ceberus

After having problems with the aux trunk battery holding charge, I just bought a brand new 70A/h glass matt about 2 weeks ago. The battery was reading 12.4V when I got it, so before I installed I topped it off using battery tender until the light turned green or 12.9V. I drove the car twice during this time period for about 30 minutes total, and now the battery is reading 12.3V and has lost over half it's charge. The starter battery is fine reads 12.6V. So I am trying to understand what could be happening here:

1. The aux battery is not getting charged by the alternator, but wouldn't this mean the starter battery would also be affected? Are there 2 separate alternators?
2. Bad luck and got a bad battery. Never seen a battery go bad in 2 weeks so I am reluctant to rip it out again and have it tested.
3. Could this be normal for the aux battery to lose over half it's charge in 2 weeks from remaining idle?
4. Or has something consumed over half the 70A/h charge within 2 weeks. Is there parasitic draw from something?

It's been on the battery tender for over 15 hours now, and it' not even back to 80%, so I wonder if I did get a bad cell. Again, this battery is maybe 2 to 3 weeks old. I am new to MBs altogether. If any one can advise using data provided and experience how to proceed that would be helpful. Again, i think this is a charging issue, but I am unclear how the starter battery would remain unaffected if that were true. Thanks!

MarkCan

Hey Cerberus,
To start with the battery tender may not be the most accurate measuring tool out there, so grab your multimeter and confirm the values.
Things to consider here thoug;
The battery is no longer without the load. Like when you tested it prior to installation.
You drove the car, than parked for few days. During that time some electronic systems where on and off. Now you came back open the car, open the hood open the trunk, several lights are on, computer is running system checks... all of it needs energy another words puts a load on your battery. Therefore your in car battery test will always show slightly lower voltage.
To confirm there is no power thief in your system you can put an amp clamp around the positive battery cable and see what is the current draw. Take a coffe break, keep the trunk open. Don’t touch anything on the car when you come back take another reading from the amp clamp. When all systems are asleep you will see somhing like +/- 0.04A depending on your car configuration.
To confirm your alternator is working properly ( there is only one) and connections are ok;
- measure the voltage of the starting battery
- measure the voltage of the consumer battery (expect both reading somewhere above 12v)
- start the car and reapet the measurements. You should see voltages go up another 2points. 14 to 14.6v is where you should be depending on your alternator setup.
And now, I left the best for last. My one week old battery reads exactly same as yours when it sits, 12.4v.

Mark

ceberus

Thanks mark that was helpful. I checked charging and it's right at 14V. I'm working to get an AMP clamp to check parasitic drain.

The 12.3V reading seems too low and was from a multimeter. According to the charts it's half drained at that voltage which seems right because it's been charging on the Tender for 24 hours and it's not even at 80% where it flashes. I will just keep it on the Tender until I get the AMP clamp but I think the fun is going to start when it shows a leak somewhere and I have to track it down.

Mr473

what other issues did you face with the dead consumer battery

ceberus

It wouldn't hold a charge. I had it load tested and it failed so replaced with new AGM.

cadbob

I had similar issue a while back, it turned out it was my Battery Control Module, it acts like a voltage drain. I bought a used on on ebay think it was like 100.00. It resides in the trunk above battery and behind carpeting

ceberus

Mark

I put a multimeter inline with my negative terminal as a substitute for the amp clamp. The initial draw was 1 amp because the trunk light and other consumers came on when the circuit turned on. After 30 min it dropped to 0.3amp. After an hour it settled to .05 which you stated was normal draw when asleep (see pic below). Daily that works out to 1.2 A/day and per week its equals 8 or 9 amps (seems like a lot) . So my 70 Amp/Hr battery should lose about 10 -12 % of its charge per week. I do drive the car about once per week, but short trips maybe 10 miles total or 20 mins is probably just enough to maintain the current charge state.

Therefore, I think it's reasonable to conclude the draw down on the consumer battery is a normal condition based on my driving habits, or actually lack of driving. The solution then is to leave the consumer battery on constant 750mA/H trickle and be done with it. I am still baffled as to why this car consumes so much when sleeping. My last sports car was a 1992 C4 Corvette. I could let it sit for weeks or a even a month and it would always crank and run fine -- and with just a single battery!

mikefamig

I keep a tender on mine whenever it will be parked for a week or more.

Mike.

