Quote Originally Posted by Lucas_R View Post
I have this Supercharge MF66HSS AGM battery fitted to my Mk7 GTI (fitted by previous owner so im not sure how old it is). Works just fine and fits in the battery tray. My only gripe is that the top of the battery is bright yellow so its quite ugly, so I have purchased a plastic VW battery cover to hide it. I have disabled the start/stop function (I dont like it plus they are battery killers) so im hoping the battery will last until after I sell the car in a few years time. I do hook it up to my CTEK charger once a week or so and give it a good charge up too, which should help maximise its life.

Yes stop/start is a battery killer. Every 3-4 years and you will be buying a new expensive battery and negating any tiny fuel savings you might have made using the stop/start system each day. The battery will last much longer if you disable the start/stop function.
My understanding is that the stop/start system itself isn't the main reason for batteries failing sooner than they used to. The problem is that VW fit a complicated Battery Management System (BMS) that is designed to (most of the time) leave extra capacity for recharging when the car is decelerating. This is supposed to save a tiny amount of fuel and reduce the official consumption and pollution figures.

If you check your battery voltage when driving you will probably notice that the voltage is often around 13.3V which is much lower than the 14.7V that is needed to keep a lead acid battery fully charged.

The end result is that the battery isn't fully charged most of the time, which reduces their life span considerably (lead acid batteries last much longer if maintained fully charged). Of course if many owners need new batteries and fitting more frequently than on previous cars then VW and their dealers are quite happy to help out, and pocketing the extra profits is purely incidental (me cynical, I don't know what you mean ? )

So Lucas is on the right track by ensuring his battery is fully charged every week or so, this will help it last longer ( I do the same with my R, it's EFB battery is 3 1/2 years old and still going strong). AGM batteries tend to last longer than EFB anyway, but keeping it fully charged will help as wll.