Devices, systems, and techniques for selecting a period for charging an implantable rechargeable power source are disclosed. Implantable medical devices may include a rechargeable power source that can be transcutaneously charged. A system may control a charging module to begin charging the rechargeable power source of the implantable medical device with a high power level. The system may then determine an estimated heat loss based on power initially delivered to the rechargeable power source when beginning the charging. Based on this estimated heat loss during the initial period of recharging, the system may select a boost period that includes a duration of time that the rechargeable power source is charged with the high power level.
