Battery goes flat.

[John77]

As we are hardly using our second car, the battery is going flat every week or so. R171 200 SLK 2006. Yes I could go out more frequently and run it, but I would prefer to solve the problem, as if we went abroad for a couple of weeks (well one day) I would come back and the car would not start. Is there anything obvious that this could be. I haven't jumped in the boot yet to see if the courtesy light is staying on. Any ideas?

 

[Rappey69]

At two weeks it could be the natural low drain of the vehicle and maybe a more mature battery thats causing the issue rather than anything being wrong at all.
I would suggest using a multimeter on amps with a disconnected battery cable to see exactly what the current draw is when the car has fully shut down.

 

[bembo449]

Clamp meter is the best way to see if you have a drain

 

[Chrishazle]

Battery conditioner when the car is parked up - I'm lucky in that I have power in my garage so keep a conditioner on the toy (used to be Porsche 968, now SLK280) when it's parked in there, been doing that for about 20+ years! There's always something causing a small drain on the battery - clock, alarm, whatever. The S204 on the driveway has a battery condition tester connected so I can see it through the tailgate, and with the very limited use it gets in lockdown the condition light goes from green to amber about every 2 weeks, luckily I can not only get an extension lead and charger into the boot on the driveway, I can also close the tailgate over the extension lead and lock the car.

 

[John77]

At two weeks it could be the natural low drain of the vehicle and maybe a more mature battery thats causing the issue rather than anything being wrong at all.
I would suggest using a multimeter on amps with a disconnected battery cable to see exactly what the current draw is when the car has fully shut down.

The battery is only a year old. Ok thanks. so just to be sure I disconnect one of the leads and connect to meter beween the disconnected one and the battery?

 

[John77]

Battery conditioner when the car is parked up - I'm lucky in that I have power in my garage so keep a conditioner on the toy (used to be Porsche 968, now SLK280) when it's parked in there, been doing that for about 20+ years! There's always something causing a small drain on the battery - clock, alarm, whatever. The S204 on the driveway has a battery condition tester connected so I can see it through the tailgate, and with the very limited use it gets in lockdown the condition light goes from green to amber about every 2 weeks, luckily I can not only get an extension lead and charger into the boot on the driveway, I can also close the tailgate over the extension lead and lock the car.

Before lock down I traveled abroad a few times a year and leave the car at the airport carpark for 2-3 weeks. I have not done this with this car, but have with our other Mercedes over the years and there has never been a problem.

 

[John77]

Clamp meter is the best way to see if you have a drain

Thank you and one of those are?

 

[Tony Dyson]

The battery is only a year old. Ok thanks. so just to be sure I disconnect one of the leads and connect to meter beween the disconnected one and the battery?

Have a read here for some basic principles https://www.bbc.co.uk/bitesize/guides/zsfgr82/revision/3

 

[John77]

Have a read here for some basic principles https://www.bbc.co.uk/bitesize/guides/zsfgr82/revision/3

Thank you I will take a look.

 

[Rappey69]

I have a clamp meter but find a multimeter more accurate when measuring low current . accuracy helps to get a amps per week figure..
Let us know how you get on.

 

[John77]

Hi I am finally going to look at this and found this interesting webpage, as I know little about electrics.

How to Find and Fix a Parasitic Battery Drain

Quick, simple, and it works! In less than 5 minutes, we show you how to find a parasitic battery drain. Find out what's killing your battery.

shoptoolreviews.com

 

[Blobcat]

Hi I am finally going to look at this and found this interesting webpage, as I know little about electrics.

How to Find and Fix a Parasitic Battery Drain

Quick, simple, and it works! In less than 5 minutes, we show you how to find a parasitic battery drain. Find out what's killing your battery.

shoptoolreviews.com

A lot of Mercedes battery drains are due to systems not shutting down. If you use the method on that website you might not find the drain as you have to disconnect the battery to put the meter in series. This means the systems lose power and switch off...
A more subtle way is to measure the voltage across the fuses. It’s only a small voltage, but any system drawing current will drop some volts across the fuse which you can see with a Digital Multimeter. This method means you don’t have to disconnect the battery or even pull any fuses.

