I'm getting the red battery warning and after some investigation I determined that it's the front battery that isn't at the voltage that it should be. It's around 11.8V when the car is on. The battery tested fine and the rear battery is at ~14V when running so I figured it was the terminal 30 fuse since the rear battery appeared to be charging just fine. I checked the terminal 30 fuse and it's showing continuity when I test the fuse. Is it still possible that it's the front battery that's the issue. It is a few years old maybe around 3 years or so. When I test the battery it shows close to just being bad but reliably reads as a good battery. Am I missing something?

 

The SL R230 rear (consumer or accessory) battery needs to be healthy for all systems to work properly. It often suffers from parasitic drains, and needs to be left on a trickle/maintainer charge much of the time. If the battery drains down too much and/or too many times, it can kill it. You can recharge a dead battery. If it won’t hold a healthy charge, often you can recondition it.

I would recharge and/or recondition your rear battery. See my Copilot AI opinions:
What is the healthy 12 volt automobile battery voltage?

A healthy 12‑volt car battery should read about 12.4–12.7 volts when the engine is off, and 13.7–14.7 volts when the engine is running and the alternator is charging.
Below is a clear breakdown so you can quickly judge battery condition.
🔋 Healthy Voltage Ranges for a 12‑Volt Car Battery
1. Engine OFF (Resting Voltage)
A fully charged, healthy 12‑volt battery should measure:

Battery Condition

Voltage (Engine Off)

Fully charged / healthy

12.4–12.7 V

Partially discharged

12.2–12.3 V

Weak battery

12.0–12.1 V

Failing / deeply discharged

Below 12.0 V


These values assume the battery has been at rest for at least 30 minutes with no load.

2. Engine RUNNING (Charging Voltage)
With the alternator working properly, you should see:
Condition

Voltage (Engine Running)

Normal alternator output

13.7–14.7 V

Undercharging

Below 13.5 V

Overcharging (dangerous)

Above 15.0 V

🧰 Quick Diagnostic Tips
12.4–12.7 V = Battery is healthy.

Below 12.4 V = Battery is losing charge; may struggle in cold weather.

Below 12.0 V = Battery is likely failing or deeply discharged.

Engine running but voltage stays ~12.4 V = Alternator or charging circuit issue.

See: Normal Voltage for a Car Battery



When my battery charger failed, I found the least expensive, quality, state of the art one. You can go to walmart.com or most stores and get an EverStart Maxx 40 amp Automatic Battery Charger. Run the reconditioner overnight to restore your batteries. While charging first press the Voltage/alt check button to read the voltage. A good battery should be 12.4-12.7 volts. You can charge it fully, let it sit for an hour, and repeat voltage check. This battery charger has alternator check, trickle charge maintainer, Lithium or Lead, and jump start functions.

 

Thanks @aegea . This helps.

I think I'm getting somewhere. I data logged the voltage from both front and rear batteries and you can see the voltage cycle between 11.9 and 13.7. After a few minutes after startup it finally settled at 13.7V. I also pulled the codes and there was a B1828 code referencing the K57 relay. There's testing procedure for it that I'm going to pass on doing tonight but i will try tomorrow or Sunday. I don't recall seeing that code when I looked at this last week but it seems I'm in the right direction now. If I need to replace this, what kind of a relay do I need to buy?

 

I would verify that each battery holds a charge of 12.4-12.7 volts at least 30 minutes after it has been fully charged, and engine has not been running or battery charged. Erase fault codes, operate vehicle, check for fault codes.

 

First clean and polish battery post and female connectors. Then coat mating surfaces with dielectric grease.

 

Are you referring to front or rear battery charging? I've replaced the rear but no idea where the front is. 2013 SL550.
Saw somewhere where its under passenger floor mat, but the carpet is solid there. Any ideas?

 

That battery voltage data is generic to normal 12 volt automotive batteries. On my 2003 SL500 the starter battery is under the front hood next to the firewall on the passenger side. Its primary function is to start/crank/turn over the engine.

The rear /accessory/consumer battery is used to provide power to almost everything except the starter.

 

The paste option messes up the cart formatting. So, let's see if a pdf attachment looks better.

 

Good info. Thanks.

 

As I read it, the OP said his front (starter) battery was at a low voltage.
The front battery is charged via the DC to DC converter in the BCM. It sounds like either the front battery is too weak to hold a good charge or the BCM is not working correctly.
I would fully charge both batteries by connecting each to a good charger as @aegea mentions above.
Then check the voltages. If under 12.4V after disconnected for some time, its probably time to replace it.

The K57 relay is not used for charging. It is only used in an "emergency" when the rear battery is too weak to power the engine for a cold start. The front battery then temporarily powers the entire car to get it started.
I would not worry about the K57 code until you are sure both batteries are working correctly.
The K57 relay is next to the BCM in the right rear under the trunk carpeting.
As many have mentioned here, it is STRONGLY recommended to fuse the BCM as documented here and on various YouTube videos. This is to prevent a major fire if/when the BCM fails and shorts the batteries to each other and/or ground. This requires access to the BCM. Some videos show removing the entire trunk carpeting (what a pain), or trying to pull up just one side (good luck). My solution was just to cut a "U" flap over the BCM area. This way I can access the BCM, its fuses, connections and the K57 relay within seconds of opening the trunk.

