Ive had my new to me, RRVSE SDV8 nearly a month now, all the time i've had it the start stop has never worked. I charged the battery (posts under bonnet) and it woudn't charge above 12.4v using a AGM 4amp charger when I first got it but was under 12v within days, yesterday I decided I would have to bite the bullet and replace the battery but before I ordered a new one (Exide EK950) Having nothing to lose I removed the battery (JLR Branded Exide) and to my surprise the manufacturing date was 50 over 20 meaning week 50 2020! The car was built Oct 2018 so its on its second and about to be its third battery in a little over 5.5 years! I decided to open it up and found all six cells as dry as a bone, I topped each cell to just above the little step inside about 100ml each, duct taped back what was left of the lid and put on the charger over night and this morning it was at 12.82v. Fitted it back this morning with a BM2 and did a 30 mile test, start stop worked without a problem and battery is holding 12.79v 80% charge 7 hours later! I think when I replace the battery I will buy one that can be opened more easily (Exide one needs a sharpened screwdriver) because it seems these things die because they dry out!

I didn't even realise you could top up an AGM battery