V12inRI

Mike, what battery tender do you use and is it one that connects to the cigarette lighter?

mikefamig

I normally use a "Battery Tender" brand device that I bought at Costco. It charges both 12vdc and 6vdc at a max rate of 3 amps. I use it because I also have a vehicle with a 6 volt system. When that charger is busy elsewhere I use a 0.75 amp Battery Tender. Don't be confused by the amperage rating on the charger, the battery will only draw as much current as it needs and the charger is also regulated to not harm the battery. The larger the number the better when it comes to amperage on a charger but the voltage needs to be correct.

The 0.75 amp charger should be enough to maintain the consumer trunk battery under normal conditions. Each car is different depending on what is using the battery when the car is parked but I can't imagine that even the Mercedes uses more than a half an amp or less. Maybe someone here can say how much current is used in your car when it is parked.

https://www.costco.com/Battery-Tende...100241973.html

Mike.

EDIT - I see above that ceberus measured 0.05 amp at rest.

Frederick NL

My 0.8 amp ctek does the trick as a tender.
I connected the cig lighter in the trunk directly to the acc battery. Fused, of course. The ctek goes in there.

mikefamig

No I do not use the cigarettelighter, it's easy enough to go in through the trunk.

Mike.

Bhopkins

Is it safe to use a trickle charger on the R230 dual battery system without disconnecting the battery leads? I know the Operator's Manual is very specific in not using a battery charger on the consumer battery while it is installed unless the Mercedes approved charger is used. And I know that some have said that certain trickle chargers are equivalent to the Mercedes charger. I'd like a trickle charger for my SL550, but the last thing I want to do is toast the battery controller.

kbob999

To be brief...it is not a problem. I do it on all my MB cars.

mikefamig

I've wondered why Mercedes wants you to disconnect the battery when charging, If I remember correctly they also say to remove the battery from the car? I think that it is to protect the car from the battery boiling and venting acid that can damage the car. The only other reason that I can think of is because the battery will charge faster if the car is not using it while you are charging it,

Mike.

mikefamig

There is no problem with charging through the cigar lighter but there is a restriction. The lighter has small gauge wires so there is a limit to the charge rate in amps that you can charge at safely. If it is exceeded the cigar lighter fuse should blow. If the fuse did not blow you can cause an electrical fire by wires heating. The rate of charge is determined by the max rate of the charger and the condition of the battery. You shouldn't have any problem with a 1 amp or smaller charger, most cigarette lighters have approx 10 amp circuits. You can look at the size of the fuse to know exactly what you have in your car.

Mike.

EDIT -

It is always safest to attach the charger directly to the battery posts. The more wires that the current travels through the more potential for a problem.

Frederick NL

When it comes to potential risk I always count with ’downstream’ current here: not from the charger but from the battery. If your lead has a short don’t worry about your short-protected charger, worry about the surge your battery can deliver.

mikefamig

You got that right! There's enough power in your battery to weld metal. But plugging a trickle charger into the lighter shouldn't present any risk.

Mike.

mikefamig

Can anyone here give any merit to the fact that you need to use a Mercedes approved charger? What exactly is a "Mercedes approved charger"? Is there a particular brand that they recommend? Are we supposed to buy a charger from Mercedes?

Mike.

EDIT
A quick search found this :

kbob999

I run a Battery Tender Jr on both my Mercedes. Less than $50.
In my opinion you are wasting your money on that Amazon MB charger.

danaw007

Right you are kbob999, battery tender brand are great and I've never seen one fail yet. I have one ctek that was given to me and the rest are battery tenders. I use them on my generator, Harley, mower, golf cart.

ray0214

On amazon, check out the c tek 3300. Same specs AND appearance for only $50. I use that if I’m not gonna be driving the SL for a while. Also use on lawn mower, boat battery etc. has a reconditioning feature.

imtheking

This is what I have- just received it a few days ago

ot1

A battery losing half its charge would be about 6 volts, 80% would be about 10 volts, you would have to call the manufacturer of the charger and ask how they set their LED meter charge scale.
A bad cell would show up as a loss of about 2.2 volts per cell. So one bad cell = ~10 volts, a bad cell cant be charged of course, so after removing the charger a DVM reading would be ~10 volts or maybe it would take a couple of minutes for the voltage to drop.
An alternator needs to put out 14.xx volts to charge a battery in a timely manner.

Battery tenders charge very slowly, so sometimes taking more than a day to charge a battery

ray0214

An AGM battery at 50% charge is at 12.06 volts, 80% charge is at 12.42 volts.... fully charged at 12.6 volts. If your multimeter shows 10v, it’s totally dead, in fact, anything less than 12v reading would not have enough power to turn over starter, and probably not power lights