I have just read the posts about stop start problems. I now have a car that has stop start (it has a Varta, 60 AH, 590) and see that people are having battery problems after a short period. Some posts are saying it is not a good idea to fit a larger capacity battery as it would take longer to recharge, this is only true if you take more out - as with any battery. A larger battery will work under less stress for longer periods and last longer, it is a bit like a heated swimming pool - it takes longer to heat it up but also it holds the heat much longer. As for batteries aging and taking longer to charge, in reality they can't maintain a sustained output for as long as when they were new, they will also take less charge so the charge rate will drop off quickly. Be wary of cheap chargers, check the output of your charger when connected to the battery after it has be on charge for some while, use a volt meter, don't let the volts come up too high and, unless you are using trickle charge, do not leave the battery charging for too long as this will shorten the life of the battery (I once had a cheaper charger that had a voltage of 18 volts, if this was left on for any length of time it wouldn't do the battery any good at all.)
I use a simple way to trickle charge, just putting a 3 watt bulb in the circuit so the battery charges at 3 watts. Doing this overnight is enough to keep the battery topped up, but it shouldn't be necessary if all the systems are working properly. I started using Optima batteries on our farm about 20 years ago, a single Optima battery often replaced two heavy duty ordinary batteries on a tractor or combine. They are a fraction of the weight and don't mind being inactive for long periods, so far I have found that they outlast anything else. The initial cost is higher, but I think they are worth the extra expense. I do not know how well they would perform with stop start but it could be worth looking into this.
I can't understand why the Fiat is having problems. A friend of mine has a three year old car with stop start, it tells him when the battery is low and the stop start is automatically disabled. This has only happened at night since the lights are on. It then automatically reinstates itself once the battery has recharged sufficiently (usually after about half an hours driving). He has never had to boost the battery himself to keep it working properly.