I’ve never stored a phone long term before, but I just bought a 128gb 7+ from a guy at work for $200. It’s in great shape, with a battery health of 90%. Apple says to charge it to 50% and keep it turned off in a cool dry place. That’s self explanatory, but would it do me any good to power it on and use it for a day every few months? Not sure I’ll ever even need to use it, but $200 was too cheap to pass up. I’m going to take it in for a $39 battery and then store it until one of my family members break our iPhone.