Disclosed are embodiments to provide a multi-battery energy storage device. One embodiment comprises a first battery and a second battery, with a first circuit branch coupling a positive side of the first battery to a positive side of the second battery, a second circuit branch coupling a positive side of the first battery to a negative side of the second battery, a third circuit branch coupling the negative side of the first battery to the negative side of the second battery, and multiple switchable devices configured to control flow of current through corresponding branches. Other embodiments comprise other configurations and operations.