The figures illustrate an apparatus (10-1, 10-2, 10-3) for a vehicle (100) comprising: first energy storage means (12) for storing electrical energy; second energy storage means (14) for storing electrical energy; and control means (16) arranged to transfer electrical energy from the second energy storage means (14) to the first energy storage means (12) when the level of electrical energy stored in the first energy storage means (12) is below a first electrical energy threshold level and the level of electrical energy stored in the second energy storage means (14) is above a second electrical energy threshold level.