Scotsman

Reading a lot about flat batteries after a couple of weeks not being used. In Europe, it's common to have 2 or 3 weeks off so a bit concerned. Don't fancy leaving it on trickle charger.
Is this really a big problem and are you all affected?

Mustard

I don't think its a huge worry.

Mine was left for 8 weeks, and I couldn't use the remote key to unlock the doors. Panicked of course.

However, the steel key both unlocked the doors and started the car. The technician turned the wheel lock to lock to reset the electronics after the car had been running a while, then used his computer to fine-tune matters, which he said would have returned to normal in any case.

Whether that would have happened I don't know.

I also can't figure why the only sign of a battery drain was an LED blippping every few seconds. What else drains the secondary battery I just can't work out. Maybe BlueSL knows.

All in all a minor blip, so to speak!

Scotsman

Thanks Mustard. Had flat batteries with my C-class and CLK.
Turning the wheel lock to lock fixed any s/w problems with them but I dare say the SL has a few more bits and pieces to reset!
With the CLK, it was caused by the phone being incorrectly wired and drained even though the phone wasn't in the cradle.

Twinturbo

Its only fact that the a battery drains over time. AS there is two batteries, maybe the one goes flat (which one is it?), is a smaller one, of lesser quality or perhaps is always powering something, like the sophisticated alarm perhaps.

blueSL

OK. all you ever wanted to know about the batteries in the SL, and more besides.

There are two batteries in the car to address the problem of not being able to start the car after a period of standing idle.

The smaller battery in the engine compartment is used to start the car, not just the starter motor, but the ignition, fuel pump and engine management and is designed to have the absolute minimum of static current drain to allow you to start the car even after several weeks. The alternator charges both batteries.

The larger battery in the trunk powers everything else and can also start the car if the small battery is dead. However, the larger battery powers everything else in the car, including those items which are never switched off and this battery quickly looses its charge when the car is not used. I estimate that after 10 days, it's lost 50% of its charge. If the battery is not sufficiently charged when you start the car, some items will not work, for example the roof and you get the "Electrical Consumers Offline" message. When the battery is charged up by the alternator, the message goes away and in theory, everything starts working. It does look like there's a problem with some part of the car when this happens though. My air-con goes hay-wire until I stop the car, switch off and switch back on.

If you put an ammeter in the circuit, you see the current drain from this battery decrease over a period of about a minute after the car is locked as the various systems shut down. When all is quiet, the car is consuming a few watts of power only but this is enough to drain the battery over a couple of weeks.

If the battery is flat, you cannot get into the car with anything other than the mechanical key because the central locking is dead. This is why they've dumped the card for keyless go because you are now forced to carry the real key with the mechanical insert to allow you to get into the car if the battery is flat.

So what is being run when the car is shut down? The interior motion sensor and alarm system, the central locking controller plus the internal network will be active.

As pointed out, both batteries will loose their charge over time anyway due to "internal discharge". Trickle charging the car should not be a problem, especially if its in a reasonably well ventilated space.

prsmith777

Any thoughts about putting a third battery back there for backup purposes.

Ive added a few electronic mods including DVD player and XM radio and Radar detector, and have noticed the battery drains even faster, even with everything turned off. THere must be a trickle current still running. When I do the custom stereo next month, Im going to see if they can add an additional battery for backup. Those bigs amps may need their own battery anyway.

Paul

blueSL

If you need separate batteries, that will be to provide the high instantaneous current required; at the end of the day, if the power requirements are more than the alternator can provide, you're never going to get it to work and the battery will drain quickly if you sit listening the music in a parking lot.

The trickle charge solution is easy, just plug it in when you know you're not going to use the car for about a week. Not much use in airport parking lots though, but who leaves their SL in those. Not me.

Mustard

So I guess sitting watching TV on a moonlight night with the hood down somewhere desolated is totally out of the question

What trickle charger do we get BlueSL?

blueSL

The normal stereo will be fine for a few hours, but the point I was making was that upgraded amplifiers can use large amounts of power which will quickly flatten a battery which is not being charged.

The trickle chargers are available from dealers or else you can make your own. All you need is a wall "brick" generating 15v at 1A and connect it to a cigar lighter connector through a 10 ohm 10 watt resistor to limit the current.

vraa

Does anyone know if the internal motion sensor and other various things are controlled by the computer or by a switch or what? I'm thinking maybe we can hack some stuff into the COMAND as per options so when you know you aren't going to need half that security you can disable it to save battery life.

mattnaz

Is there an internal motion sensor in the SL? If so, is it activated when the top is both up and down?

blueSL

The electronics of the SL is far more complex than many realise - over 40 control units, each with their own microprocessor handling inputs and outputs local to where they are situated, all networked together to form a complete functioning (most of the time) car.

