I have two batteries in my ThinkPad. One went where the optical drive would go. Re: Phone battery swap I typically have a fully charged one in my backpack. I

I have two batteries in my ThinkPad. One went where the optical drive would go. Re: Phone battery swap I typically have a fully charged one in my backpack. I use it every so often just to keep it healthy. The one on the phone gets charged when the phone does and it basically delivers 2X battery time, and can discharge first. My experience with these lithium batteries is they will degrade very slowly when kept out of extremes at the bottom and constant charge at the top. A deep discharge degrades the battery the worst, so basically do not do that. Same goes for laptops. Batteries then perform for years.