Off and on for the past several months I have been getting the no consumer battery battery message. It usually comes back with half a mile of driving, but I'm concerend that the battery is failing as it never used to do this at all.

The battery is two years old.

During the winter I have always used a battery tender on my motorcycle. It is a BMW and accepts a charge through the accessory plug. Keeping the charge topped up greatly extends battery life.

I notice that when the consumer battery goes offline the cigarette lighter (where I plug in my radar detector) is offline. This got me to thinking if it would be safe, or a disaster, to use a battery tender through the cigarette lighter (1.25 amps). Same idea as the bike.

I have seen this done on cars back in cold states like Minnesota, but never on anything so complicated as an SL.

Would this work, or will I have french fried electronics?

Any suggestions for products?

 

do you have keyless go? My friend's 03 SL55 seems to have the same issue... I believe he is on his 3rd consumer battery. His car has approx 1900 miles on it.

 

I do have keyless go. However, this has only been happening this winter. No problem last year. I have traveled a lot recdently and the car has set idle. I fear the battery has discharged too much. I take it out on long high speed drives when I can but that doesn't seem to solve the problem.

 

OK, here's the issue.

When you lock your car, there's a residual battery drain which falls to about 190 mA but that is enough to drain the battery after 7 - 10 days to the point where you get the "consumers offline" message. This is fine if you drive your car every 7 days, I do not.

As you rightly say, the power recepticle in the trunk is disconnected when the ignition is off. The solution is to trickle charge the battery while the car is out of use. This is a hassle because you have to take out the trunk interior to get at it. I've installed a connector above the trunk light which is connected (via a 5A fuse) to the battery in the trunk. I use a commercially available charger (12v, 2.7A with 3 stages of charging, the last of which is trickle charging). When I park the car and want to leave it for a week or two, I open the trunk, connect the charger and I know that the battery is then being charged optimally.

That avoids the consumer battery ever going flat which will eventually require its early replacement. Nothing to do with warranties. If you let a lead acid battery go completely flat, one or more cells will fail and you will not be able to fully charge it.

The Mercedes manual tells you to disconnect the battery before charging it. That's because the typical battery charger is a pretty crude affair and will output a voltage which is only nominally correct. Open circuit, the voltage is much higher with a high ripple (AC) content and you do risk cooking the electronics. Better to use a smart charger which uses regulated DC and which bases its charging current on the voltage being presented back from the battery. Sounds complex but isn't, my charger cost me $75. With such a charger, you can safely charge the battery while still connected to the car. For UK readers, the charger I use is from RS Components: rswww.com part number: 301 4838. The charger is actually made in Norway which makes a change from China...

Problem solved.

 

i was getting a battery tender for the car in the winter. do you mean i should charge the rear battery and not worry with the front battery?

 

Does the SL55 have 2 batteries standard? I wasn't aware of this being an option let alone standard.

 

Update...

The Sl has two batteries standard. Look in the owners manual. The rear battery is the "consumer" battery and handles the roof, keyless go, accessory power, seat heater and vents, etc. There is always a draw off this battery because of having to "listen" for keyless go or the FOB transmitter.

The front battery is mainly reserved for engine operatins and starting. There is no draw on this battery when the cars is off.

Talked to the parts department at Symthe yesterday. They informed me that there is a Mercedes charger that works through the cigarette lighter for the rear battery. 2 amp charge. $139. I'll pass along more info whn I have it.

 

There's always been 2 batteries in every R230, just as there is in the new E-class. It's Mercedes way of recognising that with all the electronics, it's impossible to reduce the current drain to zero and you risk having a flat battery after a couple of weeks. So, the high priority stuff is engineered for no current drain and runs off the battery in the engine compartment; the lower priority stuff runs off the battery in the trunk. Either battery will start the engine but because the battery in the engine compartment has near zero drain, it's the one which will get you going if the one in the trunk is drained completely.

I'm surprised at that because in my 2003 SL55, the socket in the trunk is not live when the ignition is off. There's also a separate isolating relay to isolate both it and the cigar lighter in the car when the car recognises power is low - your own trunk cooler and other stuff connected to these sockets are the first to go.

