Adding a second battery

[pholliday1]

Love posts like this!

[Greybeard]

Hey Wink, I understand the name now at least 
I was born only able to see clearly out of one eye myself, I was born severely crosseyed too and had two surgeries to fix that, they tried but they won no cigars. Much better than it was though.
 As a result I understand the feeling of always trying to catch up no matter how much we learn.

Bake, google flying cars....you might be surprised at what you find!    All you need is money at this point.

I was reading this entire thing thinking ...BD certainly know his stuff...when is he going to mention cables? I was rootin for the cables to be mentioned...Welding cables that is. The only way to fly!

One thing I don't think BD covered, and it may not be important which is why he didn't cover it; voltage drop/resisance along lengths of cable. Say for instance you decided to mount one of the batteries in the bed or under the back end of the truck, how many wire sizes would be required to account for the extra resistance per foot of wire. Maybe that's getting too nit-picky. I know enough about electricity to burn my truck down pretty fast but I've heard that using wire that is too big is nearly as bad as using it too small? IDK? But inquiring minds would like to know.

BTW-BD excellent writeup! Would you do my last two college papers please? I spend more time proofing those than when I write for fun, I just don't do much proofing ...like for here.  

[winky]

Haha yeah my welding instructor couldn't remember my name the first week we were in class. He wanted to show someone how i was welding so he said "hey winky come over here" some how it ended up sticking lol. Ive read about welding cables i thought it was on these forums? (Ive done a decent amount of reading so im not sure)
I already knew about voltage drop due to resistance caused by the length of wire but im glad you mentioned it. Im hoping this post will allow other people who aren't very experienced with wiring get a decent amount of information without having to hop around pages. I use to have a chart on amps, wire size, length with voltage drop etc.. Im sure a google search would turn up some charts if someone was interested in looking at them.
      On the wire size Ive always gone bigger to be safe. I haven't ever noticed a voltage drop but ive never broke out my fluke meter to check either. I would assume (we know what happens when we assume... )that a wire being too big would depend on if its a signal wire or a power supply? BUT I would also like to know as well. I might do a search tomorrow if the one and only BD hasn't replied

Like Ive said before, I really appreciate everyone's input it feels good to know that you have a community of knowledgeable people that are willing to take time and help out.

[bd]

Over-specifying electrical cable for a given application doesn't hurt - it just doesn't benefit.  Because of the high cost of good quality cable, the benefit-to-cost ratio tanks if you specify much beyond the actual need.  Then there is the fact of introducing unnecessary weight, cable stiffness, and bulk - none of which benefit the end result.

The standard approach to wire selection is to decide on the amount of voltage loss that is acceptable over the length of the cable, and then use the appropriate formula and table (or quick reference chart) to determine suitable cable diameter to carry the anticipated current load.  However, using commonly available charts is risky if and when the chart doesn’t clearly state the percent voltage loss upon which it is calculated.  An engineered voltage loss of <2.5% is generally ideal, balancing cost and performance.  If you're able to secure a 2.5% quick chart, laminate it for future reference.

Virtually all electrical cables have a unique resistance per linear foot of length.  Hence, for any given cable material and cross-section, the longer the wire run, the greater the inherent resistance of the cable.  Because of the relationship between voltage, current and resistance, the greater the current that is demanded by an electrical appliance or load, the more significant the inherent resistance of the wiring becomes with regard to that demand. 

Current varies directly with voltage, but inversely with resistance.  That is to say, as voltage increases, current increases; as resistance increases, current decreases.  Ideally, 100% of the supply voltage should be available to power the load.  By design, the resistance of any electrical appliance is the dominant resistance in its circuit.  But, as cable resistance increases, the ratio of cable resistance to appliance resistance shifts, and more voltage is consumed by the wiring, rendering less voltage and less current to power the load.  To manage acceptable voltage loss across a wire run, the cross-section (gauge) of the wire must be chosen with both the maximum anticipated current load and the total wire length in mind.  The further a cable runs, the greater its required cross-section to minimize voltage lost to the cable. 

