Electrical power storage for downhole tools, including a tool for use in a subterranean well and methods for activating a downhole tool in a subterranean well

A method of activating a downhole tool can include configuring the tool having an electrical power source, an electrical load, control circuitry which controls the electrical load, and a switch which selectively permits current flow between the power source and the circuitry, and generating electricity, thereby causing the switch to permit current flow between the power source and the circuitry. A downhole tool can include an electrical power source, an electrical load, control circuitry, a switch which selectively permits current flow between the power source and the circuitry, and a generator. Another method can include displacing a fluid and/or an object at the tool, generating electricity in response to the displacing, permitting current flow between an electrical power source and a control circuitry in response to the generating and, after the permitting and in response to detection of a predetermined signal, the circuitry causing activation of an electrical load.