A battery pack (28) for a portable defibrillator (30). The battery pack has a latch (34) that is biased in an extended position. When the battery pack is inserted into a battery well (30) in the portable defibrillator, the latch automatically latches into a slot (124) to secure the battery pack in tile battery well. Ridges (112) are provided on the floor (108) of the battery well to reduce the friction between the battery pack and the floor of the well as the battery pack is inserted into the defibrillator. A ridge (38) is also provided around the periphery of the top (36) of the battery pack to reduce the friction between the battery pack and the ceiling of the battery well. A right wall (44) of the battery pack is inclined from vertical so that a cross section of the battery pack is trapezoid in shape. The asymmetric cross-section prevents the battery pack from being incorrectly inserted into the battery well. The disclosed battery pack construction simplifies the replacement of the battery pack in the portable defibrillator.