A secondary problem that sometimes plagues electrical systems is the heat generated off the wiring when wires are of insufficient gauge to safely power the load.  When a wire becomes hot, it's because the wire is consuming voltage and dissipating the energy as heat.  The heat dissipated by a wire in watts is equal to the current in amperes flowing through the wire, times the voltage being burned by the wire.  A "hot" wire is a wire overburdened by current flow (insufficient gauge).  For example, a 250-amp starter current flowing through a battery cable presenting a 2.5% voltage loss (0.32-volt drop) is dissipating 80 watts of heat across the length of the cable.  Decreasing the cable size to create a voltage loss of 15% (1.9-volts drop) increases heat dissipation to ~473 watts!  At some point, the insulation will melt!

Additionally, for each linear foot of distance a cable must run between the power source and its load, the actual current path is roughly double, because of the added length of the return or ground path.  In cases where the ground path is actually wire, incorporation of the ground wire length is important to overall cable selection.  In the usual case of a steel frame, although steel presents greater electrical resistance than copper or aluminum, the cross-section of the frame in any given application is generally adequate not to be of significant concern beyond corrosion-free connections.

With regard to momentary high-current circuits, such as engine crank circuits, it’s common industry practice to specify cable gauge based on between 5% to as much as 15% voltage loss during periods of maximum current transfer (i.e., while cranking).  Meaning, a cumulative loss of as much as 0.9 volt across the combined length of starter cabling (positive cable plus ground cable) is not unheard of while cranking.  In fact, diagnostic service criteria specifies a maximum of 0.5 volt loss across each starter cable while cranking, or ~4% voltage loss per cable, for ~8% combined total loss. 

----------------------------------------

I consider greater than ~8% voltage loss across high-current cabling to be excessive.  But then I generally spec battery cables and wiring for no more than ~2.5% voltage loss, based on 115% current flow (I incorporate a 15% margin).  For a starter that draws 240 amps on a hot day with a combined cable length of about 7 feet, that equates to 2-gauge copper cable.  Roughly doubling the combined run from the bed (~16 feet total), equates to 3/0-gauge copper cable.  Calculations are based on the formula:

cmil_min=40(C_maxd)

...where cmil_min is the minimum acceptable cross-sectional area of the wire in circular mils (one circular mil is equivalent to the cross-sectional area of a cylindrical wire possessing a diameter of 0.001 inch or 1 mil), 40 is a constant with the units ohm-cmil per volt-foot, C_max is the actual or anticipated maximum current draw in amperes, and d is the total cumulative length of wire run in feet, including the return run.  The calculated cross-sectional area is then looked up in a table relating wire cross-sectional area in cmils to AWG wire gauge.

The quick reference chart is easier!

[Greybeard]

OK, Let's see the electrical engineering degree...I know you have one...cumon lets see it, bragging is allowed . 

Good stuff! Bringing up the resistance differential between a hot 98* and a cold 0* degree day was a great point.

I brought up the length of cable issue because knew a lot of fellas put the auxiliary battery in the back near the second winch mount area to help alleviate this voltage drop problem, then they increased the wire size to accommodate doubling the battery draw down between the primary and secondary battery when under a full load. It has always been way over my head, and I know enough to ask some decent, although maybe unnecessary, questions although I don't pretend to understand all of it. I did learn a great deal about wiring in the military (35 years ago) when I was in A/C and ventilation repair school. The units we learned about ran on 660 and above 3-phase voltage. I have mostly forgotten all of it now until something like this discussion reminds me of a few points.

One unnecessary question that might be irrelevant as per your cost/versus benefit ratio... it has to do with wire strands in the cable. Besides welding cable being much easier to work with than than standard say triplex wire of the same gauge does the increased number of wire strands reduce resistance? This is another one of those urban mysteries floating around at the flat edge of my memory. I seem to recall something about this however, more stands equal more actual wire density inside the insulation and therefor more actual wire per gauge size due to the reduced airspace between wire strands and also more surface contact area between strands. Of course I'm not using the right terminology but....Or is this getting way to technical for use in a truck with a winch? LOL 

[winky]

Holy crap. lol. yup. One well educated guy there.
(pssst... GreyBeard... He is a professor at Harvard)

[Greybeard]

Hahahahahaha! That explains it then! LOL... 

