I understand what you're saying, but also suggest Fiat wouldn't factory-fit a battery that wouldn't work? I assume all the electronics related to charging (including the voltage sensor connected to the battery that is part of the stop/start system) is all designed to work with the alternator battery and computer in the car.
The Panda alternator has a variable output, depending on other demands from the engine. It delivers a higher current (and voltage?) on the over-run than it does when the car is accelerating, which is why the heater blower appears to blow harder when you're going downhill. I guess this is one reason why the battery is less happy if the car only makes short trips around town. There are known issues with these 'smart' alternators if an additional battery is connected to them...
According to Exide, the battery I provided a link to is an EFB version...
https://www.exidegroup.com/eu/en/battery-finder/technical-specifications
View attachment 415187