These phone cases with an integrated battery pack goes a long way towards replaceable batteries if you keep a stack of them. They probable put less strain and wear on the built-in battery too.
> They probable put less strain and wear on the built-in battery too.
Nope, unless you can fore your phone to only charge to below 80% they strain the battery just as much if not more by keeping them at 100% all the time.
This is false. It's true that staying at 100% is harder on lithium ion batteries than staying at 40%. However, the wear due to charge cycles is way, way more significant. Your battery will see much less wear if you keep it at 100% for some time vs discharging and recharging it a few times between (e.g.) 40% and 80% during that same time.
Nope, unless you can fore your phone to only charge to below 80% they strain the battery just as much if not more by keeping them at 100% all the time.