The present invention is directed to battery systems. In various embodiments, the present invention provides a battery system having a first battery group and a second battery group. The first battery group and the second battery group can share a single housing, or be positioned within separate housings. The first battery group and the second battery group can each have a plurality of cells configured in parallel, and the two groups can be electrically integrated. When operating at a low temperature, the first battery group is configured to provide electrical energy to a heating module that selectively heats up one or more segments of the second battery group. In addition, the first battery group also provides energy for initial operation of an electric vehicle or equipment. Once the selected segments of the battery group are heated up to an operating temperature, the selected segments supply electrical power to the vehicle or equipment and charge the first battery group. In addition, the selected segments of the second battery group provide electrical energy for heating other segments of the second battery group. There are other embodiments as well.