Former

Please note - I am not a mechanic or expert in anything. Great stuff, many of those go at 3.7a IIRC, of course they do need to be used when needed and preferably before the battery is too flat, many forget prevention is better than cure and others think just driving the car will restore the battery every time. 4 years out of the factory battery shows how tight-fisted VW were and that they should have fitted a good quality AGM. The factory fitted battery, when done properly, should be the longest lasting as it's working with everything from new on the car and not used and worn for latter batteries. I thought changing at 6 years on my wife's car was very bad, though I could have made the battery last longer but as it was on my wife's car the hassle to monetary cost was too high to deal with.

Please note - I am not a mechanic or expert in anything. Three year warranty on the Banner. I've had an Enduroline battery in another car and it was fine, for my wife's 2015 Fabia I put a Bosch (labelled, apparently Varta I was told) in and I might have considered the Yuasa. Whichever you buy you might get a longer life from it, depending on your vehicle and its electrics use, by also using an appropriate battery charger minder. This is just one example, other makes and suppliers available, 4 amps so slower than many others which is generally better for a better charge. (£23.49 inc. P&P off eBay currently). - https://www.ringautomotive.com/en/product/RSC804

Please note - I am not a mechanic or expert in anything. To that I'd add carrying unnecessary weight, things like stuff in the car that's carried but not used (junk, tools, spares, undelivered items, perhaps partner or grown kids(?)) and things that reduce the aerodynamics of the car like empty or full top boxes perhaps.

Please note - I am not a mechanic or expert in anything. Treat the gauge as just a 'gauge' and not an 'accurate'. Same for the car's own computer MPG figures, think of it as being more optimistic than pessimistic, or exaggerating a little rather than understate. As has been put a more reliable measure is to fill the tank until the petrol station pump clicks off, do not try to add more, zero your tripometer. Next time you go to the petrol station fill the tank until the petrol station pump clicks off, take the tripomter reading, zero if you want to repeat. If you want MPG then divide the number of litres by 4.54 (or 4.5 if you want) to get to how many gallons and then divide the number of gallons into the tripometer mileage and you have a more accurate MPG which is repeatable for comparisons. Unless someone else uses the same system in the same way take any MPG claims with caution and bear in mind any comparisons are only rough comparisons as there are so many variables.

Please note - I am not a mechanic or expert in anything. Well if it's an electric ultrasonic siren type thing I hope it's more effective than the cheap thing my neighbour fitted to front of his property to keep animals off his front garden. He fitted, but not set it up, just before going out for the night and it went off every time a car drove passed in the road or someone walked by on the public pavement and was very audile and annoying. As he was out I disconnected it from it's power supply and left him a note. Next day I went across to help him set it up properly and turn the audio off, it was too cheap to have any real control range and made for Canada where they must have much bigger gardens and worried about moose, bears and spiders. It was very effective at keeping bears away as we never saw any all the time it was fitted, totally useless at keeping other animals off his front garden though. The clonk of the switching was so loud that you could hear it walking passed in the middle of the road.

Please note - I am not a mechanic or expert in anything. I don't know what Mader protection is but obviously an aftermarket item.

Please note - I am not a mechanic or expert in anything. Remember the computer program is a diagnostic tool so most times you have to use the information given rather than taking it as an absolute answer. Look up the codes and see what is common to them as a starting point from the codes and how they might relate to each other. P0170 VW - https://www.engine-codes.com/p0170_volkswagen.html P0106 - https://www.engine-codes.com/p0106.html

Please note - I am not a mechanic or expert in anything. Does it also sit unused a lot and parked somewhere exposed and always in the same place, position and facing the same way. How bad it is now may also depend on how bad it was last year and/or MoT and how strict or leniently (within rules of course) it was inspected. My car was out in two winter days of very moist/damp/mist and then not moved for a number of weeks and come the spring servicing (farting about getting dirty and messy) doing the 6-month service work and checks I noticed far more rust than 6 months earlier and much more progression than any other winter or year. Perhaps the garage are laying it on a bit thick for some reason but perhaps it might be for a good reason or perhaps they just would prefer not to work on your car. I'd go for a couple more opinions as I've generally only had bad experiences of the motor trade and find many to be almost as bad as each other. Of course there are also bad customers, I know that too.

Please note - I am not a mechanic or expert in anything. Well, not too productive but hopefully some reassurance (I hope). Provided it is just a twig and not some bare cables someone has slipped up the sleeving (I have seen some strange stuff done with wiring and some times by professionals, professional what I am not sure). So as with the first reply you got from ApertureS if you have no issues on the car (warning messages or lights) then the scan tool application you are using is leading you astray about this. Photo taken with 2013Fiji (battery eater) digital camera, 3.2 Mega Pixels !!

Please note - I am not a mechanic or expert in anything. You might be right but to me it looks shiny, so good conditions or shampoo used, also reflective and copper-like looking to me.

Please note - I am not a mechanic or expert in anything. Good points and accepted. ETA: Not aimed at you or anyone other than the car manufacturers. For the vast majority of vehicles' overall use thinking of these sorts of fuel savings is laughable - but something is better than nothing - I compensated by eating less until the coding was done. 😁

Please note - I am not a mechanic or expert in anything. In my single experience you can just disconnect the start/stop at the battery post until you get the 'coding' done, and clear any/all error codes.

