DC wiring - Best Practices

[markopolo]

DC wiring - Best Practices

Forum member Hein asked a great question on the Advancing Alvar topic. Alvar is Davydd's new Class B motorhome built by Advanced RV that has 800ah of lithium batteries. Davydd has reported seeing DC current flows of up to 240 amps when the battery bank is recharging. It's impressive and bound to become the new normal as more coaches get built with this technology.

Davydd previously reported that Advanced RV ran a length of 4/0 (0000) gauge wire from the dedicated second alternator to the battery bank for the positive + connection. It's a long wire run from near the front to the rear of the van. On the negative - side the alternator is grounded to the unibody/subframe near the front of the van. There would be a similar heavy duty ground wire from the rear battery bank to the unibody/subframe at the rear of the van.

Table of AWG wire sizes: http://en.wikipedia.org/wiki/American_w ... wire_sizes

Hein's question was:

Hello Davydd,

Glad to hear that you are having a great time in your new RV.

You mention that you have experienced charge rates of over 200A and you also say that the auxiliary alternator is grounded to the chassis with a short 4/0 cable. That means you are passing that current through the chassis on it's way to the coach batteries. What does the coach battery ground look like? Is it OK to have the body passing that much current? I am curious if the connections to the body warm up when under full charge.

A very informative site turned up in a search: http://www.w8ji.com/battery_wiring.htm# ... ed_battery:

With distances more than a few feet, the chassis path through any steel unibody vehicle is far less resistance than any practical size of copper cable, and any common frame. Use the chassis for long grounds, such as from a rear mounted battery to the front. The only requirement is a good connection to the unified sheet metal.

Be cautious in frame-type vehicles, because some use rubber frame-to-body isolators to reduce road noise. See my F250 for an example of an isolated body shell in a frame-type vehicle.

And: http://www.w8ji.com/negative_lead_to_battery.htm

Connecting negative leads to battery posts, and long negative leads, are almost always a mistake.

Also read this link: http://www.w8ji.com/battery_wiring.htm# ... ed_battery

And this: http://www.w8ji.com/ground_loops.htm

Vehicle Grounds

Vehicle grounds in typical unibody passenger cars are a special situation. Mechanical construction techniques that make the platform rigid also work to form a large wide-area chassis ground path with very low resistance. The welded shell forms a very low resistance ground conductor, and is an excellent point for a common ground connection for signal and power grounds. While not zero resistance, the body shell is the closest thing to it. Using a four wire resistance measurement method, my 1989 Mustang measures less than .002 ohms from my rear battery ground to my front inner fender frame rail ground. This is the approximate equivalent of 15 feet of number 0 AWG copper wire and connectors. Much of this resistance is concentrated around the grounding lugs (before current has a chance to spread), and not over the body path. If I improved the connection points, I could greatly reduce the small resistance my system has now. This isn't really necessary, so I have not bothered.

It makes little sense to run a heavy copper negative from engine to battery when the chassis is already there and the body shell, including casually made connection losses, has less resistance than a well-made cable.

There seems to be more interest than ever in high-tech DIY Class B builds now so it a great time to explore this topic.

I'm going to double check the negative grounds in my van specifically looking for undersized wire. I know my coach batteries are grounded to the body/frame.

[gerrym51]

isn't having batteries that have high CAR just as important? Davydd has lithium which have the highest CAR. Next come the tppl agm's.

then standard agm's .

one without the other foils the purpose.

[booster]

Over the years, I have done lots of ground checking on cars, nearly all unibodies, and found that if the connections to the body were good the grounds were excellent and extremely low resistance. The area is huge, so current shouldn't be an issues. The biggest issues tend to be with the ground straps from the engine to the body. Good, paint removed, greased or silver paste, connections with stainless or brass fasteners work well.

I have heard that some of the body on frame vehicles, like the Chevy van, or our Roadmaster wagon, can have some connection issues due to the rubber mounted body onto the frame not being well connected electrically. I have never done it, but I think I will put the Fluke on the van today while I am out in the shop and see if ours shows any resistance. Our ground for the coach batteries attaches to the body of van in the wiring area in the back of the wheelwell, from the shunt, the engine is probably tied to the frame or the body with a strap, don't know which. Roadtrek originally has them connected to the frame directly at the battery boxes.

The newer vehicles seem to be using fewer ground connection points and wiring to them from more places, and they also have improved the connections themselves many times. I don't see as many of the selftapping type screws anymore for grounding, as they have spotwelded in normal thread nuts with regular bolts.

One thing you may run across, which is an age old fix for shakey ground connections, especially in the rust belt where the steel gets rusty at the connections, is to have an external tooth lockwasher between the connection and the body of the vehicle. It will bite into the pitted area of the body, not turn and loosen, and show nearly no resistance. The lockwasher also gives a little extra room for silicon grease to help slow down the corrosion. Note that this should never be used on a very high current circuit, maybe 10 amps max, as you don't have much surface area of contact.

