Bad battery, alternator or voltage regulator?

[hamodi]

Hi everyone, have a 2007 W221 S320 CDI with 62k miles. Big auxiliary battery in boot went flat for 1-2 weeks as car had engine problem and was left unstarted as being repaired for 2 months. Car running again and starts and drives fine for past few weeks but I am intermittently getting the red battery symbol showing on the dash. So I entered the secret menu so I can monitor the battery voltage as I'm driving to see what's going on:

UB Voltage when getting into the car first thing in the morning is 12.2V. When I put the key on the voltage drops to 11.3V as the fuel pump primes then goes back up to 12.2V. Car starts and alternator kicks in and raises the voltage to 14.4V.

During my 50 mile commute, about 2-3 times the battery symbol will come up and just before it does I notice the 14.4V suddenly drops to 12V. On most occasions it will kick back in and rise to 14.4V after a few seconds but a few times it has taken about 3 minutes and my voltage dropped to 11.7V in the meantime. Even after my long commute, when I turn the car off, the voltage drops to about 12.2-12.3V.

Other things to note: I have taken the auxiliary battery out and it's still the battery from new (manufactured 2006!). Also, I had an oil leak from bank 1 rocker cover which may have gone on the alternator as it's located nearby. Car starts fine, reliable and all electrics in car working fine despite issue of battery warning light.

I would change the battery, as the voltage drop doesn't seem right, but I'm just worried it might still be serviceable and bad voltage regulator or alternator from oil leak is causing me the battery light as its dropping from 14.4V to 12V. Or maybe the alternator is cutting out as the battery has dead cells/short circuit - don't know if it's programmed to do this?

Any advice from anyone would be appreciated. Thank you.

 

[Tony Dyson]

All types Lead Acid batteries will develop sulphate deposits on the lead plates during a discharge cycle which is reversed during a charge cycle, this would explain the fluctuations in your charge voltage readings, the problems that will develop if the battery is left in a discharged state for any length of time is that the sulphate deposits will crystallise and become a permanent fixture thereby insulating the lead plate surface from the electrolyte reducing the efficiency of the battery. A modern smart charger will have a 'Repair or similarly named' setting that applies a higher than usual voltage in attempts to dislodge the crystals and renovate the battery, follow the charger's instructions as the higher voltages from a repair programme should not be connected to the car!

An Auto Electrical workshop would also determine if this battery was recoverable.​

 

[AMGeed]

I would start by getting the main battery drop tested. It may well be showing a full charge (12.6v+) at times but fails when under load. Halfords or any battery retailer can do a drop test and give you a printout of whether the battery is OK or not.
Do you have one battery, or a second smaller battery? That 2006 battery is surely failing now I would have said.

Once that is done, monitor the red battery warning in the cluster and look to getting the alternator tested should it reappear, even once briefly.

When I had charging problems on my E55, it started with the main battery not reaching a satisfactory CCA when drop tested, even after being fully charged. I replaced that battery under warranty but the red warning reappeared and I lost SBC, indicators, heater and all manner of warnings appeared. Asking around and visiting a Mercedes indy confirmed I needed a new alternator.

 

[Flyinspanner]

Lidl are currently selling a battery/alternator checking tool for around £3, would diagnose overvoltage/rectifier issues.

 

[hamodi]

I would start by getting the main battery drop tested. It may well be showing a full charge (12.6v+) at times but fails when under load. Halfords or any battery retailer can do a drop test and give you a printout of whether the battery is OK or not.
Do you have one battery, or a second smaller battery? That 2006 battery is surely failing now I would have said.

Once that is done, monitor the red battery warning in the cluster and look to getting the alternator tested should it reappear, even once briefly.

When I had charging problems on my E55, it started with the main battery not reaching a satisfactory CCA when drop tested, even after being fully charged. I replaced that battery under warranty but the red warning reappeared and I lost SBC, indicators, heater and all manner of warnings appeared. Asking around and visiting a Mercedes indy confirmed I needed a new alternator.

Yes I have a second smaller battery in the engine compartment which I believe is only used for the starter. Explains why there is no voltage drop when cranking the engine. I will look into load testing big auxiliary battery and alternator. Thank you.

 

[nippa]

Not quite on topic but might apply ...

I have a W205 2016 and for the last 6 months the ECO stop/start hasn't been working. I suspected that the main battery had died when exactly 3 years old and I've now been proved to be correct.

