Extending our evaluation of augmented reality (AR) in automotive vocational training, this study measures skill transfer and 90-day retention among 140 trainees learning electric vehicle (EV) battery-pack maintenance. The rapid growth of EV fleets in India has created demand for technicians able to service high-voltage battery systems safely, yet vocational institutes lack instructors with EV experience and the procedures carry safety risks that make unsupervised practice hazardous. AR work instructions, which overlay step-by-step guidance, component identification and safety warnings on the trainee's view of the physical battery pack, offer a potential solution. A two-arm experiment at three industrial training institutes in Delhi and Haryana assigned trainees to AR-guided instruction or to video-based instruction with printed manuals for a four-day battery-pack module covering isolation, module removal, cell balancing and reassembly. Skill transfer was assessed through a practical examination on an unfamiliar pack immediately after training, and retention through a repeat examination after 90 days, with safety errors recorded by blinded assessors. AR-guided trainees showed 24% higher retention with fewer safety errors than video-instructed peers: mean retention scores were 78.3 compared with 63.1, and the rate of critical safety errors was less than half that of the video group. Immediate transfer scores were also higher, though the difference was smaller. Training time did not differ between conditions. The findings indicate that AR work instructions support durable skill acquisition for high-voltage procedures and offer a practical route to scaling EV technician training where experienced instructors are scarce.