[booster]

Originally Posted by gerrym51

isn't having batteries that have high CAR just as important? Davydd has lithium which have the highest CAR. Next come the tppl agm's.

then standard agm's .

one without the other foils the purpose.

In Davydd's case, I don't think the charge acceptance rate is going to make any difference at all, because he has so much capacity Even if he had wet cells charging at .4C (which they will do, but not ideal) he would be able to put 320 amps to them. He runs out of amps available long before he runs out of charge acceptance. The charge acceptance is more of an issue if you are undersized in capacity, but even then only the wet cells drop out of the picture.

The much bigger issue is the hold time in absorption that is required for the lead/acid batteries to get them full and keep them healthy.

[markopolo]

The underlying DC wiring matters for both charging and discharging. Sometimes we'll see long negative wire runs in B vans when it isn't necessary. There may be more resistance in the circuit using wire over using the shell or frame.

(missed Boosters earlier post)

[gerrym51]

I'm glad you guys understand this stuff cause i don;t

[Davydd]

If I wasn't sitting on sharp Quartz desert rock I would crawl under and take a look.

ARV basically said the lithium ion batteries could be charged with as many amps you could throw at it. Theoretically the alternator could go as high as 300a. Spot checking it seems to be about 210-220 most of the time then drops when fully charged. Sitting with nothing on but the refrigerator, Silverleaf panel, inverter loss, Pioneer screen idiot lights, standby stuff, parasitic loss and whatnot our RV seems to drain at an average of 8-10ah when observed when the refrigerator is running. If we are parked solar is overcoming that and charging the batteries. Driving one hour will replenish that and all our use which would be lights, charging device batteries, TV and radio, cooking on the microwave/convection oven, electric induction cooktop and at least 4 Keurig cups of coffee.

That positive copper wire from alternator to batteries 4/0 AWG is 0.53" or easier to remember 1/2 inch in diameter. ARV said it was $7 per ft.

[markopolo]

I added another body ground wire when I installed the isolator/relay recently. The drivers seat base seemed suited for it. From what I could see body/chassis grounding for the house batteries was a short piece of 4 gauge wire in my van. I added a short piece of spare 6 gauge that I had.

I have 2 x 20' of 4 gauge in my spare parts inventory now. As time permits I should be able to improve on the wiring where needed. 80 amps is about the largest load I anticipate having in this van.

Note: I don't like parallel wiring generally. If one path fails then all the current will be on the remaining path.

[booster]

Originally Posted by markopolo

Note: I don't like parallel wiring generally. If one path fails then all the current will be on the remaining path.

Me either, but I did do it on our coach wiring when I added a second #4 from the separator to the coach batteries. I have 80 amp breakers on each end, of each wire, to protect them. I have seen a lot of installs of heavy wiring where they ran two wires, tied them together, and fused them as if they one wire. Probably OK for very short stuff, but would never do a long run that way.

[papab]

Why the worry about parallel wires? If they are fused & one fails &shorts, the fuse blows. How would one fail and leave the other intact?

[avanti]

Originally Posted by papab

Why the worry about parallel wires? If they are fused & one fails &shorts, the fuse blows. How would one fail and leave the other intact?

The problem is when one fails OPEN. In that case, you end up with too-large a fuse for the remaining path. I suppose you could fuse each path separately.

[papab]

How could one fail open? I suppose the lug on one end could come unscrewed or uncrimped, but for a good installation it seems unlikely. I'm poking at this because I want to use 1/0, but its so big, thinking about 2 cables.

[avanti]

Originally Posted by papab

How could one fail open? I suppose the lug on one end could come unscrewed or uncrimped, but for a good installation it seems unlikely. I'm poking at this because I want to use 1/0, but its so big, thinking about 2 cables.

No cable failure is very likely. I agree that failing open is less likely than a fail-short. But,the whole point of safety engineering is to (a) make all failures unlikely and (b) protect against the unexpected.

[booster]

I must be missing something here.

If you have a source---feed to two separate fuses that feed the two separate wires---tied together on the exit end to feed load. I don't see how any wire could be overloaded.

A disclaimer is that if you have power source on each end, like you do when you have a separator and charge both ways, you would need the two fuses on each end of the wires.

Wires fail open all the time, in my experience. The biggest causes I see are vibration and corrosion.

[avanti]

Originally Posted by booster

I must be missing something here.

If you have a source---feed to two separate fuses that feed the two separate wires---tied together on the exit end to feed load. I don't see how any wire could be overloaded.

Yes, as I said, if you fuse each wire separately, it is fine. It is just that people tend not to do that.

[markopolo]

If one of the runs fails for any reason then the remaining would likely fail assuming the current was enough to blow the fuse. Doubling up on lugs, fuses, fuse holder etc. adds to the cost of the project. Resistance in the circuit could be increased.