It's possible the wiring has been changed in later model years so that the rear power connector is always live to facilitate easy recharging. If your car is the same as mine, just plugging in the charger is not going to help you any. In my case, an alternative is to wire a diode (say, 1N5402) between the centre pin (positive) of the connector and the trunk battery positive, cathode (striped end) to the battery.

Anybody out there care to confirm their rear power connector is live with the ignition switched off? Mine isn't.

 

Mine isn't also. Build Year '02, model year '03.

 

I was told about this charger by the parts department at the dealer. I was surprised. When the ignition is off my cigarette lighter is dead (2003 SL55). However, they swear this will work. They are waiting for more information.

As soon as I have anything specific I'll post it.

 

After a while took it in and turned out there was a power drain from the glove compartment locking mechanism cable... they changed that and bingo, the problem disappeared (unless I don't use the car for a week or so in which case it takes 10 min to charge the consumers battery again)

 

Finally got some answers. Mercedes offers a smart charger, part number B6 7 54 2024. It is similar to a battery tender (at Mercedes prices, $140). The book with the charger lists several cars, including the new SLK and the McClaren, that do accept a charge through the cigarette lighter/accessory plug. Unfortunately, the SL is not one of these cars. BlueSL's info is 100%.

I rigged up a female connector in the trunk like BlueSL did. Got some help as I am a klutz with this kind of work (too many years in software). Works great. The Benz charger has a peak charge of 3.6 amps.

Tried charging the starter battery (with clips) and the charger shows that it has a full charge. The consumer battery almost always takes a charge. Had the car in for a service and they checked the battery, charging system and current draw. All was well. However, it seems that the consumer battery, for whatever reason, does not charge as fast as the starter battery and/or drains much faster.

 

Almost forgot, a couple more things...

The book says that the draw on the consumer battery when parked is 21 mA. Find this hard to believe. If true, why would these battereis get so weak so fast?

Even after driving 50 miles on a freeway could not get the consumer battery to fully charge. Battery and charging system are supposedly all fine per the dealers inspection. Any idea how long a drive it woulkd take to charge this battery if it was low? The first time on the 3.6 amp charger took 14 hours to charge up.

 

I don't believe the 21 mA for one minute. I've measured mine and it was something like 190 mA. So, with a 100 Ah battery, you're 50% discharged after 10 days. My trunk battery is shot and needs to be replaced but the only long term solution is to hook up the charger if you are leaving the car for more than a week or so. The reason to have the connector is because you need to remove the trunk lining (and nets if you have them) to get at it.

I have the charger cable on a spring return cable drum mounted above the car (just like a roller blind) which I just pull down and connect to the car when it needs charging. When I'm done, the cable recoils out of the way.

As for charging time, if you were charging a battery from 20% to 80% using 3.6A, you would expect 60% * 100 Ah /3.6A = 16 - 17 hours, providing of course that the battery charger can maintain the charging current as the battery voltage rises. If you are driving, the current available to charge is the alternator current (say, 120A) less the amount being consumed by the car but the charging current will always be limited to something like 10A. Charging at current above this can cause overheating of the battery which is not good news. Again, using that formula, you'd be looking at 6 hours. The "electrical consumers offline" message only comes on when the battery is quite low. When the message goes off, it does not mean the battery is fully charged.

Bottom line is that this car is unhappier than most not being driven.

 

I'm no expert on the subject, but batteries that drain fast and charge slow might by broken. Batteries can break just by being discharged completely.

Why not have it replaced? Or did you try that already?

I remember having this problem on my DeLorean (which I don't have anymore): A lot of battery problems (discharging) that were all solved by having it replaced.

 

I have not replaced the battery. I was just in for a service and they assured me that the battery was fine. Can I trust this? How do you reliably test a battery? Since I have had 4 consumer offline messages (all when the car was idle over a week) can I trust the battery or any test of it?

Now that I am charging it things seem fine.

 

The type of battery used in the trunk is more suited to the effects of deep discharging which in normal batteries bring about irreversible chemical changes and once one of the 6 cells dies, the whole battery is useless. Even so, I've let mine go flat too often, so that even after a full charge, the consumers message is back after a couple of days. It needs to be replaced and I could not in all honesty claim it was faulty. It's not a big ticket item anyway, aside from the fact that you have to use a Mercedes one instead of one you might pickup at Wal-Mart or such-like.

