DC wiring - Best Practices

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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.