E46 £20d battery going flat.

I have 320d which is just over 2 years old. Twice in the last month I have been unable to start the car as the battery has gone flat. I have not driven on any particularly long trips, but I would have
thought the journeys were long enough to charge the battery. I had the battery tested (turns engine for 15secs continuously) and the charging circuit has been tested ok so the alternator is good.
Last year I never had this problem. So, is this likely to be my fault as I am not keeping the battery charged, or do the batteries give out after a few years.
I'd appreciate any comments on the life of batteries in new 2L diesel cars. Thanks -- John Perry
http://www.redoak.co.uk http://www.eze-buy.co.uk
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Batteries just wear out. Their lifetime is hard to predict, but it does seem that 2 years is rather short.
The only effect of not taking long trips is that you are consequently starting the car rather more often. Hence, you will wear out the battery sooner.
What size of battery to they install in the diesel cars? Diesel engines are much harder to start than gasoline ones.
Jim
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Yes agreed

The 3 series petrol models all get 70AH batteries except the smaller cars that get a 55AH battery. My 2L diesel has a 80AH battery; the 3L diesel, my next car, gets a 90AH battery.
Your comment about diesel's taking more power to start is I think the key; it is more important to keep a battery in a diesel car fully charged.

-- John Perry
http://www.redoak.co.uk http://www.maytrees.co.uk
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How long was the car unused for when the battery was flat?
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Tue, 02 Jan 2007 08:52:27 +0000 (GMT), "Dave Plowman (News)"

Over two weeks, I'd used the car for maybe 5 journeys each of which were for no more than 3 miles. I was on holiday so didn't take it on the regular 12 miles to work and back each day. -- John Perry
http://www.redoak.co.uk http://www.maytrees.co.uk
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I'm afraid that's the cause. Modern cars have quite a high quiescent current consumption - things like the alarm system - and a couple of weeks can be pushing it, especially if the battery isn't fully charged to start with.
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wrote:

I am beginning to think you are right.

By the vehicle that came out. He rapid charged it and then got the car engine running and confirmed the alternator was charging at over 30 amps at tick over.
He also checked the current drain on the car when it went into sleep mode and it was zero amps.

He did this test by disabling something that prevented the car starting. This was after the engine had been running for 15mins after the battery had been flat.

Thanks...have to admit I'm beginning to think it was my fault for not keeping the battery charged <embarrassed face>
Thanks for comments everyone

-- John Perry
http://www.redoak.co.uk http://www.maytrees.co.uk
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Then he didn't measure it correctly. Nor can you do this accurately with a clamp meter as it will be a fraction of an amp. But all cars these days use some current when switched off. 0.05 of an amp (50mA) isn't uncommon.
The battery is approx 70 amp/hours capacity. Leave the car unused for two weeks and theory says 0.2 amps (200mA) continuous drain will kill it - and a battery won't start a car if run down by approx half this amount.
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On Thu, 04 Jan 2007 00:48:46 +0000 (GMT), "Dave Plowman (News)"

It was a clamp meter, so yes not accurate enough, but the current dropped abruptly after about 20secs when the car went into sleep mode.

You are dead right!
200mA over 1 hr is 2A over 10 hrs is 5A over 1 day is 35A over 1 week, ie half the battery capacity!
So are we saying that if we leave our cars unused for a week, we could get a flat battery? or is the sleep mode current drain less than 200mA -- John Perry
http://www.redoak.co.uk http://www.maytrees.co.uk
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I'd say it will be something between 20 - 50mA. So with a less than fully charged battery it's quite possible it won't start after two weeks - especially in cold weather.
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On Fri, 05 Jan 2007 01:06:46 +0000 (GMT), "Dave Plowman (News)"

That has been my experience. It's shocking (pardon the pun) that I can leave my car at an airport for two weeks and it may not start when I return. The car maker shrugs his shoulders in response to criticism of his car in this respect, and offers no useful suggestions.
Active alarm systems - as opposed to passive ones - are the main culprit, I suspect.
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Dan.

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Yes - I've seen this reported.

I suspect it's additive. Many things are actually on standby in the modern car even with it switched off. The radio memory. Remote control receiver. Alarm. Central locking controller. Interior light dimmer. Then we go on to things like the engine and gearbox ECU which may store parameters in a volatile memory. Only a few milliamps each if well designed, but...
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