S211 Auxilliary Battery Functions

[e220estate]

Hi All, I have to leave my S211 unused for several weeks and don't want to flatten the batteries. I intend to disconnect the main battery (below the load area floor) first, lock the car on the Aux (will this work?) with the bonnet open, disconnect the Aux and shut the bonnet. So everything will be locked - but no intruder alarm - I accept that. On return I intend to use the key to open the driver door, reconnect the Aux and use this to open the tailgate lock to get access to the main battery for reconnection - the tailgate does not have a key lock, only electric as far as I can see. Will this work? I guess the answer is 'yes' if the Aux is fully in parallel in the system with the main battery and can take the lock load? TIA

 

[grober]

Welcome to the W211 E- Class!

 

[e220estate]

Thanks for the pointer - found this:
"Tasks and function of the two batteries:
• The battery in the trunk supplies the on-board electrical system in normal operation.
• The additional battery is a short-time backup for the power supply in the event of the on-board system battery failing."

So this says to me that the Aux does all the functions of the main battery - just limited by its capacity....so my scheme should work ok.

 

[whitenemesis]

No, the AUX battery only comes in circuit if the main battery shows low charge, not no charge (disconnected). The main battery must be in circuit for the electronics to be able to call the AUX into use

 

[e220estate]

Drat! So it looks like I have to lock electrically, with the tailgate and bonnet open. Disconnect both batteries and slam shut the bonnet and tailgate. Then to 'open up' I use the key on the drivers door and crawl thru to the load area and use the manual opener to raise the tailgate to connect the main battery and then open the bonnet and connect the Aux? Again, TIA!

 

[whitenemesis]

Why not invest in a Ctek trickle/maintenance charger? Connect to the main battery in the boot, trail the mains connection cable out of the boot, close the tailgate onto the cable (I do this, no issues) and connect to the mains supply?

 

[e220estate]

The only down-side to that is that the car has to sit outside my garage, so the connecting cable will get wet/blown about but, as you say, it is an option.

 

[whitenemesis]

My car sits outside, I use a waterproof trailing socket

 

[grober]

The Ctek range were originally developed in Scandanavia amongst other things for boat owners to keep their boat batteries charged up. The MXS 5.0 is IP65 rated meaning it can with stand low pressure water jets from any direction.

 

[mersum1es]

Mercedes is selling CTEK under is own brand... A0009822921 was one part number I found quickly

 

[e220estate]

No, the AUX battery only comes in circuit if the main battery shows low charge, not no charge (disconnected). The main battery must be in circuit for the electronics to be able to call the AUX into use

Still trying to understand the electronics......! Given what has been said, am I right in thinking that if the main battery goes flat then the Aux is totally out of circuit ie it is not used for any residual drain/circuits meaning that its charge will only decay based on the state of the Aux battery? TIA, as always!

 

[grober]

Try to think of it this way. The car is basically a computer= a series of control modules which communicate with each other [ including the battery module N82] via the canbus systems Needless to say if all power was lost to these communication networks and the cars basic intelligence , there would be no way to switch anything off/ on in the event of a main battery failure. That's the function of the battery control module to protect the auxilliary battery from normal regular power demands made by the car which require high current. The battery control module is connected to the Canbus B which will tell it about the state of the cars electronics and its main battery and act to protect the auxilliary battery in the event of main battery failure thus maintaining a " way back" when main battery power is restored.
Its a bit like the bios backup coin battery on your computer mainboard

 

[e220estate]

Thanks for that - I think I understand that the Aux does, therefore, have a power drain in the event of main battery failure (or disconnection). In the event of a drained Aux and when the main battery is restored, presumably the system reboots from the Main and 're-learns' the correct ECU control settings?

 

[DSB SL AMG]

I disconnected the mains battery on my old W211 during last winter storage period, when reconnected my Aux battery was totally knackered and had to be replaced...as already stated if possible Ctek trickle charge is best...

I think MB state car can be left for 3 week period max...how long you away?

 

[whitenemesis]

This MB training doc may be of interest

https://db.tt/FIHrX837

 

[b1g1an]

Everyone is right about how it works but it is there for no other reason than so you don't lose the fly by wire brakes if there's a power failure, hence why it was dumped when they ditched SBC.

You only have to disconnect the main battery to completely kill the car.

On the SL and some others it has a completely different function though.

 

[whitenemesis]

Correct, on the SL the battery in the engine compartment is only connected to the stater motor as it's sole function is to start the car. So there is no ' backup" for the SBC function.

In the 211/219 the AUX battery provides this 'backup' in an indirect way by supporting the main system battery but should that battery go down completely, you will loose SBC function.

 

[e220estate]

Hmmm...thought so and thanks for your experience. I got the same info from Mercedes tech and just as you laid yours up for more that 3 weeks, I will too. Ref a trickle charger, I am nervous about the car being outside and what the elements may do - if the trip on the main consumer unit in the house flips I will have REAL trouble! Looks like disconnect both batteries is my preferred, least hassle solution (apart from having to crawl thru the car to manually open the tailgate to get at the battery!).

 

[e220estate]

Really, really helpful - many thanks!

 

[e220estate]

Everyone is right about how it works but it is there for no other reason than so you don't lose the fly by wire brakes if there's a power failure, hence why it was dumped when they ditched SBC.

You only have to disconnect the main battery to completely kill the car.

On the SL and some others it has a completely different function though.

....but the Aux battery continues to be drained so will go flat in about 3 weeks, earlier if in poor condition (ref previous posts).