This is a bit off topic, but I've found an interesting part with about the right properties! The TPS610986, a step-up converter with integrated load switch. After some digging, I think it comes down to finding a chip with low quiescent current Iq for for light loads, like the MCU in sleep mode. And low minimum supply voltage (to fully drain the battery), while having enough output current capability, to power the whole device... and high enough efficiency.
I've found many other parts, that might be suitable, some with interesting energy harvesting features (among others: BQ25504, ADP5090, TPS65290, LTC3331) and some "normal" boost converters with low quiescent current (TPS61099x, LT1615, SC120, AS1323, maybe TPS61222 ). Also found this interesting approach of using an LDO in combination with a switch mode supply... but that seems not really practical.
Maybe it would be more efficient and certainly easier to just hook up a battery directly, since the microcontroller will be fine with supply voltages down to 2 V or lower... But I think it's good to have a stable supply voltage, for other parts hooked up to the controller... - I do not really have experience with low power stuff at all. So any hints are appreciated :)
As for STM32L, I've found that variant with enough flash/RAM for Espruino to hopefully work. I will probably give it a try...
STM32L072xZ - Ultra-low-power ARM Cortex-M0+ MCU with 192 KB Flash, 20 KB RAM, 6 KB EEPROM, 32 MHz CPU, USB
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