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Relocating the Battery to the boot

Discussion in 'A3/S3/Sportback (8P Chassis)' started by hatmeow, May 19, 2009.

  1. hatmeow
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    hatmeow Turbo is the only way....

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    Has any tried to relocate the battery to the boot? Just thought that it might improve weight distribution as the TT's battery is in the back.

    Any things to look out for in this seemingly simple (but I am sure there are lots of issues) mod?
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  2. Fast,not furious
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    Fast,not furious Dealing with an OCD

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    #2
  3. Damo S
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    Damo S New Member

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    Unless you have a good reason to do so I would say its a waste of time and money. The difference in weight saving (for replacement lightweight batteries) and / or distibution will be negligable.
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  4. Staz
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    Staz is a retronaut Staff Member Moderator

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    I agree with Damo, it's unlikely to make any difference plus the closer the battery is to the alternator the longer it'll last. When you add in lengths of cable there will be a small voltage drop across the cable which means the battery charges at a slower rate. It's a tiny amount of difference but over a long period of time it makes a difference.
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  5. hatmeow
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    hatmeow Turbo is the only way....

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    Thanks for the tip guys. I was actually not looking at weight saving but more on improved distribution. I would suspect that moving 15-20 lbs to the back corner will make the car handle more neutral.

    Staz, what about the TT or the M3 that has the battery out back? wont they be experiencing the same problems as well? I wonder what the factory did in addition to counter that issue.

    I guess the quicker and less problematic way would be Fast and Not Furious' with the Braille battery. Lose a couple of pounds but at a high price...hmmm maybe later...
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  6. Staz
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    Staz is a retronaut Staff Member Moderator

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    They could quite easily counter it by simply increasing the voltage the alternator kicks out.
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  7. devonmikeyboy
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    devonmikeyboy As far from JBS as possible !

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    On my car just inside the boot on the right hand side there is a metal cover which if you remove you will find a proper battery box to use anyway. That is where i keep my battery now. I only put the battery in the boot to make more space in the engine bay, i wouldn`t bother else as doing it for weight distribution/handling would be a waste of time in my mind.
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  8. VWAddict
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    VWAddict Member

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    While you're basically right insofar as the longer the cable, the greater the resistance, and resistance is a bad thing, your assumption about charging is almost entirely incorrect.

    Distance to alternator doesn't matter anywhere NEAR as much as distance to starter motor. -It's a simple case of Ohm's law. Where volts = V, Current = I, Resistance = R: V=IxR. -The current used to charge the batter is utterly dwarfed by the current used to turn the alternator.

    This is fundamentally incorrect, and would in practice be quite dangerous.

    Addressing both issues; As the battery charges, the serial voltage of the cells rises. As the voltage differential between the battery and the alternator output diminishes, the current falls. The charge current only falls to the nominal trickle when the difference is a fixed amount, and if the alternator output is raised, the battery will try to charge to that (higher) voltage. Since the output of a lead/acid cell is governed by chemistry, and cannot be raised by charging beyond its physical limits, this would result in over-current, overheating, and premature death of the battery.

    Keith
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  9. Staz
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    Staz is a retronaut Staff Member Moderator

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    Well you are right about one thing, the starter motor is the hub for the connections to the battery and the alternator. So it's not about the distance to the alternator as such, it's the distance between the alternator THEN the starter and THEN the battery. But as the distance between that starter and alternator are pretty much fixed (as they're both attached to the engine) it's pretty irrelevant.

    I don't really want to go into picking holes in your post, the statement:

    "The current used to charge the batter is utterly dwarfed by the current used to turn the alternator."

    is complete rubbish for a start as there is of course no current used to turn the alternator.

    As for the rest, well there WILL be a certain voltage dropped across the length of the wire, and if the voltage output of the alternator is tuned to offset this voltage then it is in no possible way dangerous. As far as the battery is concerned it is still seeing the same voltage dropped across it and therefore will draw the same current flow. I have in no way suggested that the voltage would be increased beyond the voltage dropped across the length of the wire.
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  10. VWAddict
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    VWAddict Member

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    You're quite right, I typed 'to turn the alternator' when I meant 'to turn the starter'.

    Now of course it makes sense. And as a professional electrical engineer I can ASSURE you that the voltage "offset" is NOT constant, since it varies with current (again, fundamental Ohms law) so it is quite impossible to do what you suggest without active electronics in between.

    Once again, the CHARGING isn't the issue; the STARTER is the problem. The starter current utterly dwarfs the alternator current, and any cable which can run a starter motor can charge a battery with insignificant passive voltage loss.

