A battery system comprising: a first battery terminal (B+); a second battery terminal (B-); a first battery module comprising a plurality of first battery cells (BMa) and a first module management unit (BMMa) configured to monitor the plurality of first battery cells and connected to the first battery terminal; a second battery module comprising a plurality of second battery cells (BMb) and a second module management unit (BMMb) configured to monitor the plurality of second battery cells, and connected to the second battery terminal; a battery management unit configured to transmit data to and receive data from the first module management unit and the second module management unit; and a battery disconnection unit, BDU, connected between the first and second battery modules and between the second battery module and the second battery terminal, wherein the BDU comprises: a substrate including a first terminal connected to a negative terminal of the first battery module, a second terminal connected to a positive terminal of the second battery module, a third terminal connected to a negative terminal of the second battery module, and a fourth terminal connected to the second battery terminal; a first main switch mounted on the substrate and connected between the first terminal and the second terminal; a precharge switch (SWp) and a precharge resistor (Rp) mounted on the substrate and connected in series between the first terminal and the second terminal; and a second main switch mounted on the substrate and connected between the third terminal and the fourth terminal, and wherein the battery management unit is configured to control the first main switch, the second main switch, and the precharge switch. a first battery terminal (B+); a second battery terminal (B-); a first battery module comprising a plurality of first battery cells (BMa) and a first module management unit (BMMa) configured to monitor the plurality of first battery cells and connected to the first battery terminal; a second battery module comprising a plurality of second battery cells (BMb) and a second module management unit (BMMb) configured to monitor the plurality of second battery cells, and connected to the second battery terminal; a battery management unit configured to transmit data to and receive data from the first module management unit and the second module management unit; and a battery disconnection unit, BDU, connected between the first and second battery modules and between the second battery module and the second battery terminal, wherein the BDU comprises: a substrate including a first terminal connected to a negative terminal of the first battery module, a second terminal connected to a positive terminal of the second battery module, a third terminal connected to a negative terminal of the second battery module, and a fourth terminal connected to the second battery terminal; a first main switch mounted on the substrate and connected between the first terminal and the second terminal; a precharge switch (SWp) and a precharge resistor (Rp) mounted on the substrate and connected in series between the first terminal and the second terminal; and a second main switch mounted on the substrate and connected between the third terminal and the fourth terminal, and a substrate including a first terminal connected to a negative terminal of the first battery module, a second terminal connected to a positive terminal of the second battery module, a third terminal connected to a negative terminal of the second battery module, and a fourth terminal connected to the second battery terminal; a first main switch mounted on the substrate and connected between the first terminal and the second terminal; a precharge switch (SWp) and a precharge resistor (Rp) mounted on the substrate and connected in series between the first terminal and the second terminal; and a second main switch mounted on the substrate and connected between the third terminal and the fourth terminal, and wherein the battery management unit is configured to control the first main switch, the second main switch, and the precharge switch.