B220d Eco stop start Battery High Resistance

[PedroEves]

B220d 2016 eco stop/start stopped working in Apr 2019 and new battery was fitted by Mercedes dealer under warranty.

August 2020 eco stop/start stopped working and Mercedes dealer wants to replace the battery again following their diagnostic test. The battery is not covered by warranty but have offered 50/50 as a good will gesture. I asked why it needs a another new battery after 18 months and was told because of High Battery Resistance. The car has always started even after leaving it for two weeks while holiday.

A battery test showed 14volts after starting and drops to 12.8V after 10mins. The loom to the battery showed 10Amps but dropped to 0.08Amps after 5mins. The battery seems fine so has anyone got any ideas why the battery is not releasing the energy from the battery?

 

[Deleted member 126969]

Wrong section really - this is for MB EV & hybrids. You want the Electronics & Audio Section Electronics and Audio to get a bigger audience.

 

[geoffus]

If your battery keeps starting your car then it's OK, the reason the stop start has failed could be due to the car used for small journeys, never gets warm or just needs overnight charge. Not sure if you have the aux battery in the boot but a low charge in this can cause Stop/Start to temp fail.
If your battery is at fault then should be replaced free of charge as 18 months old and under guarantee ?

I always turn my stop/start off anyway.

 

[rf065]

If your battery is at fault then should be replaced free of charge as 18 months old and under guarantee ?

It was replaced under the original 3 year warranty, I don't think you get an extended warranty on the battery.

 

[Dickie Fix]

My understanding is that the large battery under the bonnet does all the heavy lift - starting the engine, lights, heated seats and screens, etc. The small auxilliary(stop/start) battery (in the front passenger footwell, if it's the same as mine) is used to provide a stable voltage to the car's electronics only while the engine is cranking. During cranking, the 2 batteries are temporarily disconnected from each other. When the engine starts, the 2 batteries are reconnected.

Your problem could be compounded if you only do short journeys. I've read somewhere that the charging system stops charging the batteries when it senses that they are at 80%, so the auxilliary battery may not be charging up enough for the stop/start system green light to come on. I had this problem during my first year of ownership when the light wouldn't come on for the first 20 miles of a journey. I was told to buy a Ctek battery charger and use it periodically - easily done from under the bonnet by connecting the charger to the big battery's red terminal and the adjacent earth post. This will charge both batteries at the same time. It is what I do now and I haven't had a problem since.

 

[alexanderfoti]

Yes, it does look like the battery has failed. You need to test the internal resistance of the battery but this usually involves a battery tester rather than a multimeter.

As above, if MB replace something under warranty, the new part does not get a new warranty again, only the original warranty if its still intact.

 

[PedroEves]

Thanks everyone who replied and after many battery checks the simple answer was to charge the battery and the stop start started to work again. Therefore, there was no need for my Mercedes dealer to change the battery, which is not good for the environment and my pocket.

 

[alexanderfoti]

Happy days!

 

[Gazwould]

Thanks everyone who replied and after many battery checks the simple answer was to charge the battery and the stop start started to work again. Therefore, there was no need for my Mercedes dealer to change the battery, which is not good for the environment and my pocket.

And alot of other things...