Battery drain problem!

be logical, check how much the current drain is using a digital meter. then disconnect things till you find the source . first thing would be radio, then things like alternator.

mrcheerful

All I can think of is leaky diodes in the alternator. The current would rise non linearly with battery voltage and particularly temperature, so there could be a high drain while the engine is still warm and just switched off, itself causing self heating of the diodes (prolonging the problem) as there is about 20 times the dissipation in the reverse direction, reducing to normal near zero current when cooled off after a couple of hours. Test your current drain with a hot engine just turned off.

Their is a drain of 0.03 - 0.05 amps when every thing known is switched off. This is not the problem, at that level it would take about 4 months to flatten the 100ampHour battery.

I suppose i Could keep disconnecting things when the car is parked up but since the car is used several times a day and the battery drain is not apparent in the short term , also their is all manner of electrical stuff on this car, so this is not really a practical tactic!

I thought you were talking about a longer leave time. If the car is in multiple daily regular use and the charging circuit works ok then you could even leave the lights on and the battery would still be ok.

It sounds as though it is not charging correctly.

What voltage is across the battery when the engine is running and the charge light is out?

mrcheerful

Specifically, how are you testing the battery? Are you simply putting your ammeter across the battery terminals while the battery is still connected to the car?

Rob Graham

For Amps disconecting the posative lead from the battery terminal and then reconnecting the battery to the car through the amp meter

For volts, leaving battery attached and taking the measurement at leat 5 mins after the last know drain was switched off!

I think this would melt the ammeter!

Unfortunately it is draining with the engine cold. When I put the charged battery (12.75v) on this morning at 9am the car had not been run for about

12 hrs. The battery went from 12.75 to 12.53 in 3 hrs then to 12.42 in a further 7 hours, its now another 2 hrs on 12.39v

agreed

at 2000 rpm its giving out 13.9v with headlights, heated rear window, blowers etc, all on. Which seems OK

On the other hand I don't know if it is charging properly, even on long journeys it never gets the battery above 12.5 volts, where as, I put the car battery on my van at 12.2v this morning, I have done about 40 miles in the van today and the battery is now up to 12.65V

Then again, I have had three experts tell me the charging system is normal. Albeit, the man from Mercedes noticed the 100ah Varta battery, and did suggest that the Non Mercedes part could be my problem, he suggested I should buy a Mercedes battery (£140), I suggested with a stupid comment like that he wasn't a real mechanic, he proved his qualifications by sending me a invoice for £80 for his opinion, (I havn't paid it yet and don't think I will)!

You havn't explained exactly what's the problem. Once battery voltage is above 12v it means f--all. It depends on battery and vehicle as to what voltage reading you will get with a multimeter at any particular time. What is wrong with the car? How often does it fail to start? How long is it idle before there is a problem?

John

To eliminate any fault with the alternator I would leave the alternator disconnected overnight and then check the battery.

OK ...... First of all, I was very much under the impression that battery voltage was a good indicator of level of charge, 12V is a flat battery, 13V is fully charged! Am I wrong ?

The problem ..... One day back in Feb the car did'nt start, didn't even turn the engine over, fitted new battery, a week later same problem, the wife issues a warning, "I need something reliable" ..Since then on a few occasions the dashboard lights have dimmed as the car just manages to start, luckily half a turn fires it up. I am now faced with brochures for a CLK. There has to be a simple reason for a cars battery not functioning properly, and the dealership along with other experts have been worse than useless!

From what I have read from other posts on this group, I like the theories on the alternator, I will try disconnecting this when the car is not in use, see if this cures the battery going flat problem!

Alternator is fine then.

Thats fine.

And so close to 12.5V as to be counted as the same.

How are you measuring current? Not calling you stupid but I have known people try to measure current drain by sticking the multimeter probes across both terminals instead of disconnecting one of the terminals and sticking one probe on the now disconnected terminal and the other on the battery terminal it was fitted to.

Yup to a point.

Do you do mainly short journeys under 10 miles? Don't say "yes I do but it takes me 30 minutes" because the alternator does bugger all under a certain amount of revs.

Yup, i'm doing it just as you say!

OK. Just testing.

Rob

Yes, lots of short journeys all within a built up area! I must add here though, we have had this car for five years now and the type of journeys has not changed, for the first 4½ years faultless service, it is just since Feb this year that the battery problem has arrisen.

Also, using the batteries voltage as an indication of charge, my numerous mesurments are showing that the car is charging the battery even on very small journeys. for instance

8am battery at 12.2v short 2 mile journey, now 12.31v 1pm now at 12.25v short 2 mile journey, now 12.36v 6pm 12.22v 15 mile journey up to 12.4v 8am next morning 12.15v

(all measurements taken about 20mins after use to allow for things to settle down)

Every day the voltage is less than the day before until the car just manages to start, at which point I stick the battery on a trickle charge and bring t up to nearly 13v

My van makes very simillar journeys and its battery is constantly at 12.75v (a little less on a very cold morning)

if the battery is the same one for the last five years of use then you need a new battery, simple as that.

mrcheerful

Your obviously not keeping up here :-)

The battery was the first thing we renewed!

In fact this car is on its 3rd baattery, the first one lasted nearly 4 years, the second battery was little over a year old when we replaced it in Feb, I doub't there was anything a matter with that battery now!