C220 alternator

[Finnborg-Braga]

I have a C220d, W205, Nov 2015. The alternator appears to be running all the time - when the engine is running the voltage at the cigarette lighter socket is 14+ volts, dropping to 12+ when the engine is stopped.

I was under the impression that, once over re-charging the battery after a cold start etc, and assuming no major electrical consumers such as headlights are on, the alternator should only cut in when the engine is on overrun, and be off most of the time otherwise. Is this correct?

 

[Pluto]

Nope, when your car is running it is always using electricity and the battery is always being recharged---convince yourself---do a google search.

PS: When you are charging your battery and the battery reaches full charge does the charger automatically turn itself off??

 

[190]

The alternator will always provide an output but the voltage and therefore the rate at which it charges should vary. If you measure the voltage accurately with a digital meter I would be surprised if it was a constant 14 volts under all running conditions - provided you take the measurement with a fully charged battery. If the battery is less than 80% charged the alternator voltage will always be high.

 

[Finnborg-Braga]

On a one-hour 35 mile including some motorway but quite a bit on country lanes, early afternoon last Thursday, the voltage stayed 14.5 to 14.8 volts when the engine was running. The voltage did not react to the engine being in overrun and the 'econometer' showing 'charging'.

I would be surprised if the battery were not fully charged by the end of a trip like that.

 

[jon.english68]

Voltage will remain the same it's the amperage that changes, stop looking for problems that aren't there, you'll worry yourself senseless [emoji6]


Sent from my iPhone using Tapatalk

 

[190]

The charge voltage does change on modern ECU controlled alternators. I've observed mine going up and down as I drive, with high voltage on the over run to save fuel being the most obvious trait.

Other than those observations I don't know all the specifics of MB's alternator control but it looks like it could be a complex algorithm if this generic description of alternator control is anything to go by. Note just having the wipers or headlights on will keep alternator output at high voltage.



The control module enters Charge Mode whenever one of the following conditions is met:


Under WOT conditions and when the fuel rate (sent by the ECM/PCM) is greater than 21 g/S and the throttle position is greater than 90%.
The headlamps are on, low or high beam.
The wipers are on for more than 8 seconds.
The electric cooling fans are on high speed.
The rear defogger is on.
The battery SOC is less than 80%.
When one of these conditions is met, the control module ramps up the voltage slowly to a level between 13.4 to 15.5V (depending upon the mode of operation the system is presently in) at a rate of 8mV to 50mV per second.

The control module enters Fuel Economy Mode when the following conditions are met:

The calculated ambient air temperature is above 32°F.
The calculated battery current is less than 15A and greater than –8A.
The battery SOC is greater than 80%.
The generator field duty cycle is less than 99%.
This mode’s targeted generator output voltage is 13.0V. The control module will exit this mode once the criteria are met for Charge Mode.

The control module will enter Voltage Reduction Mode when the following conditions are met:

The calculated ambient air temperature is above 32°F.
The calculated battery current is less than 2A and greater than –7A.
The generator field duty cycle is less than 99%.
This mode’s targeted generator output voltage is 12.9V. The control module will exit this mode once the criteria are met for Charge Mode.

After the engine has started, the control module sets a targeted generator output voltage of 14.5V for 30 seconds (Start Up Mode).

The control module enters Battery Sulfation Mode when the battery voltage is less than 13.2V for 45 minutes. Once in this mode, the generator battery control module will set a targeted output voltage between 13.9 and 15.5V for five minutes. The control module will then determine which mode to enter depending on voltage requirements.

In RVC Mode, the control module bases the charging voltage on battery SOC, which is estimated during a key-off event every eight hours, after three voltage measurements every 24 hours thereafter, and then monitored constantly while the ignition is on. These voltage measurements are then compared to estimated battery temperature, as battery temperature vs. battery voltage directly corresponds to battery SOC. While the engine is running, the system uses both the battery voltage and estimated battery temperature to determine the battery current in and out of the battery. The control module then regulates the charging voltage to keep the battery above an 80% SOC.