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Author Topic: Battery Power: Running both Positive and Negative  (Read 1981 times)
K7PEH
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« on: March 09, 2005, 06:42:35 PM »

I am not sure if my title is meaningful but I will go ahead and try to express my question.

All the information, advice, and literature I have found on wiring up mobile equipment suggests that both a positive and negative feed from the battery should be supplied.  In contrast to supplying positive only and taking the negative return route through the chassis ground.

I have been talking to the installers at the local Car Toys shop and they are wondering why this is necessary.  They typically wire up the positive feed and then take the negative from ground.  Of course, they are typically wiring up power delivery for high-power audio amplifiers and not RF equipment.

So, can someone list the best reasons for including both positive and negative feed from the battery and both fused?  I will put forth one that I have heard the most is that taking the negative feed from the chassis may impose a voltage drop that you do not like at high current draws.  That is, given that the resistance from the battery location to the chassis location is some non-zero (measurable ohm) value.

But, are there other reasons?

And, is it important to fuse each feed, both positive and negative?  I am currently planning on putting fuses in both feeds but I am not sure if this is overkill or not.

So, if you know more arguments for including both feeds let me know.  If someone has a feed on just the positive and taking negative from local chassis I would like to know their experience.

phil
K7PEH
www.k7peh.com
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W7DJM
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« Reply #1 on: March 09, 2005, 07:57:45 PM »

So far as I'm concerned, this is a HUGE controversy, because many of modern mobiles, EVEN THOUGH they have a negative conductor, are ACTUALLY also grounded TO THE RADIO CASE.

There is this possible scenario:   Part of the ground circuit, that is, the main car battery cable, becomes loose/intermittent--depending on how things break, and how they've been hooked up, the auto system can NOW attempt to "ground"  from the battery, down through the radio negative conductor --through whatever traces on the circuit board--and through the radio case to ground.

Since the radio needs a fairly hefty fuse to operate normally, it is very possible that damage to the radio circuit board can result--EVEN IF the negative lead is fused.
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AA4PB
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« Reply #2 on: March 10, 2005, 06:16:05 AM »

The reason to take a negative lead from the radio back close to the battery (or to the battery) is that you don't want to depend on the chassis ground over which you have not control. It could have resistance in the joints which might even be intermittant. Currents from other vehicle equipment can cause noise in the resistance of the chassis parts. I have connected a headset to two different chassis ground points and heard alternator whine in the headset.

Both the positive and the negative leads should be fused at the battery end. The negative lead fuse is in case the vehicle grounding system develops a high resistance and attempts to pass starter motor currents thru your radio ground.
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K0BG
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« Reply #3 on: March 10, 2005, 06:32:13 AM »

Delar, it depends on the radio. Some are and some are not grounded through their cases.

Let's assume the radio is grounded through the case which is attached to the chassis, and you have both leads run directly to the battery. And let's further assume the battery ground cable fails. If there is no other path except to the negative lead to the radio, the fuse in the negative lead will fail thus protecting the radio. It might be (?) argued that the ground lead INSIDE of the radio is attached to a land on the circuit board. I've never seen that, but I guess it might be possible.

Alan, KØBG
www.k0bg.com
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K5LXP
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« Reply #4 on: March 10, 2005, 06:49:06 AM »

Take the professional installer's opinion to heart and give them the benefit of the doubt they know what they're doing.  I've fixed enough smoked radios and melted antenna leads to know never to connect the negative lead to the battery, fused or not.  It doesn't take fuse blowing amounts of current through the negative lead to cause erratic or undesired operation, of either the vehicle or the radio.  For AA4PB who could hear alternator whine at different grounding points on the chassis, this is a classic case of components and/or sections of the chassis that aren't bonded well.  Connecting the negative lead to the battery in this case, while solving the whine problem, may also be bonding the affected components together and negative lead current is the result.  I've posted numerous challenges to people that insist on doing this, and no one can prove it solves more problems than it causes.  99% of the time you'll get away with it, but in the commercial radio field in which I worked where thousands of installations are involved, that 1% wasn't worth it.  


Mark K5LXP
Albuquerque, NM
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K7PEH
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« Reply #5 on: March 10, 2005, 08:22:49 AM »

Mark, K5LXP

I visited your web page and saw the photos of your Electric Truck.  You are one mobile crazy dude!

I read part of a technical note authored by GM engineers (at my Chevy dealer yesterday) on the Chevy Suburban which is similar to my Silverado Pickup Truck.  This technical note described the methods and the constraints to be followed in installing mobile transmitting equipment as might be used by federal employees (aka FBI), commercial vehicles, and even amateurs.

One thing I noted is that it specifically showed a diagram where both the positive and the negative leads must be included.  It did not go into detail on this but it did mention that both positive and negative should be fused at the battery as well.

