hornethandler

Interesting idea about charging the aux battery via the main battery posts. Did the foreman say that the aux battery is also charged when the main battery is charged from its posts?

I’ll check it out the next time I’m in the trunk and report back.

Re: charging from jump posts. As most here are maintaining/charging our batteries via the jump posts under the hood, I’m guessing the jump posts are connect directly to the main battery.

crobb65

Have you tried contacting CTEK for their advice as to what setting should be used? I'll bet they will know.

jmattioni

I have a set of shop docs now and have been studying the wiring diagrams relative to this topic.

The Aux battery (official name is: "ECO Start/Stop function additional battery") is normally isolated from the main battery and most of the loads in the car by a relay controlled by a computer. It's not clear from the drawings what happens when the car is shut down, but my guess is that the Aux battery remains isolated from the main battery by that relay. The Aux battery does have two modules directly connected to it. Those are the Ignition module (the one behind the START button) and a module on the transmission that controls gear selection. This is to allow the car to recognize the key fob and then allow shifting the transmission out of Park if the main battery is dead. Those modules are always connected so there probably is a very small draw from the Aux battery when the car is shut down.

The apparent main function of the Aux battery is to support the ECO start/stop function, which probably almost no one uses. If you're using that feature and the engine shuts off at a stop light then when re-start is required the computer opens a relay at the main battery to isolate it from all loads except the starter. At the same time, the computer closes the relay to the Aux battery so that it takes over handling all the loads in the car, except the starter, while the engine is re-starting. The stated purpose is to prevent voltage sag for loads in the car during the re-start, which would be noticeable as dimming of lights, etc.

The docs do state that the system will charge the Aux battery whenever the engine is running, but only if the Aux battery voltage is below a threshold value. It would do this by closing the Aux battery relay. Otherwise, the Aux battery remains isolated along with the 2 modules connected to it.

Bay Leaf

The only reason I mentioned that was I read somewhere not to do a battery test using a multimeter via the engine compartment because there will be resistance in the the cables. So would the same resistance be felt when using a battery charger.

hornethandler

Excellent explanation! I confirmed the same last week with a knowledgeable MB Rep. The aux takes over the light loads (not seat heating etc.) when in stop mode as well as other safety loads such as putting the vehicle in park when necessary.

I just confirmed on my vehicle that the aux battery is not charged when a maintenance charger is connected to either the jump posts under the hood or to the main battery posts, so you’re right: the aux is isolated. However, your last para suggests that the aux may charge from the above locations if it goes below its threshold voltage. I could confirm one way or another by draining the aux to say 11 volts or so, but that won’t happen any time soon.

Again, thanks for your post; It’s the best explanation I've read so far.

hornethandler

A voltmeter reading is not affected because all of the voltage is dropped across the meter. However, during charging sufficient current is flowing to cause a very small voltage drop across the length of cable from the battery to the jump post. The voltage drop is negligible for the purpose of charging the battery, i.e., it may end up a few milli volts shy of its max charge, so it’s not really an issue.

All that being said, a purest would measure and charge directly from the battery's posts as I probably would if it was convenient.

jmattioni

Yes, it's possible that the computer would enable charging of the Aux battery while the car is off if a charging voltage (>13V) is present, but I'm not convinced that would happen. I did a measurement at the Aux battery with a charger connected to the jumper terminals and saw no effect. Then again my Aux battery is fully charged and measured >12.5V. The logic for this would be in software, so only MB can say for sure.

I'm not sure about the Aux battery handling only light loads. The wiring shows it connected to the main bus during the re-start. The Aux battery probably has enough juice to handle the load for the 1-2 seconds required to re-start the engine. The overview description (copy attached) states that "Active Comfort functions are not switched off". The system does detect if the Aux battery is weak by monitoring the voltage when it takes over the load. It'll disable the function for a period of time, and activate charging of the Aux battery, if the bus voltage dips too much during the re-start (also in the overview attached - at the very bottom of the doc).

hornethandler

I haven’t completely digested the whole theory of operation you provided—which is awesome by the way can you get more of these for the vehicle?—but I see what you’re saying. Do I understand the document correctly that the main battery supplies vehicle systems power when the vehicle is off and that the aux takes up the slack (voltage drop) only for the time the starter is engaged?

