Supercap Solar Powered Mysensors nodes as cheap as possible

[NeverDie]

The overcast days are where the two diodes are burning you.

[gohan]

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 :D

[gohan]

That is looking great so far (the spike was a reboot after I installed the SMA antenna)

[NeverDie]

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.

[gohan]

@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. :D

[NeverDie]

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.

[gohan]

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

[NeverDie]

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.

[gohan]

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 😀

[gohan]

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.

[NeverDie]

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.

[gohan]

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 :D

[NeverDie]

@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.

[gohan]

do you have an example I could take a look? I never used them

[NeverDie]

@gohan Not really, but the datasheets for voltage detector chips would have example circuits.

[NeverDie]

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.

[NeverDie]

@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.

[gohan]

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.

[NeverDie]

@gohan What exactly is it that you're noticing?

[gohan]

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