Reconfigurable batteries can change their cell topology in real time, which enables them to adapt their voltage during operation. This unique capability makes interfacing power converters redundant in applications where batteries are directly coupled with other DC components or systems. The present paper characterizes a 104 kWh prototype of a reconfigurable battery for high power applications, and derives equations for calculating the battery efficiency for the complete operating area. The battery can adapt its voltage from 0 V up to 1200 V, and reaches power values of 240 kW for charging, and 280 kW for discharging. The results are presented in efficiency maps, showing the dependency on voltage, power, and state of charge. Moreover, the efficiency characteristic is compared to a conventional battery with fixed cell topology and DC-DC converter. The reconfigurable battery can operate at a wider voltage range and achieves better efficiency up to an average power of 44.6 kW during charging, and 46.7 kW during discharging. Conversely, the conventional system performs better above these thresholds. Finally, the presented model can be used to optimize the design of reconfigurable battery strings, and to accurately size such systems for specific applications and purposes.