Battery Relocation

Honolulu

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DeCoupe: do you need two batteries to start? Maybe only in the Frozen North?

To assess batteries we need to compare apples to apples. PHCA is not the same as CCA or RC, but for me in a warm climate it sounds like the spec to use as it most nearly addresses the starting load.

My coupe battery tray is tattered and visibly unsafe, though my batt hasn't dropped out of it yet, so I'm interested here.

Welding wire has more and smaller strands and is better suited for carrying high-amperage loads, but we're starting cars over a few seconds, not pushing arc-welding amps for a minute or more at a time. That said, I used the multi-strand wire back in my '02 days. Battery wire can be sourced at marine supply stores as well as welding shops. Prices, wire gage and number of strands vary.

Wandering a bit, I'm thinking about ground-side current distribution. What might be the effect of battery grounding in the back of the car? The current running back to ground, highest during cranking, will disperse throughout the metal body structure. Is there an appreciable galvanic effect to thus charge the body?
 
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Mike Goble

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Wandering a bit, I'm thinking about ground-side current distribution. What might be the effect of battery grounding in the back of the car? The current running back to ground, highest during cranking, will disperse throughout the metal body structure. Is there an appreciable galvanic effect to thus charge the body?
If you look at any OEM rear battery installation you will find a short cable directly to the frame. This is because the frame/body will conduct many times the current as would a cable running forward, and it's free. Although the ferrous metal frame has just a fraction of the conductivity of copper, it has a cross-sectional area many times larger, allowing for much lower resistance along the path.
Once the engine is running, it all becomes a moot point because the mother of all grounds moves from the negative terminal of the battery to the case of the alternator. All current from your car returns to the alternator case, so it is important to have adequate grounding from there to the frame/body, as the frame/body is the return path from most of your devices.
 

aearch

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I JUST BOUGHT TEN DOLLAR BATTERY CABLES BRAISED THGEM TOGETHER AND RAN THEM ALONG THE UPER TUNNEL SET W/ RUBBER STRAPS AND RIVETS
SIMPLE CLEAN
 

Arde

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Mike, that is a great insight I never considered before.

...
Once the engine is running, it all becomes a moot point because the mother of all grounds moves from the negative terminal of the battery to the case of the alternator.
 

jmackro

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I agree with what Mike is saying, but with two clarifications:

Although the ferrous metal frame has just a fraction of the conductivity of copper, it has a cross-sectional area many times larger, allowing for much lower resistance along the path.
Just be sure to make that battery ground connection is attached to something beefy on the body-frame. Bolting it to a thin sheet metal section won't provide that "cross-sectional area many times larger, allowing for much lower resistance"
Once the engine is running, it all becomes a moot point because the mother of all grounds moves from the negative terminal of the battery to the case of the alternator. All current from your car returns to the alternator case, so it is important to have adequate grounding from there to the frame/body, as the frame/body is the return path from most of your devices.
True, though once the engine is running, the amperage drops dramatically. The starter is by far the largest load in any automotive electrical system.
Although the ferrous metal frame has just a fraction of the conductivity of copper
I was curious about the magnitude of the difference in conductivity between copper and steel and found a chart at: https://www.thebalance.com/electrical-conductivity-in-metals-2340117 If I'm reading the chart correctly, copper has a conductance 6x greater than iron (which should be close to that of steel).
 
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Arde

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The interesting question then is who is responsible for the annoying phenomenon of all illumination (headlights, instruments, etc.) being dimmer at idle than when revving up a bit. Is the lower intensity at idle from a drop mostly on the +12V path involving copper wires, relays/switches, and connectors? Or a drop on the ground path involving iron and its attachment points?

Or is all this a red herring and what I see is poor voltage regulation where the alternator increases its output way above 12V when the engine revs up?

I experimented a bit on the Fulvia when I replaced the alternator and saw that it reaches good voltages and charges the battery even at close to idle speeds, debunking this idea that to recharge a weak battery I have to do a spirited drive... But I still do.

Another experiment would be to build a copper E9. Stan can organizeg that project if we get 10 people interested :).


True, though once the engine is running, the amperage drops dramatically. The starter is by far the largest load in any automotive electrical system.

I was curious about the magnitude of the difference in conductivity between copper and steel and found a chart at: https://www.thebalance.com/electrical-conductivity-in-metals-2340117 If I'm reading the chart correctly, copper has a conductance 6x greater than iron (which should be close to that of steel).
 

jmackro

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Or is all this a red herring and what I see is poor voltage regulation where the alternator increases its output way above 12V when the engine revs up?
It isn't poor regulation; just that the alternator can't put out 12+ volts when it is turning slowly. If the raw alternator output is <12v, then the regulator doesn't have much to work with.