Thanks again for the info. I did the Airforce and NASA thing (32 yrs) and eat this stuff up.

rmorin49

The ability to shift out of Park when the main battery is dead is an important feature. I have already heard about some C8 Corvette owners stranded with a dead battery and unable to get their car up on a flatbed because they couldn't get the transmission out of park.

jmattioni

hornethandler: The main battery is directly tied to all loads EXCEPT for the Ignition module and Transmission module, which are tied directly to the Aux battery and get isolated along with it. What actually happens when the car is off depends on what the computer does with the ECO start/stop relay, but as best I can tell the Aux battery (and its two loads) are isolated. So a healthy Aux battery is pretty important if you want to start your car or get the trans into Neutral.

There are a few overview documents in the shop docs, like the one I posted. I should point out that the simplified diagrams in that ECO overview are not entirely correct relative to the actual wiring diagrams. For one they have the batteries in backwards in those diagrams. Despite that it's still a reasonably accurate overview.

My background is in EE. I'm now retired, but did consulting for most of my career.

rmorin49

Is there any reason not to maintain the auxiliary battery when the car sits unused?

jmattioni

For storage this is basically what the MB shop docs say to do:

1. Ensure all batteries are fully charged (includes main and aux batteries)
2. Disconnect ground cables on both batteries

With the ground cables removed they recommend checking battery charge only every 6 months. So this would get you through a typical Winter in most places.

If you don't disconnect a battery then they recommend checking the battery charge every 6 weeks. The Aux battery does have a small load on it so the 6-week rule would apply to it as well if it isn't disconnected.

There's no guidance in the shop docs that I can find on using battery maintainers, though I don't see why they wouldn't be OK. The catch is that you may need to have two - one for each battery. It must be a battery maintainer (e.g., battery tender). A regular dumb charger left on a battery would eventually overcharge it causing damage to the battery.

If you store your car for the Winter it seems like the safest approach would be to follow the MB guidance and disconnect both batteries. If you rent a storage location they may require you to do this anyway. Only a depleted battery is really susceptible to freezing in most locations. Then again, if you're somewhere temps often get well below 0F you should probably consider removing the batteries. The biggest risk of a battery freezing is that it cracks open and leaks acid, though I don't think AGM batteries would leak as badly as the old wet batteries.

I drive my SL in the Winter as long as it's not snowing and the roads are clear. If you drive it at least once every 6 weeks or so you shouldn't have to do anything special to the batteries if they're healthy.

hornethandler

No, it won’t cause harm. I think the question is: Is it necessary considering AGM low self-discharge characteristics and the fact that no power is consumed from it when the vehicle is off. Just ball parking, but I’ll probably only charge if sitting for more than two/three months.

However, I may be wrong about no power being consumed from the aux. See above post from jmattioni.

hornethandler

Looks like I stand corrected. I assumed no power draw on the Aux battery when the vehicle is off. I checked with a meter (Fluke 289) and and since Aux battery only lost 1 or 2 milli volts in a week I assumed it was due to self-discharge. At that rate it would take almost 10 years to loose 0.5 volt so I wasn’t concerned.

Do you know what load is on the Aux when the vehicle is off?

rmorin49

I have 2 CTEK Model 4.3 maintainers, one for each battery. I put them on if the car will sit for more than a week. I use the under hood post for the main battery and a pigtail on the auxiliary battery that was installed by the previous owner.

jmattioni

No specs on that in the docs, but I would expect the draw to be minimal. Just enough to allow the ignition module to detect the key fob. I doubt you would see it on a voltmeter. You'd need a low range ammeter hooked up in series with the battery.

Kris@ATL

Can anybody tell me how many batteries GLC300, MY2017 has and where are they located?

Thank you.

