Two battery questions :)
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  1. #1
    Stroker Kustom Jim's Avatar
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    Two battery questions :)



    Howdy,

    Couple of recent issues got me a thinking...

    Question one: Dr [email protected]@gle says a normal 12v battery reads approx 12.7v when charged and healthy.
    A 50% semi on and it drops to 12.4v.
    A shagged one reads 12v.

    Huh!! Ok, I've never taken any notice in my 43 years of tinkering but I thought they were 12 v and when dying dropped to 11, 10, 9 etc.

    Anyhoo, on the daily ish the battery is playing up, over a year old, dies in a week, charged it seems ok, run out and it reads 18v. An hour later it's fecking 12.2. I'm gonna dot back to garage and see if it's dropped, it's disconnected so car drain is not an option.

    So what gives! If it's 12.0 tomorrow is it kaput??

    Question 2: a friend's car got caught in a flood which only just got in to car, maybe got to starter motor and obvious battery attachment. It's currently stored, so started up now and then.
    If it is left with earthlead off battery when reconnected the starter can click ten times before working. A test for the sake of it and if positive lead pulled off battery when that bleeder is reconnected the bloody things starts first time!?

    Every thing tells me to disconnect earth. Does it spark if you do a positive disconnect first!?

    Can't get my head around it. A circuit is a circuit!? Is the positive thing giving a boot up the arse to the starter??

    Thanks for reading this far

    Jimbo
    "I'm not going to take this defeatist attitude and listen to all this crap any more from all these people who have nothing except doomsday to predict."
    Carroll Shelby

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  3. #2
    Off the Xmas card list kapri's Avatar
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    First question it reads 18V whne motoring along as alt output ? If so overcharging alternator as should hit max around 13.4.volts Over voltage will buckle battery plates and stop it retaining charge.

    No idea but on a pre enaged starter the soelnoid must physicallt slide forward to move driven teeth into flywheel , maybe sticky inside due to flood ??

  4. #3
    My name is Martin 30psi's Avatar
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    Current output is what determines a batteries punch and indicates its remaining capacity.

    If a battery drops below 12.4v when it has sat for a while is telling you it's getting knackered. Below say 12.2v and you're risking a successful winter start the morning after. Anything lower then it's really not happy.

    As Kapri notes, an over charging alternator will cook the battery. It will probably cook more on a new car with all the digital systems.

    I'd double check if you did see 18v. Normal is 13-14v.

    I can remember my mates Dodge was running 16v at idle and it would increase with revs. The alternator needle was swung way over. After rolling my sleeves up, sipping a can of Grolsch while staring at the wiring diagram, I worked out that a field terminal must be earthed out. Turned out the gasket washer thing was missing behing the terminal. We finished a few more cans after.
    '62 Ford Thunderbird
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  6. #4
    Administrator blackpopracing's Avatar
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    Most altys now put out 14.4 volts. Any higher and something is wrong.

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    Regulators cut out at 14.5 volts and cut in at 13.5 volts.
    Rough guide to state of battery health as measured by voltage
    https://s16-us2.startpage.com/cgi-bi...ticache=249001
    Depending on how much you discharge your battery (Depth of Discharge DOD) gives you an idea how long the battery will last. The more you discharge it, the shorter it's lifespan.
    https://s15-us2.startpage.com/cgi-bi...ticache=689392
    That graph is for heavy duty deep cycle batteries for solar storage, car batteries are worse and if run completely flat will only last 50 to 100 cycles, and the plates will be coated with a layer of lead sulfate making them useless.
    And if you are disconnecting a battery it should not give any sparks unless there is a high current draw, and it shouldn't matter which terminal you disconnect first because, as you say, a circuit is a circuit.
    The only exception might be if you have a bad diode in the alternator, and it's allowing the field coil to stay active which will give a high voltage discharge as the magnetic field collapses.

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  9. #6
    Stroker Kustom Jim's Avatar
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    Thanks for all the great info chaps!!!

    Got back to the car. I fitted a quick disconnect as it's a weekend car and from here on in I'm gonna break the circuit. Apparently Boxsters are terrible for drain.

    Anyhoo, the battery read 12.0 so I assumed it was game over.

    Connected and blow me it started, just.

    Checked the running charge and it's bang on the money 14.6.

    Had a good run round and last check as it was put to bed 12.34.







    According to info it looks a gonna. Over the last two years the battery has drained flat many many times.

    Ps the 18v thing I mentioned must have been a Specsavers moment!!

  10. #7
    Off the Xmas card list kapri's Avatar
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    My daily toy is down having the same thing looked at today . Alternator only pushing out 12.7 max. Modern ECUs like stable reliable voltage or start throwing up false codes everywhere.

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