Okay, so I keep banging on about “you should only fit Panasonic batteries to your VAG key fobs”, well my older daughter reported having trouble after replacing the battery on her SEAT Leon Cupra (non KESSY) key fob - now it doesn’t work, so she had to use the other key fon.

I said just to leave it and I’d try to work out what the problem was.

With old battery fitted when you press any of the buttons you get a “good solid” flash from the key’s LED but the battery voltage is down to 2.9V.

With the new again Panasonic CR2025, battery voltage 3.3V, pressing the buttons all you get is a short flicker!

I’d think that this new Panasonic battery is faulty, probably ended up with high internal resistance.

I’m just reporting this here in case someone else ends up in this situation, so before replacing a key fob battery, check what the LED’s response to a key press is, then compare that to what you now have with the new battery. I’m assuming here that some people do actually replace their key battery BEFORE it has failed.

Perhaps the battery she got was well out of date or not stored correctly or counterfeit.

There also seems to be the assumption that it is the battery at fault rather than perhaps the remote key fob faulty - we've had a VWŠkoda remote key fob fail at 7 years old (but not 7 years of use it got alternate use). Did your daughter try the battery from the working remote key fob in the non-working remote to confirm it was the battery?

I've used pound shop type CR2025 batteries and they have been fine, I check voltage before installing them but of course real world testing is required as conformation.

I always suggest synchronising the key fob remote after changing its battery, this is very easy and takes literally a couple of minutes in reality, or some would say seconds.

I also suggest using "spare" key fob remote(s) not as spare but in use on the car in whatever time period rotation suits you this means you know all key fob remotes operate and there is more even wear on the batteries, keyblades and key cylinder(s) plus you are less likely to lose the other key(s) as you know where they are.

With cars that have fuzzy(funky?) logic in the DSG, the plan tends to be each regular driver has their own key.

I’d think after getting caught out like this, from a retired test engineer’s point of view, removing the battery from your only other available key fob when about to use the car or later, would not be the smartest plan.

I’ve seen “experts” taking working kit apart to fixing failed kit - with “job stopped” happening.

I’m just generalising here!

Edited 17 December, 202517 Dec by rum4mo

Fair enough, if your car has DSG (I never think of DSG if I can avoid it). You did put non-KESSY. We only have one regular VW driver in our place, it's hard for me to imagine more than one wanting to anywhere else but fair enough I will try to remember there are those unlike me.

So, you loved your daughter enough to suggest avoiding the KESSY but not enough love to avoid DSG, well that's families for ya.

Post the battery to me and I will test it in the two remaining working remote key fobs we have for full real world function or not

I tested something I tried to fix (temporarily at least if not long term) for a neighbour three times to confirm the fix and it was fine but I told him I'd fully pack my tools away and he should test it to evoke Sod's Law and sure enough it did so new component required. Cheap enough to replace as a component (rather than even former lower cost part) but another waste of resources that make up the component that is only partly recyclable, ah, the "improvement" modern fashions and convenience bring.

The car she chose to buy was a SEAT Leon Cupra, so back in 2019 there was no other option, just DSG or nothing.

Edit:- that batch of 4 Panasonic CR2025 Lithium batteries has "made in Indonesia" on them, I'm not sure where any previous Panasonic key fob batteries that I've bought declare where they were manufactured.

Frankly, from my way of think, I was shocked to find that that youngish modern person, had actually bought double the number of key fob batteries that she should have seeing as they don't "improve" with age! I thought that "just in time" was the way to do things.

Edited 18 December, 202518 Dec by rum4mo