Battery protection warning

[Black c320]

my car is a 06 w211 E280 cdi..problem is when the car is started i get the battery protection warning and the red workshop message, it goes of after a minute or so.
The engine spins over ok and appears to have a newish battery in the boot.
i also have the auxiliary battery under the bonnet, both batteries appear to be fully charged,but i did not disconnect one to check seperately, are the batteries linked? if so i will need to disconnect and check seperate.

The alternator appears to be charging ok, so assuming the batteries and alternator are ok is there any other component that could be failing?

Is there a battery management module of some description that may be causing the problem?

Any help or suggestions appreciated.

 

[rascalmaster]

Driven it on a longish journey recently? That should charge up the batts nicely...
Left it parked for a while? That would bring up said messages!
Ctek chargers (and others) help condition charge and maintain the batts...
if you still have trouble perhaps one of the batts is coming up for changing, though 2006 would suggest there should be some juice in them yet!

 

[Deleted member 37751]

It is quite possible that it's the alternator on its way out.

 

[Black c320]

I did think it could be the alternator or the aux battery failing, so i have just done more checks,Both batteries are reading around 12.7v fully charged,
so i disconnected the aux battery and it still reads 12.7v,

reconnected the aux battery and started the engine..the main battery is now charging and reading 14.3ish, however the auxs battery is still showing 12.7 and not charging.

I assume from this that there is some sort of split charge system and could this be the problem.

The aux battery is obviously charging at some point because it is fully charged.

has anyone any idea how to check the split charge unit? and where is it?