Abstract: Techniques are described for wirelessly charging a wearable or implantable medical device, such as a continuous glucose monitor (CGM) and/or an insulin pump. In some examples, the medical device includes: a rechargeable battery; a receiving antenna configured to wirelessly receive electrical power from a transmission antenna; charging circuitry configured to recharge the rechargeable battery using the electrical power received by the receiving antenna; and one or more processors configured to: determine a relative proximity of the medical device to a body of a patient; determine, based at least in part on the relative proximity, a tuning for the receiving antenna; and cause the receiving antenna to be tuned according to the determined tuning.