I don't see this being a battery issue, I think the engine is running fine. The problem with the switch spacing has already been discussed and does indeed briefly cut track power. However, the engine continues with the flywheel inertia and regains power before shutdown, thus indicating the battery is fine. On PS/2 locomotives, the lights will go out immediately on any track power interruption, they're not supported by the battery. The only "fix" here is to fool around with the switch spacing, and since it's not affecting anything but a brief flicker of the lights, he's elected to let sleeping dogs lie.