E46 £20d battery going flat.

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

How long was the car unused for when the battery was flat?

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

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

Thanks for comments everyone

-- John Perry

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

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.

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.

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

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.

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.

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...