I recently had my key-fob die on my 'new' car only two moths after buying it, no warning just stopped, I tried two new batteries (good make) but nothing.
Booked into the dealers and had to wait until the beginning of January, lo and behold (as I expected) they put two new batteries in (they changed spare as well) and it started working again, they reckon that it's not the voltage of the battery but the mAh that makes the difference.
Charged me £10 for the privilege! I got a bit angry (not usual for me) saying that changing the batteries should be part of the handover when buying an 'Approved VW' and it was a bit of a cheek on a two month old car, costs them very little but should be part of the customer service.
More importantly when I mentioned that I didn't get a low battery warning on the dash they didn't say a thing, to me that was the bigger issue as it's likely to happen again (they said I should have seen one).
I've bought a few Varta cells which seem to have a high mAh ready just in case.