An electrical device includes a memory storing a value indicative the remaining available rated capacity of one or more batteries. 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 one or more batteries will provide at least the capacity indicated by the initial stored value. This reduces the chance of failure during emergency procedures. The one or more batteries may be integral to the electrical device. An override facility is provided.