Smart Battery Charging
Thread Starter
MBWorld Fanatic!
Joined: Oct 2010
Posts: 5,141
Likes: 1,293
From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
Smart Battery Charging
So, during a long interstate highway driving today looked at the battery voltage on the Engineering Menu screen. It is in the HU information sub menu 3.16 Battery voltage information.
While driving constant 82 mph speed for tens of miles the "Supply voltage to HU" showed 12.7 - 12.8 volts while the "Battery voltage via CAN" fluctuated between 12.5 - 12.6 volts. And this was all the time until I hit the brakes or my Distronic hit the brakes. This is when the voltages went up close to 15 volts.
I always wondered when people claim that a fully charged battery should show 13.5+ volts but I was never able to measure 13 volts on the car battery. I get the 12.6 - 12.7 volts any time I check in the trunk of my car.
I think MB has built these cars to operate at below 13 volts and they charge them only to 12.8 volts and this explains my experience with the starting problem, When I started having engine start issue I had the battery replaced as it was not reading more than 12.7 volts. This MB battery from the dealer was only a couple years old. With the new battery starting got better but after about a month I started to experience the long cranks again. This was probably because the new battery voltage dropped down to what the car charges it to.
I did not get a new battery again, instead I replaced the starter and it is starting very well now with the below 13 voltage.
It seems the stronger battery charging is made to take place during braking so it is kind of a 'hybrid' and that way saving gas.
I have no idea why MB would do this? Is it to help cars with lots of city driving perhaps?
While driving constant 82 mph speed for tens of miles the "Supply voltage to HU" showed 12.7 - 12.8 volts while the "Battery voltage via CAN" fluctuated between 12.5 - 12.6 volts. And this was all the time until I hit the brakes or my Distronic hit the brakes. This is when the voltages went up close to 15 volts.
I always wondered when people claim that a fully charged battery should show 13.5+ volts but I was never able to measure 13 volts on the car battery. I get the 12.6 - 12.7 volts any time I check in the trunk of my car.
I think MB has built these cars to operate at below 13 volts and they charge them only to 12.8 volts and this explains my experience with the starting problem, When I started having engine start issue I had the battery replaced as it was not reading more than 12.7 volts. This MB battery from the dealer was only a couple years old. With the new battery starting got better but after about a month I started to experience the long cranks again. This was probably because the new battery voltage dropped down to what the car charges it to.
I did not get a new battery again, instead I replaced the starter and it is starting very well now with the below 13 voltage.
It seems the stronger battery charging is made to take place during braking so it is kind of a 'hybrid' and that way saving gas.
I have no idea why MB would do this? Is it to help cars with lots of city driving perhaps?
Super Member
Joined: Feb 2007
Posts: 601
Likes: 285
From: Republic of Texas
W203 W212 W211 R53MCS
Well, lets start by understanding battery voltage. A fully charged battery is 12.7 volts. With these new charging systems, the intelligent battery sensor constantly monitors battery voltage. If you have a fully charged battery, there is no point in giving it a higher voltage because is already charged. As you are decelerating, the electrical system takes advantage of the engine rpm to boost battery voltage because now you are using free energy since you are not pressing the gas pedal. I get customers come in my shop asking if the car really needs an alternator, because some other shop noticed that the voltage was too low and they recommend replacing it.
