Abstract: Stand-by battery equipment which is designed to come into use when the mains supply fails, is tested by disconnecting the mains supply at regular intervals by contact 14 under the control of timing device 21. After a delay caused by timing device 23, a light sensor 15 is activated for a time period controlled by timing device 24to check whether the light output of the battery equipment has reached a desired level, and if it has not it causes an alarm lamp 16 to be illuminated until reset. A timing device 22 causes an extra long test to be carried out after a number of short tests, to ensure that the battery equipment is operable for the long period and to allow the stand-by batteries to become appreciably discharged.