A high-voltage electrical system includes a high-voltage electrical bus, a low-voltage controller performing battery management system (BMS) functions, and a rechargeable energy storage system (RESS) connected to the HV electrical bus. The RESS has contactors connected to the voltage rails, a battery pack, a current sensor outputting a voltage signal indicative of pack current, cell monitoring units (CMUs) connected to designated battery cells that measure corresponding cell voltages, and voltage isolation hardware between the CMUs and the LV controller. The voltage isolation hardware includes the contactors and may include a transformer. The RESS and LV controller are connected via LV serial connections such that the LV controller performs the designated BMS functions, including monitoring the pack current and controlling an open/closed state of the contactors. A powertrain system includes the electrical system, an electric machine, and a coupled load.