The method involves determining an amount of a theoretical energy balance based on a theoretical model of a battery, where the amount of the battery taken energy and the amount of the battery supplied energy are determined using the diagnostic period. An efficiency factor is determined, which allows an assessment of battery status, where the efficiency factor is calculated as a solution of a specific equation. An action is triggered if the efficiency factor is detected below a certain amount. The method is performed in a defined repetition interval. Independent claims are included for the following: (1) a computer program for performing a method for battery diagnosis; and (2) a battery diagnosis system for a vehicle.