There is, for example, a Door Control Unit situated in each door which processes all the inputs and outputs on the doors (heating, cooling, seat position, trunk release, window switches, LEDs, window position sensors, motor control, door lighting, exterior door handle activation, door lock activation, IR receiver data, Keyless Go receiver data and so on. My guess is that the development of that control unit alone took thousands of hours of work.

The goal is to reduce wiring in the car and to avoid duplication so that any control unit which needs to know the outside temperature can find out by asking another of the control units. When the driver performs some action, information can be broadcast on the network so that each device takes the correct pre-programmed action.

There is little hard wiring between switches and the devices they control, so any ideas of hard wiring the seat coling fans to be on all the time or interrupting power to one or more control units are doomed to failure.

The issue of battery drain was likely one of the headline items when the car was being development. Later cars, like the E class probably benefit from newer technology and lower current drain. Here in the UK, the SL can be specified with a larger capacity battery (SL55 has it as standard) and I would say it's a pretty much must-have.

Scotsman

Thanks for the info guys, especially BlueSL - taken a degree on SL's?!!

Must ask about the higher capacity battery, sounds a good idea.

Hey, and guess what - wife came home the other day and asked if I'd like to get the 500 instead? Took about .2 milliseconds to reply. Means getting it a couple of weeks later but I think I can live with that.

Mustard

What a wife!

Is she Scottish?

I had the higher capacity battery and it didn't help me a lot....

Mustard

It is active at all times when the car is locked - it can be switched off when required - as I found out on a cross channel ferry when I forgot and the alarm set off the travelling dogs in almost perfect harmony.......

Scotsman

Yup Mustard, she's Scottish. And was sober (at the time!).
Of course I'm now getting "We could do with a new dining table and chairs" and "I think we'll decorate......!" Right now I'd agree to just about anything.

Won't get it until July 10th now but that would work in pretty good as we're going to the Air Tattoo in Fairford the following weekend and was going to take the week off anyway.

Larger capacity battery didn't help? Hmm, guess I'll have to wait and see then.

blueSL

Fact is, the battery in the boot will go flat if you leave the car long enough; in that case, Keyless Go and IR key will not let you in but the metal key will and the car should still start using the battery in the engine compartment.

All the larger capacity battery does is to put off the inevitable.

None of this is a concern if you drive you car for, say, 30 minutes once a week, you'll never see the problem.

Mustard

Er, hate to state the obvious, but thats a bit hard when you are 12,000 (or whatever) miles away.

I remeber my 1939 Ford Anglia never went flat after months of not starting - we're going backwards, for sure...

Silver_Lana

Tell you what. I can solve all your battery and startup related problems.
Just hand over the keys and I'll personally guarantee that your very fine automobile will be driven at least 30 minutes a day - to make sure the battery gets fully charged of course.

Also my fees are very reasonable.

internet_mafia

How come you can't just disconnect the battery if you know that you will be gone for several weeks? That way, nothing is being drained since there is no "power source" to drain. When you return, simply plug the battery up and perform the "bootup" routing (ie. turn wheel lock to lock and begin to pray).

Mustard

Kind of you. Mind if I fly you in monthly - first class of course - to make sure things are going well. If you wouldn't mind driving my car through Europe at my cost to where I am - top hotels of course - to keep that ol' battery charged - that would be so helpful. Forum members like you are so hard to find!. :p

Silver_Lana

I do what I can for fellow gentlemen.
Cheers!

vraa

Back off, this is my section of the forum

blueSL: Well, all I have to say is.. CRAP! So I guess adding new features by us would be a no-no. Well there goes my summer project.

Twinturbo

Diconnecting the battery would not be the wisest thing to do, everything would reset and go to default. Not sure if there are other backup batteries, but this could mean doing things from unlocking the radio to retuning the presets, and all the other adjustments that you made. Even so, batteries do go flat in time, however im sure it would last for months if disconnected.

Has anyone tried the sealed batteries. I do not know if the SL's have these, but the sealed batteries never require topping up and are supposed to last much longer. They are also good for 4*4's because it is sealed and is watertight, but were talking SL'd here. I think that anyone having battery problems should try out these batteries and see if it helps.

Here is a link for these type of batteries. (just one i found at random). Optima Batteries

mblover455

it works well and is super easy to use once u attch the clips to the rear. and itkeeps the battery topped off.

Here