You can easily charge your Xemio with mains power. This gives no over voltage problems. Xemios are charged with 5V voltage via USB. Via mains power this is of course more but that is not a problem because the Xemio does not absorb more voltage than it needs.
Charging via mains power is possible, for example, by using a telephone charger.
Charging via mains power is possible, for example, by using a telephone charger.