[bd]

LOL - We’ve long passed the point of relevance to adding a battery or wiring a winch!  But, since inquiring minds want to know...   

...does [an] increased number of wire strands reduce resistance?  ...I seem to recall...more strands equal more actual wire density inside the insulation and therefore more actual wire per gauge size due to the reduced airspace between wire strands and also more surface contact area between strands....

Not true!  With regard to your military training, you may be confusing some of the complex electromagnetic effects that occur in alternating current (AC) circuits with the simple resistance effects that exist in direct current (DC) circuits.

Within the scope of automotive wiring:

• Wire gauge is determined by the combined cross-sectional area of all of the individual strands that comprise the wire - not the number of strands, not the amount of interstitial space, not incidental contact between parallel strands.  For any given wire material, 12-gauge "solid" wire, being a fine example, has the same net area in “circular mils” as 12-gauge 7-strand wire, as 12-gauge 500-strand wire.  Meaning, that as far as the wires are concerned, they are equivalent in their current carrying capacity (being limited primarily by the melting point of the insulation that enshrouds the wires).  With regard to a specific conductor material from a given manufacturer, even though the measured diameters of the wires may vary noticeably from one wire to the next, a particular wire gauge has a fixed resistance per unit length whether single strand, medium strand count, or high strand count. 
  
  
• Multi-strand wire will run slightly cooler than solid wire of the same gauge, at any given current load, because of improved heat dissipation from increased surface area.  This is due to the fact that as the diameter of a wire decreases, its cross-sectional area decreases as the square of the decrease in radius (πr²), but its circumference decreases at a rate of only twice the decrease in radius (π2r).  In other words, halving the diameter of a circular wire decreases its volume to one-fourth of its original volume while simultaneously decreasing its surface area by only one-half.  So, for any given gauge of wire, increasing the number of strands significantly increases the surface area of the total volume of wire.  However, this becomes significant only at the point of demanding excessive current flow through the wire, because of poor application.  Using one of the criteria for selecting wire gauge that was presented earlier in this thread makes heat dissipation of wire a moot consideration!   
  
  
• The “skin effect” and “proximity effect” that occur in some AC circuits affect wire impedance at radio frequencies and above, but none create significant effects in low frequency and DC circuits.  With regard to high frequencies, increasing the number of wire strands may reduce the impedance (effective resistance) of the wire.  But, accounting for these effects in automotive wiring is like fretting over the aerodynamics of a snail.  It’s irrelevant!   
  
  
• A point of greater relevance, yet outside the conscious concern of most toy builders, the more wire strands in a cable, the greater the surface area of wire potentially exposed to weather and corrosion.  Stranded wire will wick moisture and electrolytes more readily than solid wire.  It’s an observable fact!  The more strands in a given gauge of wire, the greater the risk of usable wire being transformed into green fuzz.  Although copper is an excellent conductor, the oxides (dark tarnish) and sulfides (blue/green acidic salts) of copper are not!  Especially when using high strand count wire, cable terminations should be encapsulated in double wall, adhesive lined, heat shrink tubing to repel moisture.  In addition, all terminations should be tightly crimped and soldered prior to sealing.   
  
  
• The only real benefit to high strand count wire is its flexibility.  In the case of truly heavy-gauge cable (2/0 and larger), high strand count wire may be necessary to avoid cable and appliance damage due to wire stiffness.   
  

This blackboard is closed!   

If you wish to dig into the subtleties of wire any further, purchase a good reference text on practical wiring or a copy of the National Electrical Code and study to your heart’s content.  Most of the answers you seek should be contained, therein.

[Greybeard]

Well BD thanks for being a good sport ! Excellent (albeit, as you said, pointless to the issue) information. Although it can come in handy for figuring out other areas that might pose issues during a build.