It is probably true that using a single positive feed from the battery and taking ground return from the chassis would be OK.  And, it is also true that problems that could arise in weird situations may in fact arise assuming either scenario but they are really six sigma out on the bell shaped curve so not too much to worry about.

However, I think my two concerns lead me to use two feeds, one for each positive and negative.  These are:

1.  voltage drop problems if the chassis resistance is not low enough.

2.  intermittant ground connection due to the fact that the only trusted ground is actually the frame and not the other metal parts of the truck.

This second fact I learned from the Chevy dealer.  My back pickup truck bed is separated from the frame rail by hard rubber cushions.  This is designed this way to help with noise and other mechanical problems.  So, I will be strapping (underneath) the bed to the framerail to help with the antenna ground plane.  This was recommended by the dealer.
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W7DJM
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« Reply #6 on: March 10, 2005, 09:08:43 AM »

"""intermittant ground connection due to the fact that the only trusted ground is actually the frame and not the other metal parts of the truck"""

The frame on many vehicles can be a poor ground.  Many vehicles use the engine block as the main tie point, because that's where the high current starter is, and use small bonding leads to tie various points, the battery, block, body--to the frame, and sometimes these can come loose.

Back when I had my '70 six pack, this was not a problem, can you spell "unibody?"   However,  I did have the battery in the trunk for awhile, and this requires a GOOD cable from the unibody to the engine block.

There is certainly things to be gained, often, by taking a critical look at "loose" items and adding ground straps.

Often, there is very little tying the block to the frame.

Often, there is relatively small gauge wire tying the battery to the body.

Hoods, doors, trunk lids are often good candidates for bonding straps.


Back when I sold auto parts, I don't know HOW many people spent good, hard, money on a "wench",  and then came back to ask me why it wouldn't pull much----here we are, the winch is grounded to the frame, and too small cables/battery/alternator going to the winch.   There were even a small who burned up the (small) bonding cable from the battery to the body.
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K5LXP
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« Reply #7 on: March 10, 2005, 11:30:51 AM »

I too have seen the 'blessed' instructions that show a separate negative lead, not only from auto manufacturers but in the metric radio manuals as well.  I do not know the logic behind this information.  Thing is, if it works so great, why don't *they* do it that way?  Pretty much every electrical item in a vehicle is grounded to a nearby chassis tab or stud.  


> two concerns lead me to use two feeds,

> 1. voltage drop problems if the chassis
> resistance is not low enough.

The cross section of the steel between the radio chassis and the battery is many times that of any single cable you would use.  If there is measurable drop between the radio's chassis attachment point and battery negative, it should be traced and corrected.  


> 2. the only trusted ground is actually the frame
> and not the other metal parts of the truck.

W7DJM addressed that nicely.  Would you rather correct it with bonding straps, or your radio's power lead or antenna coax?


Mark K5LXP
Albuquerque, NM
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K7PEH
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« Reply #8 on: March 10, 2005, 11:45:02 AM »

OK Guys....

I am having my truck wired next Wednesday the 16th.  I want to have the best thing done -- money is not the issue, quality of power delivery, workability, noise suppression, and all the things of that ilk are the issue.

So, I am wondering still since there has been no definitive answer to the question.  Of course, the population of respondents has been small but then I suppose it is hard to garner up the interest on such a mundane topic.

I guess if whatever I end up doing does not work out I can always have it changed.

Thanks for the ideas and advice so far.

phil
K7PEH
www.k7peh.com
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K5LXP
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« Reply #9 on: March 10, 2005, 12:11:48 PM »

> quality of power delivery, workability, noise
> suppression, and all the things of that ilk are
> the issue.

> there has been no definitive answer to the question.

What's left to answer?  Depends a lot on what you plan on running from this power source, but for a typical ~20A installation I would spec a 6 or 8ga positive wire with user accessible weatherproof fuseholder near the battery, and run to the point of radio mounting with a couple extra feet to spare.  There's not much you can do to the power lead to suppress noise or improve the 'quality' of the power.  If you're getting alternator or ignition noise those issues need to be corrected at the source or specifically filtered.  Don't make this more of a project than it needs to be, run a cable between the radio and battery and go from there.  Compared to some of the high power sound systems that are out there a ham rig that draws 20A is child's play.  Doesn't matter if the load is generating AF or RF, amps is amps.  I would trust the stereo installers, they deal with the same issues as we do with transmitting equipment.


Mark K5LXP
Albuquerque, NM
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W4JLE
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« Reply #10 on: March 10, 2005, 12:25:58 PM »

Normally the Negative lead is connected to the engine block, with a small lead to the car body. The reason is to prevent heavy current from passing through the drive train bearings. If the negative lead were grounded to the body, the current path would be from positive, through the starter, then via the transmission and drive train to the body.

Using the body as a return for high power amateur equipment can possibly burn out the small ground lead (typically a #14 piece of wire) and cause all current to flow through bearings in the drive train.

