Speed750

There seems to be a lot of talk about volts but nobody is mentioning AMPS. I've seen many batteries showing 12V but under load they have nothing. I don't know who makes a battery that shows 10.8V but still has 600CCA.

bobterry99

I think you have a very good point to take up with the factory back in Stuttgart. But with respect to the R230 systems battery this point is moot, since that battery never turns the starter motor and theoretically must source less than 130 amps to start the engine.

Speed750

I'm incredulous that Mercedes suggest 7.5 volts is adequate.[/QUOTE]

Yet we look at all their manuals as the Gospel !

whoover

The system is designed to operate even if both batteries have bad cells. The only way a 12V battery can have a voltage of less than 11.8 is if a cell is bad. That drops the voltage by approximately 2 volts. The 10.8 number is a battery with one bad cell but the other 5 in good shape. Such a battery might very well be able to start the car (although it will turn over slower than normal) and be able to run the electronic consumers (which are designed to run at that voltage). HOWEVER, if either battery is showing <11.8 volts (because of a bad cell), it will damage the alternator and should be replaced quickly.

bobterry99

Forum members, the following statement is complete nonsense. You are advised to disregard it completely.

whoover

I'm not sure if the nonsense is that a battery reading 10 volts open-circuit has a dead cell, or a battery with a dead cell will kill an alternator. Again, its your car.

A great resource for information on battery testing is Battery University. Here's yet another voltage-charge table showing a fully discharged battery is around 11.9 volts (it will vary slightly with battery type and temperature):
https://batteryuniversity.com/learn/...te_of_charge/2

It's a great resource if you want to understand the technology of batteries and the proper testing thereof.

bobterry99

My SL55 has a 5-month-old systems battery. The car has not been driven in the past few days, and at this moment the voltage on the battery is 11.5 volts. That is normal and to be expected when the car has been idle for a while, but according to you this battery has a bad cell, will damage my alternator, and should now be replaced? That is patently absurd.

I've attached a screenshot from Star Diagnosis. It involves checking for circuit 30 shorting to ground. Note that the test prerequisite is that the systems battery voltage is "o.k." The test involves measuring the systems battery voltage at the battery control module's electrical connector. Note that the acceptable voltage for an okay battery is 10.5 - 14.5 volts.

alk247

The curve is true only for no load conditions, which happens in our cars only when consumer battery is disconnected from the car. There is always some parasitic load that results in lower voltage across battery terminals than shown the curve. This is due to the internal battery resistance, as was pointed out before.
So both of you are correct. But as a practical matter voltage across battery terminals could be significantly below 11.9V with no shorted cells.

whoover

Sure, a starter battery will show a big drop during start. But a large resting load will be way under 1 amp so the drop won't be more than a volt. That's another reason the electronics are designed to work at 10 volts. I think OP saw something like 130 mA resting draw.

bobterry99

My top operates with the engine off and the systems battery measuring 11.5 volts. The load is at least 30 amps. According to the way the "curves" are being interpreted on this thread that should be impossible.

danmm7

I appreciate this very healthy and informative discussion. I certainly have learned a lot.

The issue is.... my original problem is still happening. The red battery light is back on!!!

It actually came on twice in the last 3 days.

On Friday, after the car was parked for 2 days or so.... i started it for 2 seconds, realized that i forgot my phone in the house, turned the engine off. Came back in a minute, started it and the RED light came on.
Drove around for 10-15 miles. Ran some errands, the light kept coming on as soon as i started the engine every single time. Went home.... turned engine off.... measured voltage at both batteries - Rear 12.6V ..... Front 12.8V
Plugged the CTEK overnight, .... next morning the red light came on again, despite rear battery being fully charged. I pulled the scanner.... same code. I erased the code. Light went off and it did not come back on. I drove the car on Sunday too. All was good.

This morning,...started the car. No issues. Dropped kid at school. Started the car again - no issue. Drove 5 min to tire shop. Parked there for 2 mins. Then drove another 5 min to gas station. No light. Put gas in, ...and as soon as i started the car - bam, RED light came ON again.

So, the only common pattern i can see in the above is that in both instances when the light came on i had 2 back to back engine starts. aka... i started the car, turned it off... and then turned it on shortly after.

Perhaps a bad/weak battery did not like that?!?!
But didn't the light go away on Saturday after i has the CTEK plugged in all night?
I saw the rear battery was fully charged, yet the red warning persisted, until i manually cleared the code.

Few weeks ago, the red light went away on its own after an overnight charge. This is why i was more and more leaning towards replacing the rear battery. Now, i am not so sure.
And of course... the error code stored (B1828) has nothing to do with bad battery. It still points at the relay K57 (already replaced) or bad circuit 61.

bobterry99

If this were the case, then starting the engine and running it with the rear battery disconnected ought to trigger the fault -- yet it won't.

