Just had a "change key battery" message:  occurs with both keys. Batteries checked and both 2.9 v. 

Any suggestions?     

Checking batteries with a volt meter without some sort of load on the battery is unreliable. New button cells can read 3.2V.

If car reporting batteries low, I would just change them with new branded cells.

 

I had exactly the same the other day. 6 months after changing I get the dash warning. It’s just as easy to have a load of spares and just change them. They are very cheap after all.

Thanks for the suggestions. I swapped the current ones for two which had given the same problem before but still showed full normal voltage, and the problem has gone away for the moment, but I'll keep some new ones in the car anyway.   One other oddity - I had a "main battery voltage low" warning a few days ago, probably due to several weeks of little running but several engine starts,  and the cruise control has stopped working. I have seen some suggestions that low main battery is associated with cruise control failure. Battery put on a charger but cruise control is still not working.   Is there a way to re-set it?          

9 hours ago, slast said:

Thanks for the suggestions. I swapped the current ones for two which had given the same problem before but still showed full normal voltage, and the problem has gone away for the moment, but I'll keep some new ones in the car anyway.   One other oddity - I had a "main battery voltage low" warning a few days ago, probably due to several weeks of little running but several engine starts,  and the cruise control has stopped working. I have seen some suggestions that low main battery is associated with cruise control failure. Battery put on a charger but cruise control is still not working.   Is there a way to re-set it?          

 

Can't comment on your cruise control question, although I'm sure someone will come along soon to answer that one.  When I first got my MK3 Octavia I had the problem with the battery in the key fob and changed it for a new one and that didn't last too long before the message flagged up again.  The standard battery is a 2025.  I managed to fit a 2032 which is the same width at 20mm but a bit thicker at 3.2mm from 2.5mm.  So it's obviously got a slightly higher capacity.  It is a tighter fit but does go in and I've not had any problems since.