e81 bmw 118d battery replacement??

[ss 3]

the car is 2011 e81 118d 2 litre diesel

i have been getting an amber battery warning and a yellow car on the ramp warning  on the check control when i switch on the car and when switching it off.(my car does not have i-drive)

i did charge the battery with a ctek charger but the problem persists.

the battery in the car is 9 yrs old and i think  needs replacing 

so i bought a Varta one of the same spec-(80AH 800CCA AGM battery)

so does anyone know if the c-teck charger can be left connected to the car while doing the battery change to avoid losing time ,date,elecredic window and radio settings?

i am hoping to register the battery using a carly for bmw which has a  battery registration app tool.Has any one successfully used the carly app tool to register the battery?

your comments would be helpful