We've slowly been upgrading the tree stands that we use for deer hunting and recently started dropping batteries in with solar panels to top them off.  The primary use has been to wire in some lighting to make it easier to get set up in the morning and pack up in the evening.  But, it gives the added benefit of having 12v DC power available for topping off phones, iPods, or even running an iPad or netbook like my uncle likes to do during the long mid-day hours when the deer don't move around as much.