2016 GLC 300 High Voltage
 

After being warmed up and driven for a while the alternator voltage is 14.9 to 15V. The local Mercedes dealer says this is ok and normal. All cars I have owned before would destroy the battery if charged at anything above 14.5V.

Is this high voltage really normal on the GLC?

That's not high, why are you worried about it?

The reason I am concerned is in my experience charging voltage over 14.7V other than briefl at start is an indication of a bad voltage regulator and can cause damage to the battery, seriously shortening its life. Below is an extract from an alternator trouble shooting guide.

To fully charge in reasonable times, alternator output must be 14.2 V to 14.5 V as measured right across the battery posts. Above 14.5 volts charging voltage, batteries have a greatly increased tendency to release excessive acidic vapors, hydrogen gas, and to corrode things around the battery. The battery terminal charging voltage must be less than 14.7 volts to prevent excessive gassing. Charging voltages over 14.7 volts can prematurely dry the battery by boiling out electrolyte, and increase risk of a battery hydrogen gas explosion.

Are there any Mercedes Mechanics that can chime in?

I'm a tech at MB of Miami. They all charge high at times.

The car has a very comprehensive control of the power flow. There are battery sensors measuring voltage, amperage and draw. Also, what you see displayed may not be what the battery is getting hit with. The charging system is capable of running at almost 16v if necessary, depends on load and drive cycle. The cars use different tricks to recover battery power, and run the battery below full charge all the time to have some reserve capacity. There are times when the alternator doesn't run at all to reduce load/emissions.

If there's no warning messages, don't worry about it.

Also, your extract about gassing is correct, but largely a non-issue as we used sealed batteries in everything.

Thanks ItalianJoe1! That is good info and puts my mind at ease.


This is the most sophisticated car I have owned and based on my experience with other cars and also flying light aircraft, a voltage above 14.5V was a warning sign (in fact, in an aircraft the alternator would be shut off and isolated due to potential danger).


Thanks again - I won't worry about it until I see a warning light.


TJ

Yes, its normal to see the car charge upwards of 14.9 v for a short while. IIRC, specs for operating voltages are 11.5-15.5vdc.

GLC 300 - 2018
 

I have a GLC 300 getting the message Stop Vehicle - Leave Engine Running is that indicative of a voltage regulator needing to be replaced or junction block replacement?

Thanks,

Externally managed alternator ⚡
 

The subject is about "ELEVATED voltage" not "High voltage"... we're only dealing with 12V DC.

14.9VDC is the peak voltage of the quick charge period directly managed by the ECU.
The alternator is producing power based on external control for dynamic needs.
Voltage can go as low as 12.6VDC during float period after the main battery is reported fully charged by sensor.

Current in Amps is an interesting factor to measure how discharged is the Main battery. The car service menu can show you all that while driving....

To try answer your concern:
​​​​​​If you don't locate an existing TSB fix, I suggest you scan the ODB2 port for DTC faults before swapping any electrical parts. Voltage control is a huge spaghetti involving countless networked modules. Faults on German cars are engineered to be caused by multiple conditions.

The alternator brush set may need replacement around 100kMi....
Use only OEM brushes, not knockoff compatibles marketed under sweet names... they are guaranteed to destroy your alternator extra-soft copper rings with hard carbon.

Popular battery killers are "failed charging" control (<11.3V) and "high drain" while parked (Fan, SAMS,...). Using a CTEK float charger is a good insurance for your AGM.
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