The copper wires, relays/switches, and connectors have no idea whether the engine is at idle or redline.
 

Mike Goble

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I agree with what Mike is saying, but with two clarifications:
Just be sure to make that battery ground connection is attached to something beefy on the body-frame. Bolting it to a thin sheet metal section won't provide that "cross-sectional area many times larger, allowing for much lower resistance"
True, though once the engine is running, the amperage drops dramatically. The starter is by far the largest load in any automotive electrical system.
I was curious about the magnitude of the difference in conductivity between copper and steel and found a chart at: https://www.thebalance.com/electrical-conductivity-in-metals-2340117 If I'm reading the chart correctly, copper has a conductance 6x greater than iron (which should be close to that of steel).
As I pointed out in the first sentence - "If you look at any OEM rear battery installation you will find a short cable directly to the frame." You will also notice that this short cable is often of a lighter gauge than its long counterpart. OEM's know that the difference between #2 cable and #00 cable over a distance of 12" at a load of 150A is going to be about 0.012 volts, a minuscule difference. If you were to use #2 for the entire circuit the effect would be much more.

If you did happen to attach it to a piece of sheet metal the effect would be minimal. The choke point, the piece of metal that is smaller in cross-sectional area than the main conductor and presents more resistance to flow, operates under the same rules as all the conducting metal - resistance over distance. The ferrule that you use to connect to the frame represents a choke point because the cross-sectional area of the ferrule is less than that of the cable. The actual point of contact, where the tinned ferrule meets the ferrous metal is another choke point, no matter how thick the ferrous metal is.

Let's say that you did attach your battery cable to 1/16" thick sheet metal on your trunk floor and it has a conductivity of 16% that of copper. At what point will the cross-sectional area of the sheet metal be six times the cross-sectional area of the cable? Let's use 00 cable as our standard and it has a cross-sectional area of 0.1 sq.in. One thing we need to realize is that the flow of electrons goes in all directions as it selects the path of least resistance. It will flow radially from our contact point out into the sheet metal, so we are calculating the diameter of a cylinder of 1/16" sheet metal whose wall area is 0.6". It turns out to be a circle roughly 3.2" in diameter. At any point past that the cross sectional are of the sheet metal is such that it is >6 times the area of the cable and able to conduct current with less voltage drop. If you were to put a voltmeter across the 1.6" radius of this circle, you probably couldn't measure the drop with a readily available voltmeter.

Let's say your floor pan averages 50" wide and is 1/16" thick. It has a CSA of 3.125 sq.in., roughly 30 times the CSA of a 00 cable, so the floor pan by itself has 1/5 the resistance of a 00 cable of the same length. Throw in the frame rails, roof, rockers and any other paths of conduction you may end up with 1/8 the resistance.

It is absolutely true that the starter is the largest consumer of power, and the battery is the only device that can provide this power. Once the starter has done its job, which like most men is done rather quickly and with great flourish, the battery becomes just another load. The alternator becomes the prime mover of electrons in the system and the case of the alternator becomes the point to which all current in the system returns. You could replace both battery cables with some 12 gauge wire and everything would work just fine, and the battery will recharge itself in a few minutes. If you forgot to switch back, the 12 gauge wire would fuse immediately.
 

jmackro

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If you did happen to attach it to a piece of sheet metal the effect would be minimal. The choke point, the piece of metal that is smaller in cross-sectional area than the main conductor and presents more resistance to flow, operates under the same rules as all the conducting metal - resistance over distance.
Thanks for the detailed explanation. I see your point: the thin steel where the ground is connected would be a "choke point", but only in the immediate vicinity. Still, you can usually find multiple layers of sheet metal, or a heavier support, within a few inches of any point on a car body. So I would use a (copper) ground cable a few inches longer to reach such a point, rather than to connect the battery ground to a single layer of thin sheet metal.

If I was a engineer at Toyota designing a car to be built in high volume, saving a few inches of ground cable by connecting to the closest sheet metal would be important. But when you're relocating the battery on your coupe, who cares if a 10" long ground cable costs a couple of bucks more than an 8" cable if it allows you to connect to thicker metal? Admittedly, this is more a matter of aesthetics than engineering.
If you were to put a voltmeter across the 1.6" radius of this circle, you probably couldn't measure the drop with a readily available voltmeter.
Maybe, but these things just bother me. Sort of like why I clean & polish parts that only I will ever see.
 
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Mike Goble

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The pros and cons of ground cable length probably never comes up when the average guy is relocating a battery. You put the battery in where you want it, choose a spot to attach the ground and you get a cable to fit. I buy them ready made as any gain in voltage drop I may realize will probably be offset by faulty assembly on my part.
 
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