vnova94

I have a 2003 SL55 that has been having charging issues. The car is not in use much and will sometimes drain the battery, so i put on a Battery Tender to keep it charged. Recently, the car had trouble starting and it seemed like the starter battery might be bad, so i went and replaced both batteries with new ones. Everything seemed fine, but the battery warning came on... no biggie as i though it would clear itself after a bit. The next day, i drive the car and the light keeps coming on and off..... it will be driving fine... then the battery light will come on and the all of the dash gauges will turn off... then they come back and everything is ok... it will keep doing this repeatedly. I have a code reader in my OBD-II port and it has no codes related to the charging system. All of my cables look to be solidly connected... unless there are issues underneath the car (which i haven't had the opportunity to check underneath yet)? What else can I check? Can the voltage regulator cause this, or the alternator?

Thanks,

Matt

g0rsq

Check battery voltages with engine stopped and running.
When stopped they should be around 12.5V. Running they should both be about 13.5 to 14 volts.

As you know the alternator only charges the rear battery (normally) and the BCM charges the front battery (from the rear battery).

Knowing the voltages will help identify what is wrong.

Tom Manning

I wouldn't connect the two faults just yet. Failing batteries are common and so is the red battery light after replacing the rear battery. Usually it stays on until you clear the code then it's gone for good.

In your case the on and off battery light plus the gauges point to bad connections. I would pull all three fuse boxes and look for corrosion on the connections. Blocked drains in the front can lead to water ingress in the modules

MikeJ65

I believe that the cluster is fed by the driver's SAM. SAM plugs into the fuse block and can corrode at that interface.

vnova94

So far I took the measurements as suggested.

With car off: 12.6v on front battery and 12.4 on rear (it was sitting for a couple days without my battery tender on, so hence the slight drain in the consumer battery).

with car on: the front battery stayed about the same voltage. The rear went up to about 14.1v after running for a couple minutes. At first it was only around 13.

g0rsq

The consumer battery voltage should jump up to 14.5 ish volts as soon as engine is running, not take time to get there. You may have a charging issue.

Check rear battery terminals are clean, as poor connection can cause poor charging.

Tom Manning

I believe that the cluster is fed by the driver's SAM. SAM plugs into the fuse block and can corrode at that interface.
That would explain the cluster dropping in and out. I think I remember a thread on this topic; corrosion at the module was the issue.


The consumer battery voltage should jump up to 14.5ish volts as soon as engine is running, not take time to get there.
Other way around. If the voltage jumps up immediately, it indicates high initial internal battery resistance, therefore more voltage across the terminals, which is what happens as they age. New batteries have lower internal resistance, allowing them to take a charge, which is why the voltage should come up slowly from cold.

Greg Elmassian

And of course that is mitigated by the state of charge, i.e. a well charged battery will jump to 14.x volts as soon as you can start the car, climb out, and look at your meter in the trunk.

A good battery in a poor state of charge could take a longer time...

A bad battery, but recently charged will show 14.x volts (but may not have all the amp hours it did before)

So, simplistic "rules" kind of don't always apply... charge it up, note how it charges, and see how it discharges by itself and under load.

greg

Tom Manning

True.

Testing the charging voltage is more about checking the charging system rather than a measure of battery performance. A dummy load is needed to test a battery comprehensively, to see its current delivery over time.

Also these indications apply to old lead acid batteries - do our AGM batteries behave the same way?

With our cars, be aware that you may not read a charging voltage on the front battery every time you test. The BCM is the charging system for the front battery and only operates when needed.

***

With my good, new rear battery, the charge comes up to 14V within 10 seconds, after it's been sitting for a while.

Greg Elmassian

You read my mind, I noticed you did not say to look at the voltage elevation on the "starting" battery.... so the system is a bit "smarter" about when to charge it? Sort of makes sense.

Thanks, Greg

vnova94

I will look at all the fuses and try to find all possible connection points to see if there is any corrosion. I assume the alternator and regulator are behaving as they should? And the BCM as well? or we wouldn’t know unless the starter battery required a charge. Both batteries are brand new.

g0rsq

Discussions about internal resistance of a battery are only significant when talking about discharging into a load.

The problem with charging, is the battery terminal voltage will oppose the charging voltage, and effectively "swamp" the effect of the internal resistance (a few 10's of milli-Ohms (0.010 ohms)). It is there, but insignificant compared to the effect of the opposing EMF. As the battery charges up, it resists the flow of current into it, until when fully charged very little current will flow.

The alternator can give a voltage of about 14.5 volts (14.4V is optimal charging voltage for AGM batteries), and a current of up to 150 amps. An alternator is effectively a fixed voltage device, varying the current as demand needs it.

When charging a battery with the alternator, it will generate the 14.5 Volts against the battery terminals but only if it can generate the current being consumed by battery, and rest of electrical system. (If it cant generate the current needed its output voltage will drop below 14.5Volts).

However the battery will only consume the current it has capacity for, so a fully charged battery will only consume a fraction of what the alternator can generate. With the fixed 14.5V charging voltage, the charging current will decrease as the battery charges up, until it is only a few mA when fully charged.

Hence the battery terminal voltage will immediately rise to the alternator voltage (14.5V) and stay there while charging. If it doesn't, then the battery and cars electrics are consuming more power (amps) than the alternator can generate. Hence why i suggested the alternator may have a problem, as it cant keep up with the current (amps) demand.


so either the batter is in poor state of charge, or alternator has poor output.

Unfortunately, alternator charging is very crude (our cars don't have charge controllers on the consumer battery) and when fully charged will continue to supply terminal voltage of 14.5V and not a float charge of 13.5V which is ideal, and would prolong the life of the battery.

When the charging voltage is removed, the battery drops back down to its chemical voltage of about 12.8V if it is fully charged.

You can check this quite easily with a voltmeter. Of course you cant easily measure charging current. You can tell a lot about a charging circuit by the voltages you measure.

Here are screenshots of my voltmeter on consumer battery terminals as car started and stopped.

Starting



Stopping



Both batteies.

Tom Manning

What is that voltmeter and software you are using?

Interesting, mine definitely takes a bit of time to come up and always has so I assume this is normal operation, but there could be underlying issues. Everything works perfectly with no errors. I will test it again.

There was an issue with dud regulators on early cars. Mine had the alternator replaced a few months from new.

g0rsq

I have one of these on each battery, so can keep an eye on them.

They store 5 days of data, and it will export the data as well.



Later cars, I think facelift, do use a charge controller, and the output of the alternator is controlled by the ECU, so should be kinder to the rear battery.

Thrasher5621

I always contend if you don't understand how the system work... it is very difficult to troubleshoot.
I have a dual battery pamphlet that was provided to Mercedes Tech's years ago loaded on my website..... free for the downloading: https://rb.gy/10p09
Not trying to sell anyone something.... just post them there because it is easier.
If your Red warning light is on.... it has something to do with the front battery. It is not being charged. This could be for several reasons. If your rear battery is charging the alternator is working.
Could be a bad battery... but, very often the K57 relay and/or the BCM is the issue. I also always replace the K75 relay when I do the K57.
The starter battery is only charged as needed. If you have a new starter battery it will usually last a long time before not having enough power to start the engine.
What you NEED to be concerned about is a power overloading at the BCM. All those AMP's can cause a fire if not managed and routed properly. VERY BAD!!!!
Mark