It could be a fault but fiestas have a "smart charge" system. it tries to not charge the battery where it would cause an increase in fuel consumption unless the battery is below threshold values in that case it charges continuously until threshold charge is achieved. What should happen under normal conditions is the battery should recharge when slowing down in gear. Try lifting off early for traffic lights junctions etc instead of using the brake so it has more chance to charge. Driving like this you should also notice improved fuel consumption. Also check for power drains when switched off as this could cause a flat battery.
I have smart charge system fitted to my mk7.5 and on a average I do 20 1.1 mile and 2 3.5mile journeys per week with a longer journey a couple of times a month and a dash cam running in parking mode and never had. A flat battery.