Running both leads directly to the battery obviates all the problems.
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K7PEH
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« Reply #11 on: March 10, 2005, 12:35:48 PM »

Mark,

Thanks for your comments.

I will be running more than 20 amps eventually.  I plan to buy an SGC 500 watt solid state amp and put that in my truck too.  So, I need to handle peak draws of maybe 80 to 90 amps and an average of maybe 45 amps during transmit.  Thus, I am using #4 wire.

Of course, at first it will just be my little IC-706, but I will probably add the amp sometime this summer after I gain a little experience with the configuration and so on.

Other input though that I just got from SGC today.  They are local here in Bellevue (Washington) and they do a lot of installs themselves for their own mobile equipment.  The technical help guy said that they run dual cables to the battery for positive and negative.  But, he also said that he didn't know of any reason why you couldn't tap the chassis ground for negative return and limit the cabling to positive feed only.

But, he did recommend using a second battery for their 500 watt amp if the vehicle battery and alternator are not beefy enough to handle the current draw since the amplifier could peak at 90 amps.  The second battery buffers the demand and you have an average charging current between the first battery & alternator and the second battery.

phil
K7PEH
www.k7peh.com
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K5LXP
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« Reply #12 on: March 10, 2005, 03:35:21 PM »

W4JLE wrote:

> burn out the small ground lead (typically a #14
> piece of wire) and cause all current to flow
> through bearings in the drive train.

In all of my years of installation work, I have never seen anything short of at least 1/2" wide braid from engine block to ground, with 4-6 gauge cables used for starter current.  You are right though, without a good engine-frame ground current will flow through the drivetrain, exhaust, and at least one other undesirable path.  

When you're dealing with currents in the dozens or hundreds of amps, milliohms make a big difference.  No matter what, you're going to need an alternator that can keep up with that kind of power draw.  Depending on the overall source impedance of the amp's power source (alternator, cables, buffer battery) you want it to be as stiff as possible to keep the amplifier linear.  Running parallel wires for a high current load is a good idea.  The problem with a point source buffer battery is, even if it had zero impedance, won't start contributing any power to the amplifier until the bus voltage drops to about 12.8V or so.  The system source (alternator) is nominally running at 14.4V, so there's a 1.6V potential delta depending on the source impedance of your alternator and cabling.  This isn't really a problem with CW or FM (steady state) but will definitely impact linearity in SSB.  This will manifest itself in increased IMD.  You might investigate the rated power input regulation requirements of the amplfier.  The large capacitors the boom-boom stereos use won't do much for you either, due to their relatively high internal impedance.  Now that we know what you're up to, you would be wise to err on the side of too big rather than just enough.  If it would make you feel any better, you could measure your chassis impedance by running a nominal amount of current from where the amp will mount to where the battery negative is grounded, and measure the voltage drop with a DMM.  Even just 20A (say from a small heater, or lamps) will tell you within a few milliohms what you've got.  You could compare that value to the resistance of the cable you would otherwise use and decide which has the least resistance.


Mark K5LXP
Albuquerque, NM
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K7PEH
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« Reply #13 on: March 10, 2005, 03:47:54 PM »

Battery Isolators are the solution...

I have investigated the multiple battery issue.  If you run two batteries, one to power the regular car startup and accessories and one dedicated to the ham radio equipment then a battery isolator is absolutely necessary.

A battery isolator will prevent current flowing from one battery to the other and visa versa but still allow the alternator to charge both.  A cute little device and they are designed for hefty truck, RV, and marine requirements.  The battery isolator I would need would cost about $75.

Also, my truck can be easily fit with dual batteries in the front engine compartment.  These dual batteries with the battery holder designed for my Chevy truck area available at many after market dealers and I got several recommendations for our greater Seattle area.

Whereever there is a problem it seems that solutions just crop up -- given enough money of course.

phil
K7PEH
www.k7peh.com
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AA4PB
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« Reply #14 on: March 10, 2005, 05:19:44 PM »

Take the professional installer's opinion to heart and give them the benefit of the doubt they know what they're doing
--------------------------------------------------

Don't believe that for a minute. "Professional" simply means that he gets paid for doing the work, not that he necessarily knows or understands what he is doing. Even professionals have different opinions on this issue. Most professionals who install 100 watt commercial radio equipment however, take a ground wire back to the tie point near the battery on newer vehicles or directly to the battery on older vehicles - and they fuse the negative lead as well as the positive lead.

I had an alinco 2M mobile that had the negative lead directly to a PC board run. It gave me significant alternator whine (in the receiver even with the volume turned all the way down) any time the radio case was also grounded. The solution was to provide a bonding wire between the case and the negative lead at the radio in order to minimize the leakage current in the PC board ground run. The bonding wire completely eliminated the whine.
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