Sorry yes I originally put OP is in Switzerland and info was for UK viewers but events overtook me and the post was not completed and lost, will edit in now. I will look for info to help giff add in his location (later). I think there might be error codes, which will not be seen until a scanner is used, but not necessarily dash warnings, as far as the car is concerned it just a battery disconnect which you can do for various reasons and lights and warnings can go out with use of the car as the computers 'learn' things have changed/improved. The VW computer programs have their own brain-farts (not as many as me obviously) but they often sort themselves out or you 'turn 'em off-'n' on agen' in some sort of fashion. Personally I'd like to reboot them as hard and far as possible but I know they'd make me suffer if I ever raised a lip to them. 🤣

Please note - I am not a mechanic or expert in anything. VW's computer programs are complex, over complex, intertwined and over-intrusive in my opinion, so any aftermarket additions may cause issues add-on alarms systems particularly from what I have seen (I do not know, or say that the Mader protection is an alarm). My preference is to not only disconnect but to remove and make good any electrical connections from the removed add-on system as much as possible. I was not worried or over excited or upset by the Mader protection wires just that I would prefer the two cables were sleeved together as much as is appropriate, this would also offer further protection to the red live cable that is very close to the negative post clamp and take it further away from the post clamp. But those cables are insulated, the wires that I circled appear to be bare. If I get the opportunity I will photograph how the connection is in my wife's car for you. Good luck.

Please note - I am not a mechanic or expert in anything. ETA: OP is in Switzerland so following info is for UK viewers The Halfords battery is expensive - IIRC(?) the Halfords battery is Yuasa(?) if so then at time of this post Halfords reduced to £151.99 and Yuasa YBX9096 from Tayna (other suppliers are available) £136.48 including delivery which is normally next day if ordered before 6.30pm. - https://www.tayna.co.uk/car-batteries/yuasa/ybx9096/

Please note - I am not a mechanic or expert in anything. Yes but depending on how long and how much you use the car and its electrics it might have some effect on the overall battery life - we did and the Earth still turned but perhaps our battery's potential lifespan might be shortened however there are other factors like the users use, abuse and care of the battery. When I changed our battery I was told by a BMW technician that they just changed the battery on his brother-in-laws car and never got around to 'coding' it and many months later all was still fine, I can't remember if it was a car with evil BMW computer programs or evil VW programs though. Even if 'like-for-like' as put earlier definitely preferable to 'code' the battery and some say essential.

Please note - I am not a mechanic or expert in anything. Blue is EFB, silver is AGM. AGM is better so that would be my choice but it will need 'coding' in whereas you might not need coding for a 'like for like' (EFB, 70ah) battery.

Please note - I am not a mechanic or expert in anything. https://swissbatt24.ch/starterbatterien/autobatterien/varta-autobatterien/varta-silver-dynamic-agm/varta-silver-dynamic-agm/42/varta-e39-silver-dynamic-agm-570-901-076-autobatterie-70ah Generally (subject to quality of manufacture) an AGM battery will be better than a EFB for start/stop particularly but that will need to be 'coded' in, not only the serial number but EFB changed to Fleece. From Tayna for UK viewers. - https://www.tayna.co.uk/car-batteries/varta/e39/

Please note - I am not a mechanic or expert in anything. That is the battery varoom linked to, could you buy it from elsewhere and get it delivered sooner.

Please note - I am not a mechanic or expert in anything. Sorry my wife's car has just returned her to work so I can not compare with yours and I do not know what a Mader protection is or for but if the problem and error message was before its installation then obviously it was not the original cause whether it contributes to the issue now or not. I am not a big fan of the look of the Mader protection wiring though it may all be perfectly fine in installation and operation. I am curious about what is sticking out from the green/yellow and red/blue cables sleeving as that does not look correct to me and I can not remember ever noticing the sleeving being like that on my wife's, certainly not with stray wires protruding.

Please note - I am not a mechanic or expert in anything. Sounds like someone before might have done something, possibly wrong, if there is insulating tape. There are just two wires as I remember. People make mistakes, as in the photos I posted I think the battery post clamp has been over tightened, it needs to be tight so that it does not move at all but that looks squashed. If I remember correctly the start/stop function will not work if that plastic connector is disconnected, or damaged, perhaps the insulating tape was from a time when the plastic connector was disconnected to disable the start/stop function instead of pressing the button at every start up of the car.

Please note - I am not a mechanic or expert in anything. A photo of your battery (in landscape as that gives more info) might help. Below are photos I lifted from here or the web (I forget) that show the awkward (to me) small plastic connector off the negative post clamp side of the battery, this is for the battery monitoring. You could check the clip is clean inside and fully located, though you should see a dash warning(s) if it is not.

Please note - I am not a mechanic or expert in anything. Many years back I bought a small tin of Copperease that would have lasted me a hundred lifetimes but now it'll be ten-thousand. I was thinking more about this. - https://www.autotorque.news/the-aftermarket/technical-talk/delphi-does-not-recommend-the-use-of-copper-grease/ https://mintex.com/copper-slip-vs-ceratec/?lang=en-gbr

Please note - I am not a mechanic or expert in anything. More effort than I'd give the car, and great longevity, well done. Just for info perhaps, If that's copper grease it's no longer recommended for (modern) brakes.

Please note - I am not a mechanic or expert in anything. Forget WD-40 Multi-Use for anything other than using it up on unimportant jobs. This is what I use and recommend as a penetrating lubricant, it works better and lasts longer (and smells nicer) than WD-40 Multi-Use - GT85 (formerly a British company now part of WD-40 Company). I recently got three cans from Wilco for £10 but obviously also available widely from elsewhere and at different prices (DIY, bikes, other). Shake can before use, hold upright, spray, leave to soak as long as possible overnight is good, penetrating/releasing agents always seem to work well when combined with patience.