Battery keeps going flat

[Una]

Hello everyone.
I have a question concerning my 2005 SLK 200. My battery keeps going flat. It's a new battery, so I took it back to the store where I bought it and they checked and said it was in perfect condition.

If I could explain, I am a new widow who hasn't got a clue about cars. My son put the battery on charge and everything was fine for a while. The battery went flat on me on three other occasions, the alarm would go off and drain the battery. I took it to the garage when small electrical things started to go wrong. It turned out that the seal in the driver's door had completely broken down and water had got into the electrics under the driver's seat. That job was costly, but after having the door seal fixed, the electrics dried out, everything was fine again. Until last Wednesday, when it rained. The battery was flat again.

It seems, on reading posts on here, that it's a common fault for batteries to go flat.
Where do I go from here? Should I buy a battery tester thingy and take out the fuses one by one to find what's draining it?
Also, I don't go on long journeys and usually only use the car once or twice a week. Short trips to the shops. Is that where I'm going wrong?
Any help or advice would be greatly appreciated. Thank you in anticipation.

 

[Salal]

Although it is common but it shouldn’t really happen and when it does it causes you a nuisance as your currently experiencing.

You definitely have a somewhat significant draw as your battery shouldn’t get depleted in that space of time as your saying, it shouldn’t really go flat where you won’t be able to start the engine again in the space of a week.

Is the new battery that you bought the correct capacity/size?

The best course of action would be to do what you mentioned. You will need a multimeter that can measure Amps and connect it in-line between one of the battery terminals and the battery post itself. I am not too sure on an SLK but I now on most other Mercedes-Benz if I remember it be around 50 Milliamps or less. You might have a current draw of 100-200 Milliamps currently if your battery dies within a week or so.

Problem is as well it can be much more difficult finding certain current draws nowadays with data buses such as CAN bus when some lines being awake or at sleep (on or off) at different times or together. Some modules stay awake for a little while and other modules being awake simultaneously until the databus switches off and if you pull one fuse you may isolate another system where the problem lies and you might not find it then. There are different methods that can be used to help find the fault.

Also is the car standard and has it had any aftermarket accessories installed, sometimes this can cause it too. And are you sure that there is still nothing else that is wet in the area or around it where it was leaking as it could be related?

 

[MikeInWimbledon]

Take the car to a car electrician, not a generalist garage. A new battery should certainly last five years, while a Mercedes battery can easily last ten years.

From your description, there "may" be something wrong with the circuitry somewhere. (Usually because some idiot has tinkered with the wiring) Taking fuses out is a recipe for mistakes and complications if you're not comfortable with handling car electrics.

However, if you're driving less than ten miles a week, with only four starts, you'll be running the battery down, but there's no simple solution to that, short of putting the car onto trickle charge overnight every few months. (To see what mileage you're doing, compare the mileage on your last MoT with the mileage on the car today).

A car electrician can wave his wand over the car and find if the car does have a fault, or advise you how to top the battery charge simply because you're using it too sporadically.

 

[Una]

Thank you both for your replies. I may take it back to the garage that found the damp in the first place. Perhaps I can get them to have a quick look to see if everything is dry. I don't feel confident enough to mess with it myself.
I just wondered if anyone else has had the same problems. My son keeps telling me that I don't drive the car enough and the short journeys I do is draining the battery.

 

[mbcaner]

Hello and welcome.

 

[thebiglad]

Hello Una and welcome in to the forum. It definitely sounds like something is drawing current from the battery whilst not in use.

When your car is parked up at home, is it near to an electric outlet so you could run an extension cable and put the battery on charge, when not in use ?

I had an S-Type Jaguar a few years back and could never get to the bottom of the new Varta battery going flat, so eventually I decided to disconnect the battery negative lead, when the car was not in use. Never had any further problems

 

[optimusprime]

Welcome Unato the forum The car will need 15 mins of good driving to put back in to the battery whats taken out in starting the car up. Short runs wont help .

