I am currently modifying a battery case - something like this: for an iPhone to stick some 18650s in it to greatly increase the capacity. I can either modify a 3rd party case or a battery case made by Apple.
The Apple one is attractive because the battery percent of the case can be seen in the iPhone control center and the phone remains in “mobile” mode rather than “charging” mode when the case is charging it (the phone is able to draw less power during use because it senses that it’s not plugged into a wall but can still charge from the battery). However, when the phone is inserted in the case, the case always charges the phone, and there is no way to turn off the charging except to let the case run out of battery. A 3rd party case, however, can have its charging toggled on and off.
I have a choice to make now. I can use the Apple case and have my phone 100% charged all the time, or I can use a 3rd party case and toggle the charging on and off. I know that the capacity of a battery decreases over time as it is used, but it’s also bad to keep a battery at 100% charge all the time. Does leaving the phone battery charged at 100% (when it’s inside the Apple battery case) do more or less long-term damage to the cell than cycling it normally (charging and discharging in a 3rd party case that can be turned on and off)?