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Statek

X3 M54 cranking but no start

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Hi, 

As it always seems to be, the moment I start thinking about selling a car it begins to play up.

My wife's 2006 X3 E83 2.5 (M54) battery has started to become weak, as witnesses by slow occasional slow cranking. Yesterday I drove the car to the refuse centre (not to dispose of it, just yet 😅) and when I jumped back in the car it was cranking fine but wouldn't start. I tried locking/unlocking the car, getting in and out, with no success. Even though cranking was still strong, I put a jumper pack on it and the car started straight away.

I trickle charged the battery overnight, and car start great this morning, so I drove it down the road to the bakery. 3 minutes later the car is cranking again, but no start. Connected the jump starter and it fired up straight away.

Is this a symptom of a weak battery? The little inspection window shows 'green', but I'm making the assumption the battery just doesn't have many CCAs left in it.

Any thoughts?

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Just theorising... but if the car cranks over like normal but doesn't fire, maybe the ECU (or ignition system) requires a certain minimum voltage to operate ? Battery voltage drops substantially while cranking and if the battery is getting tired the cranking voltage may be dropping below that threshold. Adding the jump starter will lift the voltage above the threshold and it fires right up. Could be worth checking with someone who knows more about BMW electronics than I do :)

Cheers...

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58 minutes ago, jon dee said:

Just theorising... but if the car cranks over like normal but doesn't fire, maybe the ECU (or ignition system) requires a certain minimum voltage to operate ? Battery voltage drops substantially while cranking and if the battery is getting tired the cranking voltage may be dropping below that threshold. Adding the jump starter will lift the voltage above the threshold and it fires right up. Could be worth checking with someone who knows more about BMW electronics than I do :)

Cheers...

That's exactly what i'm thinking @jon dee - would be keen to get confirmation from someone who experienced this before, or like you said, knows more about BMW electronics then myself 😅 I hate replacing parts by guessing...

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A bit of poking around on the internet suggests that the DME cutoff occurs around 9 volts, and if your battery is on its last legs it would be getting down there during cranking. The starter digs deep to get the engine spinning and after that the cranking voltage will recover a little, but by then the DME has likely latched the "I'm not going to start" relay.

When the battery is weak two things happen. The starter being a DC motor, draws a certain amount of power (volts x amps) to do its job. So when the voltage gets lower the amp draw goes up and this equates to a bale of hay being thrown on the Camel's back :(  The second thing that when the battery plates are sulphated the battery cannot be charged properly. The battery gets a "surface charge" and when tested with a voltmeter it will read as fully charged, but as little as one attempted start will kill it dead.

My "smart" charger has a "rejuvenation" setting that is claimed to overcome the sulphation problem by pulsing the battery for 24 hours with different voltages according to some secret algorithm. I have just re-installed the 10 year old Jap battery in my Mitsi after having it die on me a couple of days ago. Tomorrow I will see how effective the rejuvenation has been :)

Cheers...

Edited by jon dee
Grammar...
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@jon dee sent my wife to get a Wof for the x3 and asked them to test the battery - they show that the battery is still pretty good but they suggest charging it as well.

I'll get my hands on a proper trickle charger and give it a go. 

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Standard type car batteries (not being deep cycle design) don't like being run flat or near flat frequently. They like to be always charged to 85% full or more and that means having a decent commute (like 20km each way) or getting at least one decent run every week if not in daily use. After killing a couple of new batteries by running them dead flat with an old style high drain car alarm system, I hooked up a solar panel battery maintainer and that was enough to offset the alarm system drain. 

Charging a battery is always a better option than driving around aimlessly for an hour hoping that it will charge up enough to start next time. If you have a car that gets little use, I highly recommend a battery maintainer. That will keep your battery fully charged and when you do use the car it is ready to go :)

Cheers...

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