Retention systems are disclosed. In some embodiments, a retainer apparatus is configured to retain a battery of the type used to power a pitch-control drivetrain for adjusting a pitch angle of a wind-turbine rotor. In a working embodiment, the retainer apparatus has a separable housing member and sleeve member configured to matingly engage each other in an interlocking relationship and to define a retention chamber having an open interior region sized to receive the battery. Adjustable pitch rotors having one or more innovative retainer apparatus are also disclosed. Innovative principles disclosed herein can be adapted to retainers for a wide variety of retainable elements, other than batteries.