I have two batteries on my l405. The aux is not related to stop/start and I’m quite unsure what it’s for. Apparently 4.4’s built in 2013/2014 had this as standard on UK cars and internationally as well if sold in cold climates (Russia for example), but was done away with in later years or replaced by a much smaller battery if fitted with Start/Stop.

I’ve occasionally had the “low battery start engine” warning and I had a flat battery earlier in the year after cleaning the interior of the car for an hour or so. As such I noticed the aux battery was factory original and over 10 years old so certainly needed replacing. I assumed the main battery would be the same but after buying two new AGM batteries, the main battery is only 4 years old and is likely just fine.

If I just replace the aux battery, do I need to tell the ECU of a battery reset? Can any harm be done doing this either way as I guess the main battery was replaced by a previous owner (as a dealer would have replaced both at the same time surely?) so I assume he wouldn’t have updated the ecu when he did it.

Grateful for any advice.

It's not essential to tell the car it has a new battery, it will soon get to learn it has a new one and adjust accordingly...  Pete

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2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-