Hello, an ordinary USB charger (USB-A cable included) is sufficient. However, the device also works with batteries.

Mine has always lit up. However, it never changed its display, so I'm not sure if mine was simply broken. In any case, it does not measure CO2 directly, so if you want to have overnight measurements, I would definitely buy another sensor (e.g. the one from Sensirion, which is great and only a bit more expensive).

Please contact customer service :)

Yes, for certain functions you even have to. You can find everything and a little more in the description on Digitec: Ventilation recommendation if air quality is too poor (coloured background lighting). For a more precise measurement of the air quality, more power is needed, which is why the air quality is only displayed in operation with a power supply unit. The ventilation recommendation can also only be given in mains operation, in battery operation the time, temperature and humidity values are visible.

Only one cable with USB connection is available for charging.

It should be air quality, I have never received an alarm for humidity. But since the display then turns yellow and later red, this is very visible.

Yes, it is in the operating instructions

Above all, the device does not measure CO2, but VOC. The designation in Digitec's webshop is grossly misleading. And apparently deliberately: I reported the mistake to Digitec months ago, but they apparently don't think it's necessary to correct it.

It has a CO2 sensor. The description here in the shop is obviously incomplete. The display of the air quality is based on the CO2 content of the air in ppm. 5/5 stars means <500 ppm. 0/5 >2451 ppm. I have no way of verifying this exactly. But when I stop the air and exhale into the ventilation slots of the unit after about a minute, the quality drops drastically for a short time. But it quickly returns to normal again.