if someone has any expiriance: if battery getting weak (discharging) faster, could it be problem of whole battery or any of cell?
It's the whole battery, not a cell as long as you don't have warning lights. It's normal NiMH battery aging that can be fixed via reconditioning learn more here: FAQ (Frequently Asked Questions) | Hybrid Automotive and here: BU-807: How to Restore Nickel-based Batteries – Battery University
