AUX battery - testing replacing

[GeorgeR]

I have spent some time testing and finally replacing the AUX battery. Let me share the knowledge here, it may help others.

Nearly all MB cars manufactured in the last 10-15 have AUX battery. (So the hybrid models have 3 batteries in total, while the regular ones have two.) It is a small, 10-15 Ah AGM type of battery. Due to its small size, it can be located under the trunk carpet, inside or under the dashboard or under the rear seats. (Places vary a lot. For example, the W212 has all 3 types of locations.)

It starts to wear out after 4-5-7 years of use. The typical error you tend to see is the "AUX battery malfunction" warning. It may cause non-working comfort functions, such as start - stop, hill hold brake, comfort lights etc. (depending on the model). If you take a long drive and do a car stop - start cycle and the above errors go away you can be quite sure that it is the AUX battery age.

You can diagnose this battery and replace it quite simply to save 150-250 GBP off the bill of an MB garage.

Diagnosing it:
You need to find your AUX battery and connect a simple voltmeter to it. The voltages you need to see: healthy (unhealthy) values.
Long idle 12.5-13.2 (below 12.5) after 10 mins of full inactivity of the car, so that the AUX battery is not connected to the core electric system of the car.
Driving above 10 miles per hour, the engine is on: 14.5-14.9 V (13.8V - 14.8V) - trickle charge peaks
Discharging during the drive: 13.5 - 13.1 V (less than 13.1V), normal drive after 20-100 secs of drive, stopping, idling, low speed
the above two phases change quite frequently during the drive of the car.
Engine start, self-test cycle step 1: 2-4 seconds 13.5-14.5 (13.5-14.5) - the car gives a short charge to the battery
Engine start, self-test cycle step 2: 5 secs after the start at low speed: 13.1-13.6 (below 13.1) - the car checks the battery voltage after the initial charge pulse. If it is less than 13.1 V -> it sends the error message to the system -> AUX battery malfunction and cancels some comfort functions.

So it has a small logic board that handles the isolation from the main circuit as well as the trickle charging of the battery. It is - usually - a small flat box sitting next to the battery with the size of 5 x 3 x 1 cm and 4 main cabling connections.

Replacing:
Replacement battery: you can go for
MB genuine for 150+ (the most typical model for most cars is the: A 000982 96 08),
Varta for 100+ (OEM provider for MB for other batteries, #AUX14 513 106 020) or any
other 3rd party AGM battery with matching size and capacity (under 40-85 GBP). If you take the 3rd option you will need to source a
AGM battery with
Capacity: 12Ah (at least),
Max amps: 200A (or above)
Length: 150 mm (max 166mm)
Width: max 89 mm
Height: 145-147 mm
Ventilation hole 6mm on the short side (at the neg pole)
If you go for a German make you may get the 6 mm ventilation hole on the right side of the battery (most possibly), so it will be plug and play. If you pick a Japanese or a US make (such as Exide EK131) you may get a 5 mm hole on the long side. So that you will need to source a matching vent tube and elbow to connect it properly.

Replacing steps are quite simple (once you managed to find the battery for diagnosis):
Stop the car and remove the key for a good 10 mins (do not even open or shot doors)
Remove the battery (simple 10 mm wrench, 3 screws)
Check the polarity and the voltage of the new one (12.5 - 12.7V idle voltage is fine)
Connect the battery firmly (using the 10 mm screws), lock it into position (with the flip and 3rd screw on the top)
Connect your vent pipe. The factory tube will have a 6 mm ending (small red elbow on a transparent tube)

Check the voltages with a 1-2 mile test drive (see the values above)
Reinstall the lining.

 

[190]

Driving above 10 miles per hour, the engine is on: 14.5-14.9 V (13.8V - 14.8V) - trickle charge peaks
Discharging during the drive: 13.5 - 13.1 V (less than 13.1V), normal drive after 20-100 secs of drive, stopping, idling, low speed
the above two phases change quite frequently during the drive of the car.

This is the important bit. I didn't see any other reference so far to the fact that modern cars have smart alternators meaning the charging voltage is controlled by the ecu and will vary in order to minimize fuel consumption. It varies according to a quite complex algorithm but in simplistic terms you should see a higher voltage when the engine is on the overrun and a lower voltage when the engine is under load. All previous comments about voltage readings being too low need to be qualified by this. You can no longer say that the charging voltage should be a minimum of say 14.5 volts, only that it should occasionally peak at a minimum 14.5 volts. The way to get your head around this is to drive the car with a voltmeter hooked up and watch the variations. I made up an adapter to connect my DVM to the cigar lighter.

 

[GeorgeR]

This is the important bit. I didn't see any other reference so far to the fact that modern cars have smart alternators meaning the charging voltage is controlled by the ecu and will vary in order to minimize fuel consumption. It varies according to a quite complex algorithm but in simplistic terms you should see a higher voltage when the engine is on the overrun and a lower voltage when the engine is under load. All previous comments about voltage readings being too low need to be qualified by this. You can no longer say that the charging voltage should be a minimum of say 14.5 volts, only that it should occasionally peak at a minimum 14.5 volts. The way to get your head around this is to drive the car with a voltmeter hooked up and watch the variations. I made up an adapter to connect my DVM to the cigar lighter.

100% agree. The AUX battery has its own little controller next to the battery. The main battery also has a regulator which is a digital relais controlled by the computer system(s) of the car. The logic can include hundreds of inputs to decide to charge or discharge the specific battery at the very moment.

Therefore since model yr 2005-08 (or so) it is not so easy to sniffle out the charging level and the performance of your battery. (This is why I put the reading of the actual testing cycles there above.) Due to the smart charging, you may not be able to measure the voltage of your battery simply through the cigar lighter. You'd better connect to the battery directly.

And also the idle voltage means nothing. (My old AUX battery had higher idle voltage than the new one.) In case of this battery role (AUX), the testing is how much voltage it holds directly after a quick charge peak. If it is above 13.1 then it is good. (There could be an entirely different logic for the main starter battery, though.)