A battery system with cell groups arranged in modules and with plurality of modules arranged in individual battery sub-packs includes a controller network configured to monitor the sub-packs. The network includes a plurality of cell monitoring units (CMUs); each CMU connected to one module for processing data for respective cell groups. The network also includes multiple voltage sensors on each CMU, with each sensor detecting voltage across one cell group. The network additionally includes an electronic controller programmed with an algorithm and in wireless communication with each CMU. The algorithm identifies when electrical power is disconnected from the RESS and directs electrical current through a selected sub-pack after power is restored. The algorithm also interrogates voltage sensors of a particular CMU, detects a change in voltage triggered by the current, and records a cross-reference between the particular CMU and the selected sub-pack when the change in voltage is detected.