The sensor will arrive partly charged but ideally you should charge it for 24 hours before use for optimum performance. The battery can be recharged via a PC, a USB +5V wall adapter, or a portable USB battery pack using the USB lead provided.
$ Please note that due to the heat generated whilst charging, WiFi devices with an internal temperature sensor will indicate a temperature a few degrees above the ambient temperature. Once fully charged, this heat generation ceases and the sensor will eventually indicate the correct temperature. This anomaly does not apply to data loggers with external temperature probes.