Without specifics, it's hard to comment. One persons parallel wiring setup could be for fully functional redundancy another persons setup could be using wire rated half of what the load would require.

From what I've read, parallel wiring runs should be the same gauge, length and and have same fusing on each.

I'll probably do a parallel run when I add two additional batteries. I don't consider it to be a "Best Practice" (topic title) but the wire kits were deeply discounted They included 60A fuses with 4 gauge which seems a bit conservative but OK for how I'll use it.

Yesterday, I bought one of those crimp tools that you hit with a maul. That's how the guy at the local Napa has put the lugs on heavy gauge wire for me in the past. I like the idea of doing it a home - easier to get the length exactly right etc.

[booster]

Originally Posted by markopolo

If one of the runs fails for any reason then the remaining would likely fail assuming the current was enough to blow the fuse. Doubling up on lugs, fuses, fuse holder etc. adds to the cost of the project. Resistance in the circuit could be increased.

Without specifics, it's hard to comment. One persons parallel wiring setup could be for fully functional redundancy another persons setup could be using wire rated half of what the load would require.

From what I've read, parallel wiring runs should be the same gauge, length and and have same fusing on each.

I'll probably do a parallel run when I add two additional batteries. I don't consider it to be a "Best Practice" (topic title) but the wire kits were deeply discounted They included 60A fuses with 4 gauge which seems a bit conservative but OK for how I'll use it.

Yesterday, I bought one of those crimp tools that you hit with a maul. That's how the guy at the local Napa has put the lugs on heavy gauge wire for me in the past. I like the idea of doing it a home - easier to get the length exactly right etc.

It is interesting about the parallel stuff. As Mark says, adding stuff increases resistance, most times. With two fuses in parallel, you double the ohms of resistance you are using, but they are in parallel, so the circuit resistance due to them actually drops if the were tied together on both sides of them. In the example here, there would be no extra voltage drop between a 5 ohm resistance before both wires tied together, or two separate wires where each of the wires had a 5 ohm resistance before them. The same thing happens with all the connectors in each circuit. All this said, you do want the two circuits to be as close to the same resistance as each other as possible, so they share the current equally. Length is the biggest factor, but a shorter run with more connectors could be the same resistance as a longer unbroken run.

Again as Mark said, if the parallel circuit is run to reduce voltage drop and heat, with low current low enough that one of the wires could carry it safely (but at higher voltage drop), you wouldn't need to fuse them separately.

With 60 amp fuses, and 4 ga wire, I wouldn't think length, double fusing, or much of anything else need to be considered, as the capacity and voltage drop will be of no consequence.

I also have one of the hammer crimpers, and have used it for all of the wiring on our Roadtrek. It works very well when used right. I have done pull test, and resistance test on every connection and never found a bad one to this point. The downside of them is that it is very difficult to use them inside the van, if you have a cable that is already in place, because you need a really substantial surface to pound on if you want a good crimp. I had to do a couple inside the back of the van for the previous changes, and I wound up taking the 2' square by 1/2" thick steel base for my bike stand and muscleing it into the van. It still took more pounding then on the big plate on the workbench, or the big vise. Handybob hated the hammer crimpers and ran them down in his many very interesting articles, but I have not seen any of the problems he worried about. Of course, most of what he was dealing with were field installs, so folks were probably trying to use them on a 2X6 laying on the ground.

[markopolo]

Good points. Ampacity + Voltage Drop. The wire should be sized to carry the load and have acceptable voltage drop. Those are two items to consider.

Two conductors in parallel would equal a single conductor 3 gauge size larger. (my understanding - please correct me if wrong)

So 2 x 4AWG in parallel would be the equivalent of 1AWG.

I refer to this voltage drop calculator: http://www.powerstream.com/Wire_Size.htm

[booster]

Originally Posted by markopolo

Good points. Ampacity + Voltage Drop. The wire should be sized to carry the load and have acceptable voltage drop. Those are two items to consider.

Two conductors in parallel would equal a single conductor 3 gauge size larger. (my understanding - please correct me if wrong)

So 2 x 4AWG in parallel would be the equivalent of 1AWG.

I refer to this voltage drop calculator: http://www.powerstream.com/Wire_Size.htm

That sounds just about what I came up with when I added the second 4 ga from the engine to coach.

[papab]

Where can I get 1/0 lugs properly crimped? I bought the cable and lugs at NAPA, but they didn't have a proper crimp tool, they had a cable cutter with crimp nubs on it. I don't think I want to trust it. Before I bought the cable there I tried 2 auto electric shops & neither one of them wanted to help. I've tried several elect contractors, batterys plus.
What do you think about this method:
http://www.instructables.com/id/Properl ... /Crimping/
good enuf?