Supercap Solar Powered Mysensors nodes as cheap as possible

[NeverDie]

@gohan said in Supercap Solar Powered Mysensors nodes as cheap as possible:

My biggest problem now it is more related in understanding why the voltage value I get from the A0 through the voltage divider is off +/- 0.3~0.5v. I checked with the multimeter and with a voltage divider calculator and I do get values within a very small error (resistors tolerances). For example on the 2.7v caps I'm reading 1V on multimeter and arduino reports 0.7v, while on the 5.5v caps I'm getting almost 0.5v more. I'm not sure what to look for. The code was from an example of @sundberg84 . Since on the 2.7v I'm running it on a steady 3.3v source (booster), maybe I could read the voltage directly from the A0 pin without voltage divider, or is it gonna kill the arduino?

IIRC, you shouldn't try to read a voltage that's higher than around 0.3v (or was it 0.6v?) more than VCC. Either way, if VCC is 3.3v, then 5.5 is well outside that range.

[gohan]

I'm just talking about the 2.7v option. Ofc I can't read 5.5v directly

[NeverDie]

Since you're assuming access to direct sunlight, you can tolerate a lot of inefficiencies because of the overabundance of available energy compared to what most sensor nodes require. I remember one guy posted who had a similar 6v solar panel outdoors, and IIRC he said his 10F supercap was already fully recharged just from the daylight before sunrise. i.e. it's bright outside. So, even if your cheap 100F supercaps lose most of what they collect through high self discharge, but they collect a much larger amount all day long because of their higher capacity, maybe you come out ahead. Seems like a worthwhile experiment. Glad you're doing it.

[gohan]

Here is the graph of the voltage: the measurements started ok at ~2.6V but now it is showing 0.66V while I read 0.936V on multimeter I wonder what it may be. My knowledge in electronics is quite poor :sweat_smile:

[NeverDie]

@gohan said in Supercap Solar Powered Mysensors nodes as cheap as possible:

I wonder what it may be.

I would guess the capacitor has a high internal resistance. Your multimeter doesn't see it unless the capacitor is under load. But your arduino is a load, so it sees it.

[gohan]

Actually it should be reading the voltage from the 0.1uF cap that is on the voltage divider of the Easy PCB; would it make sense to increase capacitance? I have a bunch of 10uF ceramics

[NeverDie]

Maybe a schematic of your setup would help the readers of your thread make informed comments. Otherwise, it starts to become 20 questions, if you know what I mean. People on this forum love to look at schematics.

[gohan]

@NeverDie https://www.openhardware.io/view/389/EasyNewbie-PCB-RFM69-HWW-edition-for-MySensors "battery operation", I use this setup with the code like on this page https://www.mysensors.org/build/battery#measuring-and-reporting-battery-level

[NeverDie]

@gohan
Well, if 0.1uf is normally sufficient, why wouldn't it be in your case?

There's an easy way to test my hypothesis: why don't you try powering your setup from some fresh batteries (or a high quality supercap if you have one) and see if there's still the voltage discrepancy? If not, then it's the ESR of your cheap supercap. If it's still there, then maybe you just need to calibrate?

[gohan]

Maybe in the WE I'll try the full range with the power supply and see, because towards the top end the values are pretty close, less than 0.1V apart

[gohan]

I have uploaded 2 pictures of the solar powered 5.5V node

[NeverDie]

@gohan said in Supercap Solar Powered Mysensors nodes as cheap as possible:

I have uploaded 2 pictures of the solar powered 5.5V node

Uploaded to where? I don't see anything. Nevermind, I see it now at the beginning of the thread.

[gohan]

I'm currently testing the single 100F cap but it seems the buck boost is barely able to charge the supercap in direct sunlight, while the solar panel connected directly to the 5.5v supercaps was working much better. I'll have to try with a normal buck converter as it seems now the boost mode is making the solar panel working on a voltage too low that doesn't produce much power.

[gohan]

I replaced the buck-boost module with a simple buck converter and it seems to work much better since the operating voltage is from 4.5V to 23V. As you can see from the below picture, the buck-boost barely charged the cap while in direct sunlight while the buck converter was able to charge the cap in a short time even with a cloudy day.

I am also very happy since I set the output voltage to 2.62V on the buck converter and I actually get 2.62V reported by the arduino.
I also forgot to say that I am using a booster like showed in @sundberg84 project link text

[NeverDie]

@gohan I suspect it's "working" because the voltage on your boost converter is higher than 2.62v from the solar panel anyway at the time that it's charging. Even on a cloudy day, it's not hard to get more than 2.6v on a 6v solar panel.

[gohan]

The booster is only from the supercap to the node to keep a steady 3.3v. The solar panel if feeding the buck converter directly so it will start charging when panel is providing around 4,5v. It is a double conversion, I know, but efficiency is not the goal.

[gohan]

I guess I might have overlooked the backward discharge of the buck converter.... It drained the supercap in 4 hours while it should have lasted 3 days. I need to put a diode in place :sweat_smile:

[gohan]

Interesting... The node is still dead... I guess there is big flaw in the setup or the buck converter is dead due to the reverse current flow from supercapacitor

[gohan]

Schottky diode to the rescue: the buck converter had a 1.5 mA drain. No wonder it didn't last long. After I put a schottky on the input now I get no drain :muscle:

[gohan]

Now voltage stays between 2 and 2.4V (it should be 2.62V but I need to adjust the buck converter and add 0.2V as they are lost though the diode now). I don't understand why the voltage is so unstable now during discharge hours