My 83 yr old mum is driving me mad with this!

[belfry]

My mother has a 2002 Merc SLK 320 auto. She calls every other day in a right grump about her car. She is getting to the point where she says she doesn't trust her car and wants a new car.

The battery goes completely flat if she doesn't drive the car every day. So flat that the dash lights don't illuminate and the interior lights don't come on.
I have done the normal parasitic drain type test using a multi meter between the negative battery terminal and the neg. battery lead. The results show zero (occasionally 0.01) drain. I have been able to find parasitic drains using this method before by removing each fuse in turn until the drain drops to almost Zero. On my mother's car there really is no parasitic drain and I have the car at my house the car overnight and checked every Four hours. I cannot find any drain and the car starts in the morning without issue.

I am sure that she is not leaving anything on in the car overnight.

I put a new Halfords battery in in January which didn't cure this issue. However, could the issue be a faulty battery rather than the electrics in the car?

Angry mothers are quite stressful, especially as I am shielding during lockdown.

Any ideas?

[steveo3002]

yeah certainly worth having it checked , is it charging properly too?

[paintman]

Might be worth a diagnostic to see if there are any stored codes, faults or issues with the charging circuit.
Faulty battery is a possible.

FWIW my father bought a Mini Clubman estate automatic in the early 70s.
Brand new car but every time there was thick fog overnight the battery would be flat in the morning.
Car was doing around 40 miles every weekday as I was using it to take my brother & I to school & back.
In & out of the dealers like a yo-yo but nothing made any difference.
After a year of this got he got fed up with it & it was p/exd with the dealer.

[GreenV8S]

You don't mention checking battery voltage when running, while parked, when the problem happens. That would tell you a lot about the health of the battery and charging system.

[geeks]

Once you have checked all the charge states etc and the issue is only happening when she has the car and not you then it is time to see what she is doing when parking up, is she leaving something on perhaps?

[E-bmw]

GreenV8S said:

You don't mention checking battery voltage when running, while parked, when the problem happens. That would tell you a lot about the health of the battery and charging system.

^^^^ Wot 'e said.

[belfry]

Voltage when running is 12.5v. 13.8v when first started.

[jeremyc]

She's not parking with the indicator stalk in the up or down position is she (perhaps having made a turn into a drive or similar just before parking)?

This might leave the parking lights on (front and back) for the relevant side as is/was common on many cars.

[eybic]

belfry said:

Voltage when running is 12.5v. 13.8v when first started.

I'd say it should be higher when running. 12.5v when not running would be ok but it should increase with an alternator charging it, certainly higher than 12.5v

[Mr E]

eybic said:

I'd say it should be higher when running. 12.5v when not running would be ok but it should increase with an alternator charging it, certainly higher than 12.5v

Yes. 12.6 when charged and engine off.
14+ when engine on and charging.

[geeks]

Mr E said:

eybic said:

I'd say it should be higher when running. 12.5v when not running would be ok but it should increase with an alternator charging it, certainly higher than 12.5v

Yes. 12.6 when charged and engine off.
14+ when engine on and charging.

Depends if it has a smart alternator doesn't it?

[Mr E]

geeks said:

Depends if it has a smart alternator doesn't it?

True. But on a 20 year old SLK?
Last battery I checked was on a K-series. That doesn’t have a smart alternator.

As it turned out, didn’t have an working alternator at all.

[geeks]

Mr E said:

geeks said:

Depends if it has a smart alternator doesn't it?

True. But on a 20 year old SLK?
Last battery I checked was on a K-series. That doesn’t have a smart alternator.

As it turned out, didn’t have an working alternator at all.

Good point hadn't considered that!

[Penelope Stopit]

belfry said:

I have done the normal parasitic drain type test using a multi meter between the negative battery terminal and the neg. battery lead. The results show zero (occasionally 0.01) drain

Any ideas?

Suspect a problem with the ammeter

[tapkaJohnD]

"I have the car at my house the car overnight and checked every Four hours. I cannot find any drain and the car starts in the morning without issue."

But the battery drains when she has it. She has to be leaving something on, not closing the doors?

John

[M4cruiser]

tapkaJohnD said:

"I have the car at my house the car overnight and checked every Four hours. I cannot find any drain and the car starts in the morning without issue."

But the battery drains when she has it. She has to be leaving something on, not closing the doors?

John

Agree with this. Sometimes you just have to be there and watch what the user does. Maybe she leaves something on which would turn off if she locked it, but she doesn't lock it?

[belfry]

Yes, she never locks it!

[DaBuGi]

That could be the issue on some cars locking the doors is the final part in the shut down procedure. So she could be leaving the system awake causing a drain while she has it.

[Penelope Stopit]

"The results show zero (occasionally 0.01) drain"

No way, this isn't possible, there's something wrong with a zero drain machine

[kev b]

Zero parasitic drain seems suspicious, try another ammeter?

I suspect the problem may be the car is not entering sleep mode because it is left unlocked, parking light is worth checking too though.