 

[MancMike]

There's a few possibilities. The battery may be too small for the job, despite being in good condition, the alternator could be goosed, or there's a wiring problem leaking current.

Given what you're saying about it being connected to water ingress, it seems there's somewhere else where water is causing a short to ground.

 

[190]

The car will need 15 mins of good driving to put back in to the battery whats taken out in starting the car up. Short runs wont help

I agree that short runs won't help and specifically won't put back into the battery what is lost by a car not being regularly used. A battery charger may need to be used every now and then to keep the battery fully charged.

If it were true though that it takes 15 mins to put back the energy consumed by one operation of the starter motor then stop start as a concept couldn't work. I once did the sums on a petrol engine and concluded the starting loss was made good in a few hundred yards. That assumes instant starting which ever modern petrol engined car should do.

 

[Alex225]

Hi Una, firstly sorry to read that you have recently become a widow.

With regards your car, which model is it? I have an E Class (2003) which hates sitting around. Even with a new battery, after 2-3 weeks with a couple of cold snaps it will show me the convenience features are switched off until it’s driven for 10 minutes+.

I don’t know if my car has a battery drain but I know when driven every 1-2 days it’s fine. May be worth looking at a trickle charger if you have the option.

I use a C-Tek charger with a convenience plug. Basically means it has a a socket connected to the battery so you just plug in to the car and to power. Saves faffing connecting direct to the battery.

 

[MikeInWimbledon]

My son keeps telling me that I don't drive the car enough and the short journeys I do is draining the battery.

It would help if you could give us a rough idea of how many miles and journeys you're doing.
How many miles has the car done since it's last MoT, so we can work out average mileage per week.
And how many separate journeys each week.



My wife does 2,000 miles a year, call it 40 miles a week, usually with around eight separate stop starts. Her car is still on its 14 year old battery.

I used to have a couple of SL's which I would leave in the garage unused for a couple of months at a time. They would always start after being left, but you could tell their battery had become drained from lack of use. Both cars had their original over 10 year old batteries.

 

[Una]

It would help if you could give us a rough idea of how many miles and journeys you're doing.
How many miles has the car done since it's last MoT, so we can work out average mileage per week.
And how many separate journeys each week.



My wife does 2,000 miles a year, call it 40 miles a week, usually with around eight separate stop starts. Her car is still on its 14 year old battery.

I used to have a couple of SL's which I would leave in the garage unused for a couple of months at a time. They would always start after being left, but you could tell their battery had become drained from lack of use. Both cars had their original over 10 year old batteries.

Welcome Unato the forum The car will need 15 mins of good driving to put back in to the battery whats taken out in starting the car up. Short runs wont help

 

[Una]

Hello and thank you everyone for your replies.
My son put the car on charge on Sunday 14th October and left it on for 24 hours. The car wasn't fully charged (as the charger indicated), but it started and on Tuesday 16th, I went on a five mile journey stopping once (grocery shop). On Saturday, 20th the car door wouldn't even open with the key fob. It's now been on charge since Saturday morning and I don't intend using it until tomorrow.
I need to make a 60 mile return journey. My son is not happy about it. He says I may not be able to start the car after the first 30 miles.
I have breakdown cover, but I'm nervous because I can't trust the car. I checked inside the car to see if there was any damp under the driver's seat, but couldn't find any. The alarm didn't sound this time. The car was just dead. There are no indications on the dash to say that something electrical is at fault.
I probably do about 1000 miles per annum, maybe less. Just a weekly or maybe fortnightly grocery shop, which means just about 10-12 miles. Occasionally I visit a friend who lives 10.4 miles away. So as you see, I hardly use the car at all. My trip tomorrow is unusual, but I thought it would be advantageous to travel that far, but my son has put the beejesus up me now.
Perhaps I should just get a new battery? The one in the car now is a year old and is the correct size. THEBIGLAD mentioned disconnecting the negative battery terminal after each journey. At least now I've been shown how to attach the battery charger, so I know which is positive and negative!!!
I am at a loss. I rely on the car even though I don't travel far. When my husband passed away, I suffered from agoraphobia. I am trying to battle this, so I do need the car. Hopefully, after having three full days of charge, the battery will be fully charged.
Once again, thank you all for your advice.

