I thought it would be a nice experiment to put the Funky in a solar-powered garden lamp. I got one recently with the intention to use it for that particular experiment, it is quite cheap, costs a little over one euro. Inside we have a rechargeable battery of unknown type. I’d assume it is NiMH, it is size AA. Since my version of the Funky has a step-up circuit, I can directly hook it up to the battery it will get itself the 3.3V. So I removed the LED that was there so it won’t eat battery and soldered some wires to the battery terminals to power the funky. At that time the battery measured mere 0.5V and could not power up the Funky yet. I left it on a shiny place all day, and the battery now measures 1.3V. Transmissions started roughly 15 minutes after I left it outside. I have the Funky run the internal temperature sensor sketch, it reports the readings every minute. Of course the readings are quite high in direct sunlight, but you can use longer wire to put the Funky to a more shadowy place.
I can’t measure the rechargeable battery’s voltage without a dedicated voltage divider, because the step-up circuit always brings it to nice 3.3V and internal band gap reports so.
I’ll experiment a few days, I am interested to see if it will make it through the night, especially in a cloudy day. From practical aspect, this may be useful for projects that are in less accessible places, it is somewhat self-powered remote sensing node.