2004 SL55 auxiliary battery

Mercedes » SL-Class » 2003-2012
Problem Description

Which Battery, under hood or in trunk is the auxiliary battery? And.. what alternative to Mercedes battery do I need to get to replace it? Getting battery offline warning first 3 minutes after I start my 2004 SL55 which I just bought less than a month ago so I’m not completely familiar with vehicle or replacing consumable parts.
Thank you,
Debbie