So, I really love my new D500. Still learning all the features but really enjoy the camera. One question is, my factory battery is only good for 2-3 hours of shooting. Now I am using a long lens (200-500) and I know that the auto focus on a lens like that can draw down a battery pretty quick. Is it worth investing in one of the battery packs I see on line? Invest is a second factory battery? I do a lot of wildlife stuff so I am out with the camera for hours waiting for the birds to show up or wildlife to present themselves.