 

Thanks @mrvedit . So you're saying if I try a new battery, the code will go away? I've got a spare k57 relay that I can try. I going to try that first and if that doesn't work, I'm going to see if I can borrow a starter battery to see if I can get that code to go away.

 

wireless,as we said, fully charge your battery, after 1/2 hour check the voltage across the positive and negative battery posts. tell us what the voltage reads. If it is less than 12.4 volts, then you may want to try a different battery, or recondition and check voltage again. Before you throw parts at the problem, if it is easy to evaluate the part, verify that it needs to be replaced.

 

Exactly what @aegea said above; follow his steps precisely. If still a K57 related code, try erasing it first with a scan tool. Only if the batteries are both reading good and the code comes back might the actual relay be the problem.
Unless you have a flap cut into the trunk carpeting over the BCM area, replacing the K57 relay is a frustrating many hour job.
Do you have fuses on your BCM? If not, one of the active members here made this detailed YouTube on how to fuse the BCM:

 

I let it sit for ~30 minutes and the voltage for both batteries is at 12.6v. Because I had a spare relay I changed it. You can get to it pretty quickly front the top. The code still shows up.


The code is still showing up. It shows a test sequence that I don’t quite understand how to implement.


Unless I’m misunderstanding the battery readings, they appear to be fine. I appreciate everyone’s help on this. The car is also my test bed for learning so figuring out why this is happening is fascinating.

 

I also have the kit to add a fuse that I’ll be doing at some point.

 

12.6 volts is great. So,it looks like you don't need to swap out a battery.

 

Does this point to a bcm issue or are there still other things I need to eliminate? Batteries appear to be good, terminal 30 fuse is good. K57 fuse is good. What’s left? Alternator is charging the battery when running so I’m guessing that’s good. Maybe the voltage regulator? But would that cause the B1828 code? Would a bad bcm cause the code?

The fluctuation of voltage at startup had me a bit stumped. But after a few minutes it stabilizes at a normal voltage.

 

Gonna toss this out to see if they are connected. I didn't they were but they electrical related so maybe they are. I also started experiencing that the left door controls (all of them, window, seat memory, seat heating, and the lighting to the switches) would work. They would eventually come on and it would work normally. This never occurred when I didn't have additional load like the headlights. Now I only noticed this when I would drive it at night and now since it's getting a little cooler here in Texas. The overhead lights would also pulsate.

 

If you haven't already, you will want to read and study (!) the MB "R230 Dual Battery Electrical System" document.
It is available here on this forum:
https://mbworld.org/forums/attachmen...370d1591305151

I just read through that document again and still don't understand how your B1828 code could be determined. The K57 relay is never energized except during an "Emergency" when the rear battery is too weak (under 10.8 V). Perhaps the BCM runs just a small current through the relay, not enough to energize it, but enough to determine if it is there.

 

Here's my thinking on this. If the voltage regulator isn't providing enough power, would this put the car into emergency mode (albeit temporally?). As a result causing the K57 to kick in? The other thing that is pointing me towards a voltage regulator issue is that the voltage fluctuates. They are good batteries, one of them I just bought last month because it went bad. I'd had that one for since 2019 (or at least that's what the sticker said)

 

No, that is not how the MB document describes it. When the key is turned, it signals the BCM to wake up. It then reads the voltage of the rear battery and if below 10.8 V, it goes into Emergency mode and closes relay K57 for about 5 minutes. Note that the decision is made before the engine starts and therefore before the alternator is active. Now, might K57 stay on if the alternator is weak? Don't know. But you said you have 14V on the rear battery with the engine running which indicates a good alternator.

 

Mrvedit, do you have a picture of your trunk carpet where you made the cutaway to access the BCM? I'm thinking of getting fuses installed and would like to go this route to provide easy access. Don't want to cut in the wrong place

 

Here you go. Here is a pic of the carpeting "U" flap I cut for easy to the BCM and the fuses I added.

 

And here are pics of installing the fuses.
First pic shows how the the big wires that used to go to the left bolt on the BCM are now bolted together with a fuse line and with a large piece of shrink tubing ready to go on.
The second pics shows the final assembly. The big wires to the right bolt are similarly bolted together with a fuse line. (Forgot why I used cloth tape and not shrink tubing).
Notice that the other end of the corresponding fuse lines now go to the BCM. 30A fuses are installed. The max current that is supposed to flow through the BCM is about 20 Amp (15A ?) when charging the front battery through the DC-to-DC converter. Hence 30A fuses are plenty.
Now if the BCM fails internally causing it to short the batteries to ground or each other, the fuse(s) will blow instead of catching the BCM on fire.
For diagnostic work, such as chasing down battery drains or testing the BCM, it is now trivial to measure how current is going through the BCM. (For reference my BCM draws 0 mA when the ignition is off.)

 

On the subject, here is a very useful diagram from the MB document mentioned above. I know the diagram is listed in other threads, but to correct the Red Battery light, it helps to understand this diagram.