    Keith
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  11. Staz
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    Staz is a retronaut Staff Member Moderator

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    The offset it not constant no, as yes it varies with the current demand, and that causes the voltage output from the alternator to fluctuate. BUT the alternator voltage is regulated. So when there is a (resonably) high load there is high current flow to the battery, but the voltage will remain the same and therefore the voltage dropped across the length of the cable will do too.

    So, bar fluctuations in the voltage due to changes in the load, the voltage essentially stays the same and therefore they COULD increase the voltage output from the alternator to counter this.

    The starter current does dwarf the alternator current yes. And the cable can of course handle the starter current. But what difference does that make to the resistance of the cable? It has a certain resistance, dependant on length, no matter how thick it is!
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  12. Igdos
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    Igdos Member

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    :huh:

    Head hurts after reading this thread!
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  13. VWAddict
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    VWAddict Member

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    It means that ANY cable which can handle the HUNDREDS of amps to turn the starter motor without any significant voltage drop across its length must -by mathematical inference- be capable of passing the few amps required to charge the battery. -Remember that the typical alternator can only generate in the region of one hundred amps maximum, and the battery charge rate is usually a small fraction of that.

    Thus ohms law tells us that the charging voltage drop must be TINY, if the cable can turn a starter motor.

    THAT's what 'difference' it makes to the analysis.

    An increased resistance in a charging circuit does NOT in any way alter the voltage to which a circuit component such as a battery or a capacitor charges. -It limits the maximum RATE at which the component can charge.

    For a capacitor, (capacitors are simpler to analyse than batteries which are a more complex component) the standard is given as T=RC where T is the time to reach 63% of the supply voltage in seconds, R is the total resistance in circuit in ohms, and C is the capacitance in Farads.

    A battery is more complex because it involves a chemical transfer of energy, (which in a capacitor can be considered analagous to dielectric absorbtion) but PART of the model is a complete resistance to increase the charge voltage across its terminals beyond a (chemically-defined) fixed number, and to convert any excess into heat. In a 'capacitor' model, this would be like a reverse-biased zener diode in parallel. -Any attempt to charge beyond a certain point (no matter what the series resistance) is dissipated as heat.

    ...and you DON'T want excess heat pointlessly dissipated through a fully charged battery.

    Now let's briefly consider the scale of the numbers involved:

    For a load to pull 12 Amps DC from 12 volts DC, its resistance has to be 1 ohm. For a load to pull 120 amps, its resistance must be one tenth of an ohm. If the cable in series has even so much as one tenth of an ohm of resistance, it would comprise 50% of the total circuit reistance, and as such would dissipate 50% of the power, and the starter would only see six of the original twelve volts. -That doesn't just go for the cable's resistance either: contact resistance, cable indictance, -everything- in short, the TOTAL impedance of the circuit to and from the starter motor must be under a hundredth of an ohm, or you're going to get a slow crank.

    Now, if a battery INITIALLY charges at 20 amps (for example after a significant discharge from a long period of cranking, trying to start the engine with some sort of problem) then the voltage drop across the one-hundredth of an ohm at 20 amperes is 0.2Volts.

    _So you MIGHT raise the voltage of the alternator by 0.2 volts, and that would fix it, -right?

    -Wrong. -Because as the battery's internal charge was restored, its charge current would drop to a trickle. -Say it would be around 1 amp, just for some easy mathematics. -Now the voltage drop across the one-hundredth of an ohm total circuit impedance would only be a hundredth of a volt... so we'd be overcharging by 0.19 Volts. (which is actually a significant amount to a battery) and the current would in fact never drop off to that small degree, because the battery would be trying to 'sink' the extra voltage.

    Adjusting the alternator voltage to address this problem would be entirely the WRONG thing to do. Minimising the circuit impedance is the RIGHT thing to do.

    I've relocated batteries on a couple of race cars before now (to the front in a rear-engined aircooled VW and to the rear in a front-engined Porsche; to the spare wheel well in both cases) and Welding cable works well enough for a racecar. in Neither case did the starter motor slow down at ALL appreciably, and the batteries charged just perfectly, and to the correct voltage, and with absolutely NO measurable difference in charging current WHATSOEVER.

    Super-low-gauge welding cable is perfect for an occasional track car, but I'd say it's NOT recommended for daily street use, but not because it can't handle the current... Because it's usually VERY finely stranded (which makes it flexible and easier to use on welding jobs... though such soft supple flexibility isn't much of a benefit in a permanent install like on a car) and as such has a high surface-area-to-sectional-area ratio. At a car battery, there are frequently lots of nasty corrosive gases in the environment, and they tend to gain ingress to the cable through the end. Once sulphuric gases get into the end of welding cable, that end of it quiclkly turns into a green, brittle, high-resistance nightmare. -Depending on the type of isulation (often a rubbery compound) that too may not cope well with either the inhospitable chemical environment near to the battery, or the heat at the engine bay end. -but like I say, it's GREAT for a racecar, where changing it out is just part of the 'cost of doing business'.

