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Alternator working all the time?

glasgow

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Jul 18, 2014
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47
Car
W231 SL500, W211 E280, W124 E300, W171 SLK200
E220 coupe, 2014 (C207), 40K miles, under MB extended warranty. Battery reading was low (11.7V with engine off and down to 10.2V during engine start). MB put a new battery.

Now I noticed on driving, the voltage reading is always between 14.2-14.9V even if the car is driven for 1h with no AC/Heaters. This suggests the alternator is more or less working all the time when the engine is on. When the engine shuts down (for example start stop function is activated) the battery voltage reads 12.4-12.7V.

My understanding with smart alternator, they only work for a short period then stops when not needed (at least that was the case with the old battery which after 10-15 minutes driving the voltage is around 12.4-12.7 with no electric load on).

Could the dealership done any change in the alternator setting to make work more harder? Apart from poor fuel economy, is there any other risk of the alternator working all the time?

Thank you for your help.

A
 
i don't know anything of Smart Alternator, but my understanding is -

The Battery is used to start the car, then the alternator takes over and runs all the electrics on the car and keeps the battery charged up. As far as I'm concerned the alternator should always be working when the car is running. I'm happy to be proved wrong if technology has advanced.
 
That doesn’t look too bad. on the overrun, the ‘intelligent’ charging will allow the alternator to charge up to around 15 volts. Conversely, when accelerating, the alternator output is reduced to around 12.5 - could be more or less depending on the software/make/model.
this would leave the average anywhere between around 14.2-14.5
Also if you use a volmeter dusplay in the dash, these can be horrendously inaccurate and are driven by software, not a recognised way of measuring voltage. My Jeep in particular only shows in .8 volt increments e.g. 12.4,13.2,14,14.8 - it changes to 14.8 at a true 14.3(approx).
If it were really overcharging at max, it would read 17/18/19 volts. You would smell the battery, and see the headlights brighten considerably as you raised the revs.
Personally it looks pretty much ok to me. Unless you have it measured with a calibrated meter then I wouldn’t be concerned.
 
The smartmess is in the ECU rather than the alternator and it's more complex than just high charge voltage on the overrun.

This not an MB specific description of how they work but I suspect all manufacturers use a similar approach. You can see there are several conditions that might cause the charge volt to remain high or in charge mode as it's called here:


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.
 
E220 coupe, 2014 (C207), 40K miles, under MB extended warranty. Battery reading was low (11.7V with engine off and down to 10.2V during engine start). MB put a new battery.

Did MB charged up the new battery before they fitted it to your car?
Did you charge up the battery using a battery charge?

It is possible that the charge in the new battery is well below the threshold (80%?) and the smart alternator is working hard to charge up the new battery properly. It is very unlikely that MB made modifications to your alternator without charging you or at least documenting it on the invoice (if the modifications were done under instruction of a recall or directive from MB).
 

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