I have an old iPhone 4, which I haven't used in over a year. The battery has been dead since last January. I was going to use the iPhone again, and of course will have to charge it before use. Since it hasn't been used in so long and hasn't even been turned on in so long, will it be okay? Is it safe to use, or could the battery have deteriorated? You hear horror stories of iPhone batteries combusting; is this a threat here? Use it or just leave it like it is?