I had this this weekend, our Skoda showed a key battery low warning, so I purchased 2 Duracell 3020 batteries from Mr Sainsbury's (after googling the size, as the handbook omits this piece of useful information).
Got home and fitted a replacement, didn't work, took it back out and removed the bitex sticker and put it back, then worked, mini takes it out for the evening. Next morning it didn't work again.
I took out the battery, my multimeter and my 4.7kΩ shunt, just under 3V, try the other one in the pack, 3.2. so 50% failure rate in Duracell batteries with a production date of January this year.
I think the shunt thing I learnt from Kim on here, but it really helps to identify batteries on their way out.