This study presents a standalone diagnostic device for HEV high-voltage battery packs that communicates directly with the BMS outside the vehicle and enables quantitative verification of BMS SOC and SOH outputs. The prototype, developed for a Renault CMA HEV pack, activates the BMS via the low-voltage harness, reads key variables such as SOC, SOH, cell voltage and temperature, and pack voltage and current over CAN, and safely controls the pack’s high-voltage relay. Using a pack reported as 100% SOH by the BMS, constantcurrent discharge at about a 0.1 C-rate was performed in the SOC range from 30% to 45%, and for 5, 10 and 15-minute segments the usable energy estimated from the BMS SOC and the rated capacity showed mean values around 1.54kWh with a coefficient of variation of approximately 2-3%. The proposed BMS-linked evaluation equipment estimates the usable capacity within a tolerance consistent with the manufacturer’s nominal specification and can serve as a practical basis and tool for second-life evaluation of used high voltage battery packs.