How to determine age of Varta battery?
 

Just picked up an '08 A8 4.2 that has the Varta battery. No records were available for any replaced battery. I have a long trip coming up and want to be sure I shouldn't replace the battery now. Put her on a 2 amp trickle charge for 6 hrs..battery seems to be holding at 13.02V
The Varta has engraved numbers of 00 978 401 and a paper sticker that has numbers under the bar code ( 4E 09 151 05A ) but Varta doesn't seem to code the manufacture date as domestic batteries do.. so, does any one know how to decode that number for getting the age of the Varta battery? Vatrta's web site is absent this data.
Thanks!!!