    So wht's the summary? -For the cable resistance to appreciably slow the charging rate to in ANY way have an effect ob battery charge, the cable resistance would have to be so apprecialbe that your starter motor wouldn't turn... -simple as that.

    Different types of battery may require slightly variant alternator output voltages for reliable charging, but for this hypothetical analysis, you don't need to change it at all. The voltage drop across ANY cable that can run the starter motor will be so miniscule that it will fall WELL within the alternator output's window of regulation, even when in perfect condition.

    Keith
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  14. Staz
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    Staz is a retronaut Staff Member Moderator

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    Well I agree that lower cable resistance is always a better thing. You certainly know your stuff. I'm a little confused by the concept a reversed biased zener diode. Surely that's just a forward biased diode isn't it?!

    If you say any loses are negligible then they are.

    Something isn't sitting right with your maths though, but I've had along drive and need an early night.

    :thumbsup:
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  15. VWAddict
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    VWAddict Member

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    I'll take another look at the figures, but I don't usually make THAT many mistakes... typos though; yes!

    Zener diodes... very similar to 'avalanche' diodes: they act like regular diodes in that they conduct in one direction (forward-biased) but not in the other direction (reverse biased)... bot only up to a point.

    When a reverse-biased Zener sees more than it's 'avalanche' voltage, it begins to conduct, but only sufficient to 'clamp' the reverse-bias voltage across its terminals.

    So, for example, if you take a 10V Zener and wire it in series with a 1kΩ resistor, then stick 11V across the series chain of both components, 1mA will flow, so that the 1kΩ resistor will drop 1 Volt, and the Zener will maintain 10V across it. If you reduce the voltage to 10V or less, nothing flows, because the Zener is non-conductive. -If you up the voltage to 20V, the Zener again begins to conduct, but clamps the voltage at 10V, meaning that the 1kΩ resistor disspiates the other 10V. (10mA current).

    Zeners form the basis of most voltage regulator designs, and can be cleverly incorporated into CCS (constant current source) designs.. they're handy little gadgets, and often excellent sources of white noise, too. (Many simple white noise generators use them, and pink noise generators frequently use them followed by a simple filter.)

    Handy little buggers:
    http://www.allaboutcircuits.com/vol_3/chpt_3/11.html

    Keith
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  16. Staz
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    Staz is a retronaut Staff Member Moderator

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    Ah I see what you meant now. I know all about zeners it's just when you said reversed biased zener I thought you mean reversed reversed i.e. forward biased, because zeners are of course always reverse biased.
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  17. AuldReekie
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    AuldReekie Member

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    Wah alert :busted_cop:

    Time out. There is no way that statement actually means anything. I think this is an elaborate wind up between you two upon the rest of us mere mortals!

    Tell me, are you wearing tin foil hats? :moa::salute:
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  18. Staz
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    Staz is a retronaut Staff Member Moderator

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    LMAO!
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  19. VWAddict
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    VWAddict Member

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    heheheh...

    Yup.. we're making it all up.

    Now, can anyone help me rewire this three-handled family gredunzer? -I think there's too much end-play on the fonortner rods.

    :)
    #19
  20. Fast,not furious
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    Fast,not furious Dealing with an OCD

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    Can't help you with that one mate!

    Hatmeow: Bet you wished you'd never asked now?
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  21. VWAddict
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    VWAddict Member

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    There's the issue of corrosion to consider, wherever you put a battery.

    It seems to be less of an issue on vehicles made from fully-galvanised sheet metal, but over long periods of time, un-vented gases accumulating in a not-particularly-well-vented area tend to do their slow-corrosive work.

    Again, in a track-rat car, this is less of an issue, but I mounted the batteries in lightweight (non-corrosive), gasketed, glass-fibre boxes, secured to the vehicle, and then vented gas buildup outside of the vehicle via a rubber tube...

    I still got some mild corrosion around where the vent tube exited after a year, but -again- part of the "cost of doing business".

    In both cars it was nice to rebalance the vehicle, though. (mind you, my sixteen-stone weight and ham-fisted driving skillls make any small efforts at rebalancing pretty difficult for me to notice! :)

    Keith
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  22. bigchewie
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    bigchewie Member

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    3.2V6 A3's come with the battery in the boot, took me forever to find it. it doesn't compromise boot space either
    #22

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