 

[mattkh]

....... This means the systems lose power and switch off...

But when you connect one probe to the battery and the other probe to the the thick cable plug, the power to the system gets restored and the system wakes up, does it not..?

 

[Blobcat]

But when you connect one probe to the battery and the other probe to the the thick cable plug, the power to the system gets restored and the system wakes up, does it not..?

Usually not as the systems have gone to sleep. They normally come back on with unlocking or ignition on, not just with battery reconnection.
Comand systems are one of the most common drains due to not switching off, and that doesn’t switch back on with reconnecting the battery generally.

A safety issue with connecting your DC ammeter in series is if you inadvertently switch the ignition on you’ll draw a great deal more than 10a and fry your meter...

 

[mioba]

My w220 is sluggish to start if I leave it for a week. Bear in mind the alarm, tow away etc are all live when the car is not in use. You mention you have a relatively new battery is this the correct spec for the car, it could be an el cheapo battery fitted by the PO.

 

[Tony Dyson]

Largely dependent upon the quality of your test instruments, a series connected ammeter will be more accurate than a clip on ammeter and it has to be said that not everyone carries the latter in their DIY toolbox, but the alternative method of looking for a potential difference across the fuses is a fast easy and accessible place to start. The best way to maintain a 12vdc supply to your car while disconnecting a battery, whether you're changing a battery or connecting an ammeter between the battery and your car is to clip a PSU to the jump start terminals under your bonnet maintaining power to your car while required.

 

[SL63 Mark]

I am not an electrician, but, On the R230 to check for current drain I connect a meter in series from the consumer battery earth cable to the battery negative post. Make sure you have the ignition off, and the doors, windows and bonnet open. Make sure you are in the 10 amp range, and have the polarity correct. You will need to click the door, boot and bonnet catches closed to make the car think it is asleep. After half an hour, the car will be fully asleep. Do not switch anything on, or the meter will fry, although a good quality meter will have cut outs and fuses, I would not want to try it. Certainly do not start the car. I use one of my AVO meter analog multi meters, just because I like them.

Once the car has gone to sleep you can swap to the milli amp ranges, and start to do some serious measuring. I found my current draw was only 40 mA at rest, so well within tolerances. Maximum draw should be no more than 50 mA, anything over that and you have a problem, and the battery will go flat within a week.

It is interesting to watch the car go to sleep as systems switch off. There is quite a bit of pulsing on my AVO meter, which you can clearly see with a sensitive analogue meter.

Eventually it settles down to around 35-40 mA.

Now you have access to the fuse boxes, and the car is asleep, you can start to do some testing of fuse circuits. You do not need to remove the fuses to do this, there are test points in each fuse. Just place another multimeter across, or an amp-hound or similar device if you have one. There are different types of testers available on eBay. I found a slight problem in one of the rear window motors, but that was it, so I live with it, as it is only about a 5-10 mA draw, so not really an issue.

 

[John77]

A lot of Mercedes battery drains are due to systems not shutting down. If you use the method on that website you might not find the drain as you have to disconnect the battery to put the meter in series. This means the systems lose power and switch off...
A more subtle way is to measure the voltage across the fuses. It’s only a small voltage, but any system drawing current will drop some volts across the fuse which you can see with a Digital Multimeter. This method means you don’t have to disconnect the battery or even pull any fuses.

Thank you and you do this how exactly?

 

[Blobcat]

Thank you and you do this how exactly?

With the car off, fuse box open, DMM on DC Volts, one probe on one side of the fuse and the other probe on the other side of the same fuse.
Blade fuses have small cut outs in the top that you can connect to with the probes.

 

[John77]

My w220 is sluggish to start if I leave it for a week. Bear in mind the alarm, tow away etc are all live when the car is not in use. You mention you have a relatively new battery is this the correct spec for the car, it could be an el cheapo battery fitted by th

My w220 is sluggish to start if I leave it for a week. Bear in mind the alarm, tow away etc are all live when the car is not in use. You mention you have a relatively new battery is this the correct spec for the car, it could be an el cheapo battery fitted by the PO.

The battery is a Exide premium Car battery, so no not a cheapy.

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