A battery system for powering a vehicle is provided. The system may include a first lithium-ion battery pack having a first total energy capacity and a first power to energy ratio (P/E ratio) and a second lithium-ion battery pack connected in parallel with the first lithium-ion battery pack and having a second total energy capacity that is higher than the first total energy capacity and a second P/E ratio that is lower than the first P/E ratio. A method of controlling the battery system is also provided, and may include controlling an operation of a vehicle according to a total power capability of the first and second battery strings, wherein the total power capability is the sum of a first battery string power capability and a second battery string power capability at a same voltage.