IOS – fix things when iOS 13.0 disconnect smart battery case on iPhone 6S

It depends on how the batteries are maintained since Lithium-ion polymer batteries need to be used and recharged regularly. There will definitely be some reduction in capacity as time passes, regardless of whether the batteries are regularly used/charged or not.

From Battery University - Is Lithium-ion the Ideal Battery? (emphasis mine):

Aging is a concern with most lithium-ion batteries and many manufacturers remain silent about this issue. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. The battery frequently fails after two or three years. It should be noted that other chemistries also have age-related degenerative effects. This is especially true for nickel-metal-hydride if exposed to high ambient temperatures. At the same time, lithium-ion packs are known to have served for five years in some applications.

Manufacturers are constantly improving lithium-ion. New and enhanced chemical combinations are introduced every six months or so. With such rapid progress, it is difficult to assess how well the revised battery will age.

References:
Apple - Lithium-ion Batteries
Battery University

Problem Solved
The battery was fine and the problem was with the charger. It seems that the charger couldnt transmit enough power to charge the computer while it was in use.
I opened the casing of the charger and observed that 2 capacitors were swelled. I changed them with new ones (I found them in a video receiver). Now its working like the first day I got it.