Abstract: To provide an easily removed and supervisable electrical battery unit for a bicycle, for selective connection to a bicycle generator, a unitary housing (1) retaining battery cells (4) is mounted on the handle bar support posts (17, 19) by rearwardly extending wing-like projections, through which a bolt (23, 28) is passed reaching around the rear of the post (17, 19) to releasably and firmly secure the housing to the bicycle. The top wall of the housing carries two monitoring lamps, one indicating the charge state of the battery and the other proper functioning of the illumination circuit and, especially, of the tail light of the bicycle. The battery unit, thus, with the monitoring light, is continuously within the field of vision of the bicycle rider.