E250 charging voltage varies?

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ChrisHGTV

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Hi all

I wonder if anyone can cast any light on a couple of issues, or maybe just one issue!

I swapped out one of the power sockets in my 2011 s212 E250 for a triple usb with a voltage indicator. I added these to two other cars and they work really well, with 2 USB C and a usb.

In the Merc the resting battery charge is 12.1v and when idling it goes to 14.7. On my iCarsoft it comes up with no alternator faults.

I noticed today when driving that the indicator on the charging point drops to 12.1 when under load and then goes back up to 14 or so when I’m coasting. And it varies depending how much throttle I have on.

So firstly is this normal? I would think it should be charging at 14.7 regardless?

The other issue which I thought could be related is that I get a fault code for the aircon saying it’s not getting enough power and it blows cold intermittently. A10D21 - The power supply is too low. The signal amplitude is less then the minimum amplitude.

Any ideas? Could they be related?

Many thanks!
 
The voltages are correct for ‘intelligent’ charging (with stop start).
It’s also a good sign that your battery is healthy and the system is working correctly.
Once a number of criteria are met - battery in good condition, decent state of charge, engine temp ok, ambient temp ok etc etc, the ‘intelligent’ charging will kick in.
You should see the voltage drop as low as 12v when accelerating (creates less electrical load on the engine), and up to 15 volts on the overrun for a period of time.
Can’t remember the exact figures and conditions for Mercedes, but my Jeep was spot on for when it would cut in and out, the Mercedes (slc250 2017) was a lot more variable - it seemed to take a lot longer to activate and wasn’t so accurate when it came to increase or decrease the voltage - almost as if the software was buggy.
 
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Just for info a resting voltage of just 12.1 Volts is pretty flat battery.....should be around 12.6V. So if its showing that an hour or so after switching of the engine its probably not got long to go.

1718033214074.png
 
Just for info a resting voltage of just 12.1 Volts is pretty flat battery.....should be around 12.6V. So if its showing that an hour or so after switching of the engine its probably not got long to go.

View attachment 157866
Hmm interesting thanks. We’ve just got back from a long drive (oop North in North Yorkshire to Surrey) and on my multimeter the battery is reading 12.47 and 12.2 from my usb charger. I’ll re-measure later to see if it’s dropped.

Actually I should check the aux battery as well, I’m not sure if the aircon uses the aux battery or the main one?
 
I've tested a smart alternator and the voltage didn't fall as low as 12.1 volts under acceleration but it did get into the twelves.

The smart charging algorithm is far from being as simple as don't charge when accelerating and charge when coasting. This isn't MB specific but it gives an idea of the complexity and things being monitored.


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).

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.
 
I've tested a smart alternator and the voltage didn't fall as low as 12.1 volts under acceleration but it did get into the twelves.

The smart charging algorithm is far from being as simple as don't charge when accelerating and charge when coasting. This isn't MB specific but it gives an idea of the complexity and things being monitored.


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).

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.
Blimey!
 

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