Combined primary and secondary units

A system comprising a primary unit (1) and a number of secondary units (16), which may be stored in receiving means in the primary cleaning unit (1) or be coupled to the primary unit. The primary and secondary units are configured as movable cleaning or grounds maintenance units capable of performing at least one cleaning or grounds maintenance operation. The primary unit (1) is powered by a power source (2), which also powers the secondary unit (16) or charges the power source (17) in the secondary unit (16). Both units comprise a communications module (13, 25) capable of communicating with each other and/or a remote location. At least one of the units (1, 16) may comprise one or more proximity or near-field sensors and/or one or more sensor sensing the performance of the unit (1, 16).