A battery pack (10) for an electrical device (60) has a cover portion which forms part of the assembled electrical device (60) and a seal (78) for forming a waterproof seal between the battery pack (10) and the electrical device (10). Thus, every new battery pack can form a fresh seal with the electrical device (60), allowing effective waterproofing. A battery pack (10) for an electrical device (60) includes a memory storing a value indicative the remaining available rated capacity of the battery pack (10). The stored value is changed in use to reflect reducing capacity. The initial stored value is chosen so that there is a very high (e.g. >99.9%) confidence that the battery pack will provide at least the capacity indicated by the initial stored value. This reduces the chance of failure during emergency procedures. An override facility is provided. A laryngoscope (60) comprising a handle comprising a demountable battery pack (10) comprising one or more batteries (20) and an arm (49) operable to retain a demountable laryngoscope blade.