I did have a spare battery, but I had misplaced it. I keep it with all my other assorted gun stuff at home, but it had apparently hidden it's self under a lens cover. All is well now.
This was year 3 on that battery, so I'd kind of forgotten that it might die.
As I generally hunt just out behind my house I prefer keeping spare parts in one place as apposed to making sure they're in the correct pants/vest every time I go out. But that only works when I can actually find stuff