Detachable auxiliary power systems are disclosed. In some embodiments, the power system comprises a housing with a cavity, and a detachable portion configured to be received into the cavity. The detachable portion includes a power source such as a rechargeable battery, and may include additional electrical circuits to manage the charge and discharge of the battery. The housing includes a cable for electrically connecting the housing to an external electrical system, such as a vehicle electrical system. When the detachable portion is inserted into the housing, it is electrically connected to the cable, and so able to be charged from the external electrical system, and selectively provide power from the power source to the external electrical system, such as for jump-starting a vehicle.