2001 E39 525i battery/alternator/ charging system

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  1. 2001 E39 525i battery/alternator/ charging system 
    #1
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    rjc01's Car Details
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    Hi there

    I have an E39 525iA 2001 saloon.

    I had a discharged battery problem in April this year. I checked for a drain and after the system entered the sleep mode, I measured 20mA. The initial reading of the alternator output at the battery connections was 14.15v at idle. I therefore concluded the battery was at fault as it was 8 years old and after recharging on a number of occasions was again empty after a short time.

    My local UK BMW dealer provided me with an OEM replacement (Varta) and I fitted it in April.

    I was surprised to see the battery indicator being black after a daylight drive, from the UK to Germany, with only the air conditioning being used for over a 350 mile journey.

    I have not had any starting or electrical problems whatsoever but visited my local BMW dealer in Germany regarding another problem. They informed me that the alternator was fine. They used a Bosch tester on the battery and confusingly stated that the battery had a 102% starting capability but that the battery required urgent recharging. The technician I have always found to be very competent told me not to worry about it as the starting capacity was the crucial specification to consider.

    I further checked the alternator and again at idle produced 14.1v but fell to 13.5 when I activated the headlights and heater blower. I was under the impression that the constant voltage regulator fitted to the E39 was to maintian a constant 14v charge in all conditions or would it to be expected that this charge rate would fall at idle to 13.5v?

    I'm curious as to the explanation and any advice or comments would be appreciated as if there is a fault with the battery I will obtain a replacement from the UK outlet?

    Many thanks.
    Last edited by Joe9087; 07-11-2009 at 01:43. Reason: title amendment
     
     

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    #2
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    coupe-boy's Car Details
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    Does the battery light come on and then go off when you put the ignition on?
    From what I remember the light actually plays a part in the chaging system as it makes one of the circuits.
    Also make sure that when you are driving that the battery light is off, may sound daft but I had a similar problem years ago with a different car but when you were driving at night you could see the battery light was on but it was very faint.
    Turned out that the battery/alternator were fine it was just a diode (or something) in the actual alternator causing the problem.
     
     

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    hi coupe boy. yeah all warning lights function normally and there is no faint glimmer either. I know exactly what you mean, my E30 had a charging problem and you could only see the warning light glowing v. slightly in the dark. It was also the diode pack at fault. I had it double checked on an oscilloscope and I just installed a used alternator to solve the problem but thanks fo the suggestions anyway.
     
     

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    Dr of BMW gildersol's Avatar
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    Do you mean the battery indicator on the battery was black? That doesn't sound good after a long run. Is the battery ok now?
    I consider the technician to have meant that the battery was healthy although required charging.
    Your reading from checking the charging voltage of the alternator seem acceptable to me as the voltage will drop a bit under load at idle. You would want to check the charging voltage at 2000 rpm with the load applied. It should then maintain at 14v+.

    Difficult to say much more, however, a perfect new healthy battery with indicator (is it green for healthy?) should not be black at any point.

    I've had a brand new Bosch battery fail after a week with the green indicator not showing and took it back for a replacement.
     
     

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    #5
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    Thanks for the reply.
    Yeah the "magic eye" indicator was black after a daylight run of 350+ miles and that was the only reason I started to investigate it because there is no deterioration with the car starting and no other electrical malfunction at all. The indicator is still black now and after using a battery charger after 1 day it's black again but theragain without deterioraton of performance.

    As far as I understand:
    green=adequately charged
    black=needs charging
    yellow=kaputt!

    When I explained the situation to the techi at the BMW dealer in Germany he tested it with a Bosch tester and it showed a 102% capacity but also read urgently charge, which confused me. He concluded it didn't need recharging as it would not give a 102% reading otherwise and said that the %reading was the crucial indicator to assess the performance of the battery. He said there was no obvious defect bearing in mind I have no electrical or starting problem.

    They also checked the alternator and said likewise there was no problem. I'm not sure if they connected the DIS to complete the tests or just used a multimeter.

    When I checked with the multimeter I obtained the readings in my first post and at 1500 rpm it read 13.5v. I carried out the check directly on the battery terminals and also on the charging points in the engine compartment with only a 0.02v difference in all results overall.

    There would be no problem replacing the battery if it is defective as I bought it from a UK BMW outlet in March this year but I just want to be 100% sure there isn't a charging problem otherwise any new battery will end up in the wastebin.
     
     

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