I wondered if the battery was 'smart', as making them so is a trend. Putting intelligence inside a battery allows it to regulate itself more accurately than an external unit. The downside is fitting a new battery may require it to be told stuff so it matches properly with the load and charger.

However, the instructions on this site suggest to me that the necessary control is in the Fiesta. It says the reset procedure is:

To completely reset the start-stop feature on a Ford Fiesta, start by turning the ignition on and waiting 10 seconds. Then, press the foglight switch five times in succession. Next, depress the car’s hazard light three times.

Once you do this, you should see the battery LED light flash on your dashboard. This will indicate that the battery monitor (and thus the engine start-stop feature) has been reset. You can now shut the ignition and hazard lights off and restart your car.

This sort of hoop jumping is typical of a reset control sequence, as used by computer systems to do a full reboot, more than an ordinary on/off does. As such it's likely to do whatever is needed to configure the battery, either an old one that's lost it's marbles, or a new one.

Dave