The invention relates to a method for detecting the capacity of a storage battery. According to the method provided by the invention, a controller is adopted to detect static voltage of the storage battery when a certain condition is satisfied and controls a relay to enable the storage battery to discharge for a short time through a test resistor, the controller detects discharge voltage of the storage battery at the time, and the discharge voltage is compared with a voltage threshold pre-stored in the controller for judging whether the storage battery is good or bad. When the discharge voltage is higher than the threshold, the static voltage of the storage battery is compared with a storage battery capacity-voltage corresponding relation database, thereby determining residual capacity of the storage battery, and sending the residual capacity of the storage battery to an instrument to display. If the storage battery is monitored to be damaged soon, the information is also sent to the instrument to display. The monitoring method provided by the invention is low in cost, and effectively solves a problem that a storage battery electricity quantity sensor is high in cost in monitoring the residual capacity of the storage battery.