I have a 2020 Fabia Monte Carlo and  have a  problem with having to change batteries on the ignition fob every 2-3 months. Usually get message to say  battery is low but today the car wouldn’t start without any warning. Fortunately I was close to home and my wife was able to bring out the spare which worked fine. I had already tried pressing the end of the key against the ignition start with no success. 
I replaced the faulty?battery which read 3.06 v on my meter with one which read 3.2 v and the car started ok. I checked the battery in the spare which read 2.99 v so why should this work fine but the original battery in the normal fob not. Is there a fault with the normal  ignition key?

Thanks

Maybe measuring the open circuit PD of a cell battery and using that as a Yes/No as to if it will work when loaded by the key electronics, will only work if all cell batteries are of the same quality and so same internal resistance. I plan to only ever fit Panasonic coin cell batteries to car keys, as do VW Group.

 

Edit:- when I've changed batteries, but always just playing safe - no other reason, the open circuit PD across the cells has been same as you have measured, but none of my cars have kessy.

Edited 3 March, 20224 yr by rum4mo