Dr. Manhattan

You may have better luck here:

https://mbworld.org/forums/glc-class-x253-203/

Kalco

I live in Massachusetts. The car sits from November until April. In that case I would leave a trickle charger on each battery. During the spring and summer, sometimes the vehicle sits a week at a time. Which battery would you suggest I leave the trickle on for the spring and summer months? 2016 SL550.

rorywquin

If you connect your trickle charger to your jump start points it will keep both batteries in good condition. No need to muck about with 2 chargers, extra wires etc. We've been through all these lockdowns and that is how I've maintained my batteries (and also my wife's car also a Mercedes) without issues. MB also point out that the jump start connections are the place to connect the charger. Most important thing is to use AGM mode.

jmattioni

If you're driving the car at least once every couple of weeks then there's no need to use a charger, especially in warmer weather. If that short a period of time is causing problems then it would indicate a defective battery.

hornethandler

Drain on the Aux is so low--I measured it at about 1.5mA a week—that external charging during the summer months, especially if the car is driven every week or two, is not necessary.

The question of whether the Aux battery (Aux) accepts a charge from an external source (trickle/maintenance charger) connected to the jump posts under the hood is much discussed. From the posts at 23, 24, 28, and 30 above, it appears we’re getting close to understanding. What is confirmed is that the Aux battery does not accept a charge from the battery jump posts under the hood for a trickle/maintenance charge; however, there is speculation the Aux may take a charge from the posts if the Aux falls below a set minimum, but that is not yet verified.

However, there is a solution that makes our battery concerns completely go away: Drive it like you stole it and you'll always be fully charged!

rorywquin

@hornethandler

Apologies for the large type - tried a few times to change - no joy.

My understanding from the below is that if the aux battery voltage is too low, a relay closes and puts the aux into the charging circuit. Ergo charging from jumpstarter posts will also charge a low aux. This has been my experience when not using my car for a while and the amber ECO light comes on (i.e. Stop/Start not available because of low AUX battery), I'll put the car on charger and the amber light will eventually be extinguished indicating that the aux has been charged....

Why would MB have a setup with two batteries where a Heath Robinson setup is needed to get the AUX batteries charged while the vehicle is not in use for long periods? Does not make any sense.

Charging from the jump starter posts is proven to work for me.

C Dual Battery Systems
The dual battery system isolates all power supply sensitive electrical components which may be affected by low voltage from the primary battery during the engine starting phase.



Two contact switches are used to change the power supply into two separate circuits when an engine start is required.



Electrical power is then supplied to the sensitive electrical components from the secondary battery when an engine start is in progress.



The primary battery supplies power to the starter motor and maintains essential power to the Engine Management System (EMS) which is essential for engine starting.



Primary and secondary battery voltages are monitored to ensure sufficient voltage is available for the next start event and charge can be supplied to the secondary battery when required.

IC Dual Battery System Components

System components:

1. Starter motor
2. Primary battery
3. Power & EMS loads
4. Field effect transistor
5. Contact switch 1
6. Contact switch 2
7. Secondary battery
8. Sensitive loads
9. Alternator
IC Dual Battery System Operation

System Conditions – Start Required

• Contact 1 open
• Contact 2 closed



The primary battey supplies the starter motor and the secondary battery supports power supply sensitive components

System Conditions – Engine Running

• Contact 1 closed
• Contact 2 open



The primary battery is being charged by the alternator and the secondary battery is isolated from the circuit.



If the system detects a low auxiliary battery voltage contact 2 will remain closed once the engine is running to enable the auxiliary battery to be fully charged by the alternator.



Once the auxiliary battery is fully charged contact 2 will open to prevent damage to the axuiliary battery and reduce alternator and consequently engine loads to save fuel and reduce emissions.

hornethandler

I think you just confirmed that the Aux will charge from the jump posts under the hood if/when the aux falls below a set minimum. In your case “the amber ECO light comes on”. It was speculated that may be the case so thanks for sharing that. I considered manually discharging the Aux to see if it would connect to the jump posts, but it looks like you’ve confirmed it will.

Also, thanks for the diagrams, I’ll add them to the one provide by jmattioni in post 32 above, and provided here again for convenience.