As this thread turns into my personal highlights of silly mistakes: Another word of caution for anybody who wants raw battery values sent:
sendBatteryLevel only uses 8-bit: i.e. value from 0-255. Don't try to send your raw 10-bit sensed input from e.g. A0 directly with it.

bool sendBatteryLevel(const uint8_t level, const bool ack = false);

Make sure you divide by 4 or use something like:

int sensorValue = analogRead(ANALOGUE_SENSE_PIN) >> 2;
    sendBatteryLevel(sensorValue);

@Nca78 Yes, that should have been clear to me by now.

I just lost some time trying to burn the boot loader as always I got this error:

avrdude: stk500_getparm(): (a) protocol error

Checked, tested and even redid the soldering to exclude any wiring problems. Turns out adding a cap (0.1 µF in my case) between the programmers GND and RST did the trick.

Thank you all!

@gohan But how do I handle the different voltages? The Nano has 5V powered from USB, and I supplied the Mini Pro with 3.3V. But can't connecting the SPI pins lead to havoc? I read stuff in the ArduinoISP sketch like :

MISO °. . 5V (!) Avoid this pin on Due, Zero...

Should I just leave out the MISO connection?

Ok BOD as in brownout detection. I'll follow you into the rabbit hole and try to burn a new bootloader. Trying to figure out if I can use my 5V Arduino Nano to burn the boot loader onto the 3.3V Mini Pro...

@neverdie said in Solar cell support with non-rechargeable batteries:

Did you remember to remove the LDO? Because otherwise it gets backfed, which is not what you want.

I don't understand, the LDO is needed for charging the supercaps?

@neverdie said in Solar cell support with non-rechargeable batteries:

Also, yes, in theory you are running it out of Atmel's official spec if running at 8mhz at 1.8v, but I don't know and haven't read of even a single person who has ever had a problem with doing so. Doing this is very common.

I would be very glad if I got to lower voltages, I am just trying to understand why I can't get below ~2.9V (at the Arduino).

@Nca78 said in Solar cell support with non-rechargeable batteries:

@gunther did you update fuses to lower BOD ?
Still getting to grips with the terms. You mean the 1N4148 diodes? And BOD refers to lower voltage difference?

No, not yet. I just plugged together parts, I could get my hands on to try to understand everything. I will then order the parts that are ideal.

@NeverDie I promise to read all comments in existing threads!

One thing that got me sidetracked is that I do not understand the analog read:
I successfully confirmed that the Arduino Nano when attached to USB uses 2^10 channels with 1.1V reference.

For the Arduino Mini Pro I also see 2^10 = 1024 channels with 1.1V reference when the power support comes from USB, what I assume to be a high quality power supply.
However, when I run it off the capacitors + solar cells giving 3.3V I only get 2^8 = 256 channels with what appears to be ~1.1V reference. That is when I apply a voltage >1.1V I always get 255.
I added various capacitors from 0.1-4.7 µF in parallel but that didn't change anything.
How can that be?

Thank you all for the information!

@neverdie Ok, so now I have desoldered the Arduino Pro Mini power regulator and I now see a sleep current of 6µA with 3.2V.

One thing I see is that the node only works down to 3V. With 2.5V it is not stable anymore. This is just the Arduino Pro Mini with attached NRF24L01. Maybe this is expected? I checked visually that I have indeed the 8MHz variant.

I just tried to test it with parts I could get my hands on.

Could someone please tell me why the Battery Powering page advises to cut Vout? And what is it? It seems to me that this disconnects the Vcc on the shorter end? To what consumption does the connection lead. I find it convenient to have another Vcc pin available.

@neverdie Will try! But that means the NRF24L01 is also out? And am I correct to connect the 2.7V to VCC not RAW?

@neverdie How do you get from 2.7V to 3.3V? With another DC/DC booster?

@johnrob Ok, just like with a resistor. Thank you for clearing this up. I assumed but wasn't sure.

@gohan So only good weather updates? From a practical standpoint solar cells are probably more of a hassle but the beauty of the concept...

@gohan Thank you for clearing this up! Couldn't find anything on the consumption of these diodes.
Using just 2 AA would indeed be the easiest, however I wanted to tinker around