This week I put a cheap digital multimeter on to the battery and got 11.7volts when cold ; yesterday my Merc Dealer got 12.3volts having driven 70 miles. To my mind both of these figures indicate a battery change is needed ..The alternator is fine generating about 14.6v.


At the Dealership the MB computer showed no battery faults or any other problem … completely clear of faults.

The engineer seemed happy with the state of the battery given the computer readouts and saw no compelling reason to change it.

However . I bought a new battery at my expense ( £245 agreed price ) and immediately the ECO system worked as expected.

The odd thing is...back in the day you could tell from dimming lights and poor starting when a battery was on the way out.
Not now.
My car has started perfectly every time whatever the weather..... it was just the stop/start system that played up and one other thing.
6 months ago I got an email from my car ( Mercedes Me app ) saying that the battery needing charging.

Progress eh!

 

[sonic]

Not quite on topic but might apply ...

I have a W205 2016 and for the last 6 months the ECO stop/start hasn't been working. I suspected that the main battery had died when exactly 3 years old and I've now been proved to be correct.

This week I put a cheap digital multimeter on to the battery and got 11.7volts when cold ; yesterday my Merc Dealer got 12.3volts having driven 70 miles. To my mind both of these figures indicate a battery change is needed ..The alternator is fine generating about 14.6v.


At the Dealership the MB computer showed no battery faults or any other problem … completely clear of faults.

The engineer seemed happy with the state of the battery given the computer readouts and saw no compelling reason to change it.

However . I bought a new battery at my expense ( £245 agreed price ) and immediately the ECO system worked as expected.

The odd thing is...back in the day you could tell from dimming lights and poor starting when a battery was on the way out.
Not now.
My car has started perfectly every time whatever the weather..... it was just the stop/start system that played up and one other thing.
6 months ago I got an email from my car ( Mercedes Me app ) saying that the battery needing charging.

Progress eh!

Modern cars & particularly MB's are very voltage sensitive. This is appearing more & more on this site as a fault issue.

 

[sonic]

All types Lead Acid batteries will develop sulphate deposits on the lead plates during a discharge cycle which is reversed during a charge cycle, this would explain the fluctuations in your charge voltage readings, the problems that will develop if the battery is left in a discharged state for any length of time is that the sulphate deposits will crystallise and become a permanent fixture thereby insulating the lead plate surface from the electrolyte reducing the efficiency of the battery. A modern smart charger will have a 'Repair or similarly named' setting that applies a higher than usual voltage in attempts to dislodge the crystals and renovate the battery, follow the charger's instructions as the higher voltages from a repair programme should not be connected to the car!

An Auto Electrical workshop would also determine if this battery was recoverable.​

Thats impressive! I remember being taught that about 50 years ago.

 

[Tony Dyson]

Thats impressive! I remember being taught that about 50 years ago.

The basic principles haven't changed at all have they, I'm just a few Years behind you and I surprise myself on occasion when often I can't remember what I had for Dinner the day before!

 

[hamodi]

Hey guys, sorry to bring this thread back. I have finally had the time to replace the big auxiliary battery in the boot and I'm still getting the same issue. I replaced the old Varta AGM unit with a new Bosch AGM that had the same Ah and CCA rating.

It will randomly drop from 14.4v to 12.4v and the red battery light will come on. Sometimes it kicks back on but the other week with the old battery I was stuck in traffic in London and it got so low to almost 9v and it disabled the power steering, ABS, etc. Seems to not come up as often when I'm driving the car only on the motorway. I never have issues starting the car.

Where should I be looking next? Replace the alternator? The car has only done 70k.

 

[Jason63]

It may be possible to just change the regulator and brushes on the alternator. It could also be the wiring to the alternator.

On your car I'd expect that charge voltage to hold really steady at around the 14.4V mark if all was well.

 

[Craiglxviii]

Look at the diode pack... but the best thing is to take it to an experienced Indy and have them do a proper fault finding job...

 

[alexanderfoti]

Yes change the regulator (which will come with brushes) the oil will have turned the brushes into conductive mush.

 

[Misterdog]

Bought a 2010 A160 W169 this summer, replaced the main starter battery for a Varta AGM as per spec. but this only ever shows 12.2 V with my DMM.
Auxiliary battery reads 12.8 V.

I have read that these smart alternator's keep a reduced charge on the main starter battery to allow for brake regen charge, would anyone expect the battery voltage to only be 12.2 V or could I have an issue somewhere.