 

[190]

I think you are adopting the correct strategy by fully charging the battery which really can take several days. If the problem persists after that then it's either a faulty battery, or a current draw that shouldn't be taking place with the car switched off. First and cheapest step would be to have the battery tested. If it's not the battery then you need a good auto electrician to track down the fault.

 

[MikeInWimbledon]

Thanks for the clarification, Una. That kind of usage certainly will flatten a battery.

I still think you should get a relative take the car to a car electrician, not a generalist garage. A specialist car electrician - they're easy to find - they often operate on a mobile basis: a man and his van.

A car electrician can wave his wand over the car and find if the electrical circuits do have a fault. It's a separate piece of work, if required, to track down the cause.

Chances are, you've just got a completely flattened battery, but an electrician can put your mind at rest. In the overall cost of running a car, it's not expensive.

If you were a bloke, I'd tell you to recondition the battery using something like a CTEK, but in your case, let's keep it simple

 

[stephenmercedes]

I have a car which use to be my sole form of transport, but since acquiring my Mercedes earlier this year, it is rarely driven. When I did take it out, the battery (which was brand new this year) was always drained when I went to use it. There is definitely something 'wrong' as it shouldn't drain that quickly, however, out of sheer frustration, I just initiated a workaround. I now have the car on a permanent charge, as seen in the photo.

The charger charges the battery to full charge, then when reached, it simply maintains that charge (12.8v as seen on charger display) so that whenever I want, I know I have a car that starts. I even keep the alarm on as the charger compensates for the current drain, and fully charges and maintains. (The steering wheel sign is evidence of a fading memory and to stop me driving off absent mindedly without disconnecting first LOL). The 'jump leads' are used just to act as an extension lead for the charger to car, and a bit of safety floor rubber to stop the postman from tripping!

 

[grober]

You have a 13 year old Mercedes convertible with a recurring/intermittant electrical fault when it rains. You are on your own and need a reliable car to get out and about. You are not mechanically or electrically minded so DIY is out. I think you know where I am going with this? By all means get the car checked out by an electrical specialist equipped to diagnose Mercedes systems but if that fails- I would be looking for a replacement vehicle.

 

[Major774]

This is all pointing to a battery draw.

It needs to be diagnosed by an auto electrician or you can DIY diagnose if you are handy with a multimetre.

 

[Chrishazle]

Question 1 - do you have mains power easily available where you keep the car? (Garage?)
Question 2 - you obviously know where the battery is and which terminal is + and -, so if the answer to 1 is yes, I'd also suggest a battery conditioner that you connect to the battery every time you put the car away. Cheapest option if they still have them is your local Lidl, £13, my local one had some last week but they're only occasionally available. Failing that, the cheapest CTEG conditioner will cost about £45.

FWIW I have a Sunday toy - Porsche 968 cab - that gets parked up for anything between a day and 3 months. I always connect the conditioner when I put her away in her garage, and the only time she's failed to start was when the battery had cratered, and the conditioner sort of warned me of that as, when I turned the mains off, it indicated 2-3 reds instead of 3 reds and 2 greens, so I knew something was not right. Other than that, this is what I have done for 7 years with the cab and 12 years with the 968 coupe I had before that. I'm also convinced that it seriously extends battery life, as the battery on the coupe was 10-11 years old when I sold the car, and still going strong.

OK, this will not investigate the cause of the battery drain, but it will ensure that the car will start (at home, anyway!).

 

[stephenmercedes]

Anyone else curious how the OP (Una) got on, or just me?