The SL is not my daily driver and may go for weeks at a time without being used which is why I think it's important to trickle charge the battery like this when not in use.

Normal batteries suffer from being charged too rapidly and being left to go completely flat. Use them regularly and charge them moderately and they will last for years.

 

Tried an experiment.

Assumptions...

BlueSL's number of .19 amp per hour drain is correct.
My SL had a fully charged battery after driving (I had charged it previously).
The specs on the battery charge are accurate and you can believe the charge complete light.

Let the car set for 20 hours. That means it should have drained 3.8 amps (.19 * 20). Started charging at 12:30. Charging rate 3.6 amp per hour. Charging should complete wihtin about an hour. At 14:00 the charged light came on.

From this I conclude my battery isn't shot as actual charging behavior fit with prediction and the .190 amps per hour drain number that BlueSL measured is accurate and the stated .021 ampes per hour from Mercedes is wishful thinking.

 

I agree, your battery seems fine. If the capacity of the battery is 100 Ah, it would be more than 50% discharged after a couple of weeks, by which time the electrical consumers message will be on. I get the message after a couple of days which makes me think the battery has had it.

I think Keyless Go is the main culprit and I noticed that they seem to supply a higher capacity battery as standard now if KG is specified. In my 911, the remote unlocking is disabled after 5 days to save the battery but it doesn't have KG.

The solution to all of this is to either use the cars most days or, if it is not your daily driver, be preparated to trickle charge the battery while to car is not in use both to prolong the life of the battery and stop the functional oddities which can happen when the electrical consumes come back on line - odd messages, aircon on max temperature, that sort of thing.

 

Was over at the dealers today. Ran into the fellow who measured my battery drain (he knows his stuff). He said that he used a Fluke/SnapON load tester and that the load measured with the car just sitting was only 30 mA. I told him about this discussion and he can't account for the difference.

How was the 190 mA measured? Could there be a non-regular usage over time? That is; measured for a few minutes it is very low but measured for an hour there is a power hungry spike?

I am assuming that both the shop measurement and BlueSL's measurement were accurate. What is casuing the discrepancy?

 

It will depend on the options - do you have Keyless Go? In the UK, most of us also have something call Trakker which is an anti-theft satellite tracked/paging device buried in the car somewhere which will also be increasing the consumption. There's also the interior motion sensor which is in the bin between the seats. You also have to make the test when the car is locked, which makes it more difficult because the alarm then goes off with the trunk open.

I don't argue with the 30mA figure, but that would suggest that the battery would be 50% discharged after about 1000 hours which is about 6 weeks and experience suggests it drains quicker than that.

I measured mine again when replacing the battery yesterday, and the current drain is about the same as before. I was interested to note when playing with the German car configurator checking option prices there that if you select Keyless Go, it forces you to have the higher capacity battery option - 100 Ah, a real brute of a battery. Turns out mine is only 70 Ah and I could not see how the larger one would fit in the available space.

It's an expensive battery as well - you cannot use an after-market equivalent, price from MB was nearly $200.

 

I do have keyless go.

Priced the consumer battery here (California). $240.

Only thing I can figure right now is my battery has trouble. It acts like it is being drained at 190 mA but maybe it's just discharging too fast. Feel like I'm missing something.

When was your car built? Mine was built 12/2002. Wonder if newer SL's have same issues?

 

When I do the exact calculation, it's the same, $247 here. My car was built 09/2002. If your battery is draining faster than you expect, it means it has diminished capacity, so replacement may be the only answer. Mine was completely shot when I exchanged it.

 

Would it be okay to use a deep cycle type battery for the rear consumer battery?

 

Questions:

1. Can I use the CTEK MX5.0 Trickle charger for the Starter Battery as well as the Consumer Battery?

2. Will it be in order if I leave batteries connected in the vehicle whilst being charged?

3. Also, my charger supplier wants to know what AMPS the Starter Battery and the Consumer Battery are.

Please help.

Thanks.