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.

what I meant is that they don't actually go to 0v even if you short them as you will always be able to measure some small voltage. I am no expert but my bet is that that 0.5/0.6v is kind of their bottom line where they don't hold any more useful energy below that.

For balancing there are SAB Mosfets that help to keep all the supercaps on the same voltage, but the reverse charge issue is the same for all series circuits, also LiPo worn out batteries can get to 0V and then getting reverse charged with all the consequent risk of fire.

On the cheap supercaps I have they hold the 0.6v, they don't go to zero

The 1.1v reference could be + or - 10%, but a small ceramic cap on near the analog pin should help to stabilize the reading. Also taking 3 measures and then sending the average is a good way of minimizing errors

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

Ok, after almost a year 3 out of 5 5.5v supercaps started leaking... Rip

The send function returns a value, so you can check if it was successfully sent or not and in case retry

Is an esp8266 powerful enough for a bme680?

@korttoma if you have a library that can read the values from the sensor, you can easily add the mysensors code

you are at the limit of the radio range I'd say so randomly it looses a message, so you need to either increase power or get a better antenna (I hope you aren't using any buck/boost converter as power supply)