Charging devices, modular adapters, and circuits are provided. One charging device including a body including a rechargeable energy storage device and a housing. The housing includes at least one first coupling portion. The charging device further includes a modular adapter including a second coupling portion configured to be mated to the first coupling portion of the housing. When mated to the housing, the modular adapter is configured to be electrically connected to the rechargeable energy storage device. When the modular adapter is mated to the housing, the body and the modular adapter form a single integrated and substantially rigid structure.