daylight is getting shorter every day and clouds are making even more difficult to get a proper charge. I haven't done many changes so far because I wanted to see how it behaved in the worst conditions that we get during winter. Now the next steps are: adding some code to make sleep time increase as the supercap voltage decreases and making a board with the 3 supercaps I have in order to avoid as much as possible the voltage to drop below the 0.8V.
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Three supercaps? Sounds like you're leaving the zone of "as cheap as possible." I'd also be leary of a solar design where falling below "x" voltage leaves the system in a state it can't recover from without human intervention. Murphy's Law. Stuff happens.
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Well, I said 3 because I have 3 and I paid 3.35€ for 2, so it is not that expensive. The problem is that I would need to add something that would wait for the voltage to reach 1V after it has dropped below 0.8V before giving power to the booster: that is where a charge controller may come handy. Unfortunately I have no sufficient knowledge on how to solve this problem, so for now the code and 3 supercaps would have to do
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@gohan You can solve that using a voltage detector. 0.2v hysteresis would not be uncommon. So, it would cut the current to your boost converter when the voltage drops below 0.8v, and it wouldn't allow it to come back on again until your supercap voltage reaches 1.0v.
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do you have an example I could take a look? I never used them
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@gohan Not really, but the datasheets for voltage detector chips would have example circuits.
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They're commonly used in brownout detectors and also circuits that do automatic resets if the voltage drops below a particular threshold. Not sure whether Aliexpress has premade circuits you can just buy, which seems to be your preference. Maybe. I haven't looked for that.
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@gohan said in Supercap Solar Powered Mysensors nodes as cheap as possible:
do you have an example I could take a look? I never used them
I'm just remembering now that I used a voltage detector in this project, where in the comments I gave you a detailed walk-thru explanation for how it worked:
https://www.openhardware.io/view/396/Simple-Solar-Supercap-Charger#tabs-instructions
The design files include a schematic.
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I believe I found another problem: the discharge I am seeing in the supercaps, I am suspecting it is due to the self discharge as it is not affected by the sleep time and the adding of the second supercapacitor.
As usual the Chinese stuff it is just worth the money you pay for it.
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@gohan What exactly is it that you're noticing?
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Looking at how quickly the voltage drops over time, I'm seeing no big difference between sending data every 10/30/60 minutes and even adding a second supercapacitor so I'm assuming they are just junk
I'm seriously considering ditching the supercaps and go with a LiFePo4 battery and leaving just the mcp1700 to keep the battery at 3.3v
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@gohan Maybe before giving up entirely, perhaps you want to try a quality supercap in place of the shoddy one?
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Do you have any suggestion on something of decent quality and not too high price? I mean something of at least 100F but that doesn't cost like 10$
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@gohan do you really need 100F? How much energy does your node consume? Maybe it is easier to get the energy consumption down than to find a cheap huge capacitor wih low self-discharge?
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Well it probably doesn't need that much, but I plan to add more sensors and I need to compensate for the winter days when it's cloudy and the daylight is shorter. Of course I could expose the solar panel to direct sunlight to optimize the charging but it would be exposed to harsher weather conditions and the sun will cause the temperature reading to be higher.
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@gohan said in Supercap Solar Powered Mysensors nodes as cheap as possible:
Do you have any suggestion on something of decent quality and not too high price? I mean something of at least 100F but that doesn't cost like 10$
AVX is a good brand. At least worth a shot to test your idea. If it doesn't meet your expectations, then it's not the supercap's fault. If it turns out to be overkill, you can downgrade from there. Or just start with the higheswt capacity AVX supercap that you think is affordable and make your give-up decision after first testing with that.
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@mfalkvidd said in Supercap Solar Powered Mysensors nodes as cheap as possible:
@gohan do you really need 100F? How much energy does your node consume? Maybe it is easier to get the energy consumption down than to find a cheap huge capacitor wih low self-discharge?
