A battery system includes a cell group in a lead-acid battery power system (battery system A) coupled to a battery power system (battery system B) having a different chemistry, such as lithium ion-based battery. Each battery system, A and B, are interconnected at specific cathode and anode contacts chosen based on the voltages range present at those points to prevent damage to the different battery systems. The worst-case voltage range of the battery system A at the connection points is higher than the worst case voltage range of battery system B. The battery system includes a battery management system (BMS) that monitors voltages at each cell level and that monitors current at the pack level. The BMS can also be configured to control conductivity between packs A and B, and to protect battery systems A and B from going out of normal operating conditions.