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Thread: battery charging rates at idle and high speed

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    Regular Member justin32's Avatar
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    Default battery charging rates at idle and high speed

    Following on from my prevous thread about non compatable alternators i decided to test my battery charge at idle and while driving.At idle the battery is charging at 13.8v and the same regardless of speed i.e. 30MPH or 80MPH.

    My mate who has the same car has different readings,his car charges on idle at 13.9v but while driving its charging at 14.5v and if he goes fast the volts drop off to 14.2.

    Which results are correct as i suspect i have rong/non compatable alternator and i want to be sure before i try and take it back.

    Thanks

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    Regular Member Critch's Avatar
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    Does charge drop when u turn load items like head lights. Rear window de most etc?

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    Regular Member ftm100's Avatar
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    http://www.bcae1.com/charging.htm the basics of a car charging system might be a bit of help to you

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    Regular Member justin32's Avatar
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    Quote Originally Posted by CritchvxD View Post
    Does charge drop when u turn load items like head lights. Rear window de most etc?
    Only by 0.1 of a volt.I went for a spin earlier to get the little'un off to sleep and when i brought up the meunu on the BC it said it was charging at 12.5v with the engine running.

    Voltage drops to about 11v when i crank the engine to start the car up......is this normal?

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    Regular Member TonyP2020's Avatar
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    The link above provided by ftm100 is good.
    The 3 phase output of the alternator needs 6 diodes to turn it into (a bit rough) DC.
    1 stator coil open or short circuit will reduce the available current by 1/3.
    1 diode open circuit will reduce the current available by 1/6.
    2 diodes open circuit will reduce the current available by 1/3.
    This will result in a lower voltage being available to power the car.
    A good auto-electrical person (or someone with an oscilloscope) should be able to tell if there is any problems, but dont rule out a slack drive belt either.
    Also the diameter of the alternator pulley is important.

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    Grab a jump lead and stick one clamp on the engine and one on the suspention top bolt / something on the body and see what your readings are.(dont drive like this though )

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    Regular Member Exclusive BV's Avatar
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    Quote Originally Posted by roswell View Post
    Grab a jump lead and stick one clamp on the engine and one on the suspention top bolt / something on the body and see what your readings are.(dont drive like this though )
    You suspecting a bad earth?

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    Regular Member justin32's Avatar
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    Quote Originally Posted by TonyP2020 View Post
    The link above provided by ftm100 is good.
    The 3 phase output of the alternator needs 6 diodes to turn it into (a bit rough) DC.
    1 stator coil open or short circuit will reduce the available current by 1/3.
    1 diode open circuit will reduce the current available by 1/6.
    2 diodes open circuit will reduce the current available by 1/3.
    This will result in a lower voltage being available to power the car.
    A good auto-electrical person (or someone with an oscilloscope) should be able to tell if there is any problems, but dont rule out a slack drive belt either.
    Also the diameter of the alternator pulley is important.
    The problem is i dont know the output of the alternator.I purchaced it from my local auto factors and did a swap for my old unit which the matey put into the box.I do have a part number but when i googled it nothing came up.He assured me that although it had "drivetec" on the lebel it was infact a more expensive model with thier own braned label on it.he did say it was definatly for my make and model or car and to be honest ive brought loads of parts from them before and never had a problem

    The belt seems to be fine and not slipping,it was hard work getting the tensioner locked(using a ratchet handle and makeshift extension bar) so i think that its nice and tight.

    Quote Originally Posted by Exclusive BV View Post
    You suspecting a bad earth?
    Funny you should say that because i was only reading a post on here the other day about the extra earthing locations on the engine mount and also on the suspension top(looks like a small 10mm nut) i have both of these so will try it in the morning.

    What should i look out for when checking the voltage while doing this? a rise or drop in voltage?

    Thanks

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