The Aranet4 runs on two AA batteries and the smartphone apps are still well-supported. The apps also support CSV export of trend data which is nice. In addition to CO2 ppm, it also records temperature, air pressure, and relative humidity (these other readings are probably used to adjust the CO2 sensor readings to account for temperature/humidity/air pressure conditions). I carry one of these with me while traveling and its high alarm beep (I have it set to sample every 60 seconds) is a convenient reminder to circulate air or consider wearing a mask if I am in a public space. The air pressure reading can be used as a crude altimeter, especially if you have two of the devices: one in a stationary location and another on your person as you take a hike; subtract the two values and you get a pressure difference that is a function of elevation.
