Abstract: A battery interchangeability system having an application device requiring a battery of a particular size, an insert device that can be inserted into the application device in place of the required battery, and a battery of a second size that is smaller than original size so that at least one smaller battery can be inserted into the insert device. It is also possible for multiple batteries of a smaller size can be inserted into a single insert and used in parallel or series. In particular, the larger cell size can be D, C or AA size, while the smaller cell size can be C, AA or AAA size. A second, smaller insert device can be used which is inserted into a first, larger insert device with the battery or batteries inserted into the smaller insert device. Optionally, several smaller batteries can be placed around the axis of an insert device in a turret arrangement. Batteries in this arrangement can be placed into the device from either the top or side.