I own a GLE450 about 9 months old 5,500km (~3,000mls) but only acquired it 3 weeks ago. Due to the present zombie apocalypse it doesn't get driven much so I use a M-B badged CTEK 5.0A charger which is rated as suitable for slow charging both lead acid and lithium ion batteries. I understand the 12V and 48V batteries cross charge via a DC/DC converter. I left the charger attached to the engine bay points for 4 days and the full charge/float light was lit. I took it for a drive and the MBUX Info "engine" display showed 12.2V. After starting the engine, it showed 14.8V for about 20min driving then it changed to 12.8V. It stayed at 12.6 - 12.7V for rest (45min) of highway drive. On returning, I reattached the charger and it only showed the first light (~50%) but changed to 80% after a couple of hours. Four hours later it still shows 80%.
Does this make any sense? Does anyone have any information about the battery management in the GLE 450? My thoughts are that the 12V battery cross charged the 48V lithium battery in the background and then the voltage dropped to the lead battery operating level. If so, why the apparently low level on reconnection after a reasonably long drive?

 

Having had a fault with the EZS not going to sleep on my car and draining the 12V battery recently, I have some experience of what goes on. The voltage as measured across the terminals in the engine bay appear to be a direct measurement of the 12V battery voltage. I also have a CTEK MXS 5.0 battery tender, and had it set to Car / AGM mode. I followed the battery voltage from < 11v through to 11.5v, and then finally up to 13.6 volts in condition 7 on the CTEK charger. As soon as the charger is disconnected, the voltage drops to around 12.7v, and then steadily overnight to about 12.6v. I think this corresponds to about 85% charged in an AGM battery.



My understanding is that the ISG is the only thing that actually charges the 48V lithium ion battery. Certainly I saw (on a diagnostic screen brought up by the MB tech who came out to my car when it was dead) that the ISG was kicking out well over 100 amps when the engine was running, and that current level dropped fairly rapidly down into the 90's and kept falling. Apparently that's how it's designed so that it doesn't damage the battery. It appears to me that the 12V battery powers up most of the cars electronics, but while the 48V battery is the thing that starts the engine, runs the A/C, water pump, oil pump and other stuff, without the 12V battery, the electronics can't fire up the 48V battery to start the engine. So even if the 48V battery is 'full', if the 12V battery is dead, you're out of luck with respect to starting the car.

I also believe that the DC/DC converter works the other way around than you'd think - it only down-converts the 48V output from the ISG to 12V to charge up the 12V battery - and not the other way around; i'd be happy to be corrected on this point if someone knows better

Anyway... what you describe is exactly how my car behaved recently. The on screen display in MBUX on my car shows 14.1V while the battery is still being charged, and between 14.4 and 14.7 on a long drive, but measured across the terminals in the engine bay, about 12.7V appears to be 'normal' when the engine isn't running.

Hope this helps.

 

Thanks Wiggis, very useful. Your point that the 48V lithium charges the AGM battery is supported by the 14.8V level. This is the float voltage that is usual in battery chargers for lead acid batteries. Does that mean that the 14.8V I saw occurred during block charging of the AGM battery, and when it was fully charged, the display voltage displayed reverted to displaying its actual voltage i.e. a float voltage about 12.6V? As an aside, this means what you indicated, the only way that the lithium battery can be charged is directly by the ISG. This leaves me with the question...why do M-B say that only their lead acid/lithium compatible charger be used, if it doesn't charge the lithium battery? Why not just use the CTEK MXS 5.0 charger set to AGM, at half the cost? M-B state that the lithium battery wont play nicely with the float trickle charge from standard chargers. Why should that be an issue when the lithium battery obviously is not connected to the posts. None of this affects the function but I like to understand things.

 

There are a new generation of 12V LiPo batteries that are slowly replacing the 12V AGM batteries in some (not many) newer cars. They have a different voltage vs capacity graph, and so the charger needs to be adapted for 12V LiPo batteries vs 12V AGM batteries (otherwise the charger will not correctly determine the state of charge, or the voltage / current to apply at any given moment). I believe that a "Lithium compatible" battery charger has absolutely nothing to do with the 48V battery in our cars, and is intended for cars with a 12V lithium polymer battery - which ours do not have.

 

Those SOC values for the AGM battery apply to a battery that is disconnected from any load and also has rested for several hours.

 

Are these battery maintainers/chargers just hooked up to the jumper connections under the hood or directly to the battery under driver's seat?

 

Under the hood.

 

Yes, that's the "resting potential."
The SOC voltage chart for LA or VRLA is pretty similar, but the point is that it's not a good idea to leave your battery less than fully charged.