Paul Gtx

I have a 2013 XF . The boot was full of water , I’ve sorted that issue and it’s bone dry but I’ve got battery not charging warning light on. I had both batteries out dried cleaned , seemed to sort problem , drove 80 miles and it came on again , alternator fine and charging , batteries good no issues. Can anyone tell me what the unit is under the aux battery? The unit the aux battery actually sits on , would appreciate any help please