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  1. #1
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    Alternator or Battery ?

    I've noticed on most occasions when I start the '94 M3 with a warm engine it fails to fire-up the engine on the first attempt. First thing in the morning when the engine/battery is cold has never been a problem. I've being thinking fuel supply, spark-plugs?

    today I had to crank the starter motor at least 5 or 6 times to eventually get the warm engine to start up (so I assume the battery had plenty of AmpereHours in it).

    I did notice this time when I turned on the Alpine Radio it had lost all it's saved settings and stations, a sign of loss of supply voltage to the radio. So I'm now thinking the extended number of starter motor crankings had lowered the battery voltage long enough for the radio to do a power-reset.

    So is the battery I've got shot or is the M3 alternator not generating the right charging voltage?

    What voltage should I be seeing on the battery terminals when the alternator is charging it, 14V with close to 0 AC Volts on it?

  2. #2
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    BMW Battery ????

    The Malayasian aftermarket battery that is currently in the beast (the first cell always loses water out the drainage hole) is going.

    I've noticed over the past 18 years BMW batteries seem to last for almost a decade in the Australian climate, so I'll get a Genuine BMW battery but which one? BMW and my Indy Mechanic say there are 2 suitable for my 94 M3, a 65Ah for AUS$280 or a 75Ah for $230?


    Why is the larger capacity battery cheaper? What is the difference between the 2 apart from the price and the 10Ah?

    The M3 user manual states a 12V 65Ah battery, so the original designers of the car obviously decided a capacity of 65Ah was enough for the E36 electrics to stay alive between starts and the Alternator is specified as 140A, 1960W (=14V, go figure), so the question I've got is which battery can handle the 140A charing current better. This is normally specified as a Battery Charging rate, 1C, 1.4C or 2C etc where C is the capacity in Ah, so for 140A from the Alternator I'd want a 65Ah battery to be rated >2.16C and a 75Ah battery to be rated >1.87C. The maths are telling me why the 65Ah battery may be more expensive, it needs to handle a higher Charging rate, which may equate to more metal, bigger plates who knows - BMW.

    I've asked BMW Australia Customer Service what is the charging rate of the 2 batteries that are suitable for my car, let's see what they say, they have 10 minutes to call me back before the close of business (which was their promise about 3 hours ago when I called them). They may even tell me the CCA value of the batteries (Cold Cranking Amps) apparantly the maximum amps the battery can pump out when cold and at a sustained output voltage I assume that equates to a working starter motor perhaps. Again I think the higher the CCA value the better the battery as I need it to crank the starter motor to start the engine. The BMW designers already did the work for me in deciding the 65Ah battery is enough, I don't need the extra 10Ah, I need the battery to handle the 140A from the alternator when flat and the cranking current to start the motor.

    5 more minutes for BMW to call, but I guess it will be tomorrow.

    Anyone else got any ideas about the batteries - capacity, versus CCA/ charging rate ?



  3. #3
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    Yes | No

    bigger is better but weights more

    but wow, 10 years. We get about 4-5 here in the US, BMW or aftermarket. If the batteries are the same size, the plates could be thinner in the larger one, or maybe it's just supply versus demand, with most folks buying the original 65ah spec.
    Odd thing is it turning over slower when hot though... Usually, there's more residual pressure from the fuel pump when hot, less resistance to turn since everything has had oil pressure more recently, etc.
    Most alternators run about 13.8 at the battery, Bentley manual quick-check says 12.6v engine off, ~ 14v running (between 13.5-14.5 depending on temp, conditions).
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  4. #4
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    Re: bigger is better but weights more

    My first BMW was an E36 318is Sept 92, the battery in it lasted till May 2001 from memory, when I "cooked" it doing a 400Km freeway trip. The close to 9 years running made me buy another BMW battery for it (plus it was cheaper than an aftermarket one at the time).

    My other car a 97 E39 528i had it's battery replaced in 2007, so that is 10 years of life. It has a bigger capacity and current rating so that's why I think it got more years out of it, it is able to supply the current loads with less stress on its internals.

    Also temperature plays a part I guess in the chemistry of batteries and their cycle life or longevity, Australia is pretty warm all year round.


    But I'm stumped; capacity versus price? But the CCA and RC figures that are written on the BMW batteries will be the decision making figures for me. The BMW Customer Service girl is waiting on the Technical Team to get out of a meeting (long meeting), so I asked her to get the storeman to take a photo of the battery labels and email them to me - I just want to know what I'm buying and why, but it all seems to be too hard to get the answers.

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    61218381714 versus 61218381745

    Well these are the part numbers from BMW, and it looks like the prices are based on "apples and oranges".

    The 65Ah (61218381714) for AUS$280 is labelled as "filled" on all the databases

    The 75Ah (61218381745) for AUS$230 is labelled as "empty"

    So BMW may have added the acid and filling labour charge to the cost of the 65Ah filled battery, and don't seem to want to charge extra money for the 75Ah filling (61218381744)- WOW


    So I've ordered the 75Ah one from my Indy guy, he already said to me that it is empty so his cheaper price includes the filling and battery charging. I eventually got an answer from the BMW Dealer, not BMW Australia, on the CCA values; 300A for the 65Ah, 380A for the 75Ah (seems low, they may be quoting the CA value). I'll have a look when I see the battery label.

  6. #6
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    Yes | No

    61218381745 Empty - but not any more

    Battery is in and all feels good. The BMW battery is bigger than the old "no name" one so I'll hopefully get another decade of life out of it. The old battery shat out sulphate or whatever the chalky crap is again so it was well on its way out - it is now. The battery pan is cleaned up and primed again with battery terminal paint, some rust was appearing on the battery holder.


    From the label the CCA is 630A so I don't know where the Dealer guy's 380A came from or what it is. The reserve capacity is also 2 hours and 10 minutes of 25 Amps at 80degF, that should be plenty for an overnight or two.



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