Hmm...It is true that the battery could've been changed later. (Like at 550k miles) Still, it's bad for a battery to die after 5 years or something.
Maybe, the battery pack was used so little (since it was "99% highway driving") that the battery pack went bad?

I think it's unlikely.
But anyways, it's good to hear about a HCH still going storng after so many miles!
