Hi, I'm contemplating going on a sort of off-line holiday, where I might not have access to a charging outlet for days - or even a week. I'd take an iPhone (SE probably) as camera and use that to take pictures. I was wondering if that was a sane idea to begin with (I don't own a DSLR and my compact camera from 2007/2008-ish has probably long been surpassed in picture quality by a previous iPhone generation. Plus the battery in that isn't that great, either. How long do powerbanks keep their charge? Has anybody done any research/tests about that?