David
With the CTEK you can buy a easy plug in adapter (I have one on my S-Type) which will show you the battery charge with a series of traffic lights colours, and allows a quick plug in when you want/need to use the CTEK. I find it adequate to control/identify the state of the battery. I do though religiously put mine on the CTEK at least once per month and for at least twelve hours.
https://www.ebay.co.uk/itm/304658209966?_trkparms=amclksrc%3DITM%26aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D20200818143230%26meid%3Dfea948544cb1430bb818233e7baa26d0%26pid%3D101224%26rk%3D2%26rkt%3D5%26sd%3D275466040484%26itm%3D304658209966%26pmt%3D0%26noa%3D1%26pg%3D2047675%26algv%3DDefaultOrganicWeb%26brand%3DCTEK&_trksid=p2047675.c101224.m-1
Best Wishes and Regards, John