There is one area I would like to reference Winky and that is what BD alluded to; connections. 

At our work we had a good supply of the heavy adhesive lined shrink tubing in multiple sizes bought from Napa. We had a small wire brush to clean as much of the inside of the connector as possible then pinched the eye between some wood in a vise (wood is not a good heat sink). Heat the eye till a good solder of your choice (60/40 is what we used but it may not have been the best choice) melts by touching the inside of the cup with it and fills the cup about 3/4 full then feather the heat and start slowly inserting the wire as the heat builds in it. (the wire should be preheated and fluxed BTW and still hot) Do this slow because if it's pushed in faster than it can take in the extra solder it will blow out of the cup into your face. Not fun, ask me how I know...never mind! LOL.  The shrink can be put on after since the eye will likely be smaller than the ID of the shrink. They sell red and black (maybe more colors IDK) shrink, an easy way to keep track of cables. One thing we didn't do was crimp first. That's your choice.

There might be easier and safer ways to do this but this is the way I was taught. A cable made like this will last forever as long as the insulation does not get perforated. However, forever, seeing as that's a really long time, is relative.

This is the way I make my battery cables on both ends. I used 2/0 cable (because we had a LOT of 300+ amp worn out welding cables at work) with 2/0 post style battery ends and eyes.

[bd]

I know... I know...

I was taught to assemble battery cables the same way you were, and it worked most of the time.  I agree, 60/40 tin/lead rosin flux solder is a good choice.  Silver solder also works, but requires more heat.

This is what I learned through observation and experience:  (a) Preheating large-gauge copper cable with a torch (which is pretty much necessary to supply adequate heat, since the cable itself acts like a sink) causes the copper to anneal and oxidize.  Annealing weakens the metal; oxidation retards solder penetration and adhesion in spite of the flux.  (b) If you don't assemble a lot of cables to maintain decent technique, you run the risk of a cold solder joint because you're stabbing a too cool cable into a pool of molten metal, quenching the pool before the solder wicks up between the strands, resulting in a poor connection.  (c) Having repaired & wired many commercial vehicles, I observed high draw and connection problems that actually melted solder, allowing cables to separate from soldered only terminals.  Not good when its the starter end of a four battery set!   

I discovered that in order to lessen the potential for subsequent service problems, when I securely crimped terminal ends to cables to create a strong mechanical connection before soldering, the crimp would support and retain the cable secure in its terminal, so that the solder merely had to penetrate and seal the electrical connection from unwanted penetration by air, moisture and contaminants, to maintain good electrical conduction over the life of the cable.  Heating the outside of a crimped terminal shields the copper strands from direct flame while quickly and evenly transferring heat to the cable.  When the terminal and cable are sufficiently heated, flux core solder melts and wicks into the connection, filling the voids and sealing it.  When connecting large parts together, it's best not to rely on solder alone to retain those parts, especially if they might be subjected to subsequent high heat cycles. 

But, ultimately, as you posted, whether you crimp, crimp and solder, or just solder is individual choice.  All we can do is provide the data.

-------------------------------

Can't say that I ever had solder tears on my face, that must of hurt!   

[Greybeard]

Hey BD, Sorry it took so long for me to reply, I have two more weeks till I graduate so homework is fierce right now. But I got my project in 30 seconds before it's deadline tonight so I'm happy.

I agree and see your point. I want to point out though that by pre-tinning the cable the solder only has to remelt only a slight amount to adhere. I can see this plain type of joint being pulled apart under a high heat situation though so crimping is a good idea. On our semis we always had the battery ends (the cold ends) clamped near the batteries. The starter ends where left to chance. We did not use our trucks OTR however because we were a construction company in a small town in Iowa. So maintenance was an ongoing thing for one reason or another. Old early 80's vintage trucks didn't help much.

The one concept to remember is to put solder into the cup and then let the hot solder sort of draw the cable in (it does require pushing but it's balanced) and don't actually put too much heat to the cable, let the solder do the majority of the heating. I always thought of this as an art and prior to learning the art, a few burns were acquired.