Can anyone tell me what the secondary battery behind the right hand rear trim panel is for?

I have a 2013 4.4 autobiography Without stop-start?

Thanks,

Dave.

It powers the ecu and some other electronics so that during a cold start the voltage to the electronics dosent dip and log false error codes . Some cars have motorcycle sized battery and others have a full size battery .

As in the cars service battery its a "AGM" type its cheap to replace if its the small type and needs no resetting of the battery management system .

Thanks Charlie

Just connected a c-tsk led lead to it for monitoring purposes, it is about 2/3rds the size of the main battery🥴

This is the battery fitted .
https://www.amazon.co.uk/Exide-Ek151-Agm-C...amp;sr=8-1