Unfortunately, not all electric mains provide constant 230V voltage. Even minor variations from the mains can cause the battery to be undercharged or to gas (boil). With undercharging, the negative plate is not fully converted and thus some of the material will be inactive. With overcharging, the battery boils, causing it it to dry out. In addition, the heat generation destroys the lead plates.
CTEK's primary switch mode battery charger is not affected by variations in the mains voltage, enabling it to charge the battery effectively.