I agree, but I think he just wants to quickly test his proof-of-concept first without thinking about it too hard. Kinda a first-fit rather than best-fit search pattern. We've all done it, I'm sure.
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@neverdie 8$ it is not bad but still 6x the initial price I was thinking. Also 8$ is the cost of 3 or 4 LiFePo4 AA batteries that are less temperature sensitive than lipo but can still hold a lot more energy than a supercapacitor and have the right 3.3v working voltage avoiding the need for a voltage regulator/booster . I'll think about it
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@neverdie said in Supercap Solar Powered Mysensors nodes as cheap as possible:
Or just start with the highest capacity AVX supercap that you think is affordable
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I always like to stay on the safe side and plan ahead. Now I need to find out quickly if the solar panel is actually able to provide enough power to keep the node running even with short daylight ( I am thinking about using the LiFePO4 battery charged at 3.1V and see if it can reach the 3.3V or if it drops below 3V); meanwhile I'll try to do a runtime test on the supercaps and try to see how long I can power a node without sleep and make an estimate for a target capacity of a good quality supercap
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The LiFePO4 battery setup was unable to charge the battery (maybe because I didn't study how to charge those batteries). I am now back to the 5V supercaps but with the HW radio to see how it works and I noticed the TX power dropped from 40% to 0% so the power amplifier is actually doing its job
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Also the buck/boost module has some low voltage issues when voltage drops below 1.4V it start drawing 130/140mA and I needed to increase voltage to 1.7V (where it was drawing 240mA) before it started to supply the correct voltage for the node to boot and have the current back to a normal 20/40mA.
I need to find a solution for recovering the node after the booster goes crazy and that would require for sure using a voltage detector like @NeverDie suggested but that is going to require making a custom PCB with integrated booster for the 3.3v output and the supercap charger mentioned earlier.
Is anybody willing to help?
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Ok, after almost a year 3 out of 5 5.5v supercaps started leaking... Rip
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@gohan that's a pity. Do you know what the root cause could be? Temperature? Charging cycles? Bad part quality?
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Andreas Spiess did a whole video on how to properly charge supercaps in series. Gohan's supercaps were just soldered together without those protections to get their 5.5v rating. Maybe it was that. Or, since it was outdoors, maybe corrosion got to them.... It would be interesting to hear what Gohan's post-mortem reveals. As Edison would say, Gohan learned a new way of how not to do it. That still leaves a lot of ways that it could be done though.
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No idea really, the box was sealed. I suspect they might have gotten unbalanced. About charge cycles I don't think they were a problem since they probably had less than 400 (1 cycle a day). They were cheap Chinese parts, so poor quality for sure given the self discharge rate they had. Given the cost of a LiFePO4 battery compared to supercaps, I think I'm going to take the battery route and ditch the whole solar power. I have an identical node running on a single AA LiFePO4 and in 8 months it barely discharged even if it is sending 4 values every 10 minutes
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I've forgotten, but didyou do any protection circuit for your supercaps to keep each one from overcharging? Julien Ilett tested some and found that not all of Chinee circuits stopped the charging at 2.7v, but instead allowed them to go to 2.8 or 2.9v instead:
https://www.youtube.com/watch?v=r7M6Pb2roLs&t=180s
So, maybe that's what happened to the yours that leaked.
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I couldn't because they are packed together, but it was something I was expecting anyway. I had them completely discharged every now and then when the arduino went crazy and drained them out.
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Small update after some time running on 2 100F supercaps in parallel
Voltage seems consistent over time
Here you can see the transition from the 5.5v supercaps to the normal ones, the odd thing is that it seems the supercaps took a few days to accept higher charge voltage
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I may misunderstand how they work, but I've had the idea that (super)caps are better in short term high current situations. Like regenerative brakes for robot lawnmowers, not slow drain low power devices. It sounds to me like the life boat is the lifepo4 batteries. I'm all for experiments, so please dont stop, but unless the environmental conditions are intolerable for the batteries, I'd say you found your answer.