In an earlier post I think I explained that circuit 61 is detected by the passenger SAM and how you might troubleshoot.

danmm7

You did and i ran some diagnostic test via STAR software and everything came out fine. No issues. System functioned as expected, per the STAR test sequence i followed.
Also, the issue later disappeared, after fully charging the rear battery. Perhaps it was a coincidence.

Rudeney

I occasionally get the red battery warning light. It seems to happen when I sit in the car with the engine off but radio on, or I stop and restart the engine after a short period of time. In my case, I am fairly certain it's a lazy voltage regulator. A few years back. the alternator (likely the regulator) failed and I was able to get a lifetime warranted rebuilt one from Advance Auto that day, so I did. What I have seen is that it takes some time after starting the engine for the regulator to stabilize. If I have run the radio or done a brief engine stop, it takes longer. I chock it up to cheap Chinese parts on the rebuilt alternator. I keep a cigarette lighter volt meter in the center console to verify.

danmm7

I am 90% sure my red light comes on under the same conditions - stop and restart the engine in a short period....or run accessories without running the engine.
My alternator was also replaced by previous owner....unknown part to me. Likely it's not OEM unit since it was replaced by independent mechanic with so-so reputation.

One of my very first guesses was that i need to replace the voltage regulator with a OEM or Bosch part. Some of the people on this thread however were very convinced that the error code i get has nothing to do with the alternator or voltage regulator. My charging values are very healthy at both batteries (14.3V Rear & 14.5-7V Front) once the engine is on, but who knows what happens in the first few seconds when i start it.

Since i am still getting the red battery light on and off and no one seems to have a clear idea what exactly is causing it .... looking at the low cost of a new Bosch voltage regulator (~$40) ... i may just replace it. Worst case.... i will be $40 out. This is cheaper than a new AGM trunk battery, which may not be the issue either.

I really hate troubleshooting by swapping parts, but the SL has not left me with many choices.

whoover

Circuit 61 is the alternator charge indicator. An intermittent or otherwise non-spec alternator output most certainly could raise this error.

Rudeney

I kept my OE alternator and still intend to buy a new regulator for it, but I am lazy and haven't done that yet. I will eventually.

danmm7

The new voltage regulator (Bosch) is in the mail. $40 shipped is not too bad, IF this ends up being the solution to this mystery red light.

Speed750

I just spent the last 3 hours hanging out at my buddies "indy shop" and laughing about this thread. He's been in business for 25 years. I've seen him work on anything and everything. From weed wackers to SL600 AMGs , 750il BMW's etc. Just a total motor head.

You can not diagnose any electrical problem unless you have KNOWN GOOD BATTERIES. Not "voltage looks right" , but load tested batteries. All the codes your reading are relying on proper volts and AMPS.

For somebody to say "10.8 volts means you have one dead cell" Is hogwash ! It might be that OR 6 low cells or any other of a thousand things. There're no absolutes. This whole " manual says this and that's the bottom line" is just not real world diagnosis.

Yes, the car might run on 10.8 volts. They never said if would start on that. Then tap the brakes, lights come on , voltage dips and codes flash.

This is just in referance to all the guys with their Chinese code readers. The code reader relies on proper voltage and amps.

Every 10th post says "maybe I need a new battery" !
You can have auto zone load test the batteries for free.
If they test good, cool. Now you've taken that out of the equation. If it test bad, then you've spent 3 weeks and 90 post trying to figure it out.

For my sanity , would you please have your batteries load tested ????

danmm7

Wow. I am glad we provided so much entertainment for you and your expert buddy.

Just for your sanity, please note that I personally load tested both batteries with my own Schumacher BT-100 tester. That was 2-3 months ago. Both were in the green zone.
I did it again tonight, just in case. Still green.

Yet the red warning light persists.

any other ideas from the expert?

Speed750

Well thank goodness ! I don't recall any where in this thread where it said you load tested the batteries. Now we know for sure that's not the problem. Carry on.

Speed750

The other "expert" advice is , replacing with a new part is not the same as replacing with a KNOWN GOOD PART. It's very common to get defective parts that are new in the box.

rneaul

was hoping to hear the ultimate conclusion (if there is one)... my red battery light is always on too

danmm7

Of course it's common to get a defective new part from time to time. This is the state of "quality control" nowadays and the result of cost cutting everywhere.
I've been working on various cars for the last 20 yrs too, so i had my share of brand new parts being bad while still in the box. Fan clutches, CV axles, thermostats, and water pumps.... just to name a few.

If you have any tips on how to test a new Bosch voltage regulator while still in the box - please do share.

bobterry99

I worked in the engineering department of a Bosch manufacturing facility. The regulator is tested before it is put in the box, so in that sense it is as much a known good part as a used part which worked the last time it was used. The likelihood of a new Bosch regulator being defective is infinitesimally small and not worth considering.