What determines battery cable size?

[johnm1]

Hey all,

I'm looking into replacing the factory battery cables in the spring to hopefully get more efficient (as in shorter) generator charging time. My question is, what do I "need to do" vs. what would be "nice to do"?

1. I do not know the camper's wire size.
2. There are at least 14 crimp connectors between the 2 (6V) batteries and the "power center".
3. I have replaced ~8 of the very poorly crimped on ring connectors with real crimp battery lugs.
4. Out of the 8 replaced, I easily pulled 7 of them off with pliers with very little, to no effort.
5. I do not anticipate any heavy, or fast, battery draw downs. No inverter, no power hungry 12vdc appliance, etc.
6. I've replaced the WFCO with BestConverters Boondocker 75amp unit.
7. I have 2 Champion 2k (1.7K) invertor gennys with parallel kit to recharge as needed.
8. Right now the neg cable is screwed (self tapping screw) right to the frame near the battery and the same right near the power center.

What to do?
A. What wire size/gauge would be appropriate for my needs?
B. Should I run wire from the battery directly to the power center or replace the self tapping screws with something else more substantive?
C. The pos cable runs from the battery to some relay or circuit breaker thing (??) that's attached to the frame and exposed to the elements. Should I move it to more weather resistant area?
D. Should I install a fuse someplace to protect the batteries? If yes, what kind/size and where to put it.
E. I may, or may not, go solar. Depends upon many factors but in where we camp in NYS parks, there isn't that much sun (lots of trees) so it's not on my short list.
F. The distance from the batteries to the power center is around 20-25ft one way. This includes from the batteries down to frame and back up to the power center.

Hopefully I've listed all of the info that's needed to make proper recommendations. Let me know if I missed something.

Thanks,
John

[johnm1]

tenbear wrote:
If you have a generator you may want larger wire than #4. I have a 3000 watt generator and it can draw 80 Amps to start. The OEM used #2 wire for the battery connections.

My generators are portable and I use my son's to do the starting.

Sam Spade wrote:

johnm1 wrote:

I don't expect the truck to recharge the batteries unless it's a long day's driving and I don't even know what voltage the dual alt configuration of the 2013 GMC Duramax puts out.

To repeat:

What do you expect to GAIN by making these changes ?

Seems to me like it's impossible to answer that since you state that you don't really know what your starting point(s) are.

If you spend a ton of time and money in making "improvements", how will you know if you have really accomplished anything useful or not ???

What I expect (hope) to gain is more efficient charging from my portable generator(s). To me, efficient means faster charging and having to run them less often. It's less of a hassle and I suppose a bit of a cost savings on fuel, etc. I don't much care about the cost of upgrading the wire. It'll be an incremental cost. I'm going to replace it. So, 4 awg costs me $1.22/ft and 2 awg costs $1.54/ft. That'd be a "piddly" $16 more. BTW, 1 awg is only $2.22/ft. If I spend a little bit more to begin with, I might be able to enjoy my camping time a little bit more. That's what I'm looking for.

[wa8yxm]

What determins wire size is max amp load

What improves charging time

Larger wire will pass more current with less voltage drop

A DIRECT connection from the converter to the battery bypassing those 14 connections you mentioned.. That will help even more.

[Sam_Spade]

johnm1 wrote:

I don't expect the truck to recharge the batteries unless it's a long day's driving and I don't even know what voltage the dual alt configuration of the 2013 GMC Duramax puts out.

To repeat:

What do you expect to GAIN by making these changes ?

Seems to me like it's impossible to answer that since you state that you don't really know what your starting point(s) are.

If you spend a ton of time and money in making "improvements", how will you know if you have really accomplished anything useful or not ???

[tenbear]

If you have a generator you may want larger wire than #4. I have a 3000 watt generator and it can draw 80 Amps to start. The OEM used #2 wire for the battery connections.

[BFL13]

enblethen wrote:
You must consider the length of the conductors and their resistance.
The positive would be copper with more resistance then a welded frame. The welded frame will have no voltage drop that you would need to consider into the calculations.
Blue Sea is most likely figuring a boat where both the positive and negative are using copper conductors.

The frame isn't quite that good, but it is usually lower R than the long pos wire run.

(To answer a question earlier, you don't need the pos and neg paths to be of equal R, but they both "count." It is the total R of them both making the circuit that matters. Any reduction in R anywhere in the circuit helps.)

I had a thread about this a while back, where I was using Salvo's method of measuring R on a path, and posted a bunch of different results from things I tried. Don't have my notes anymore on that, but it must all be in the archive somewhere.

First I tried to reduce the R on the neg side (mostly frame) by adding a second neg path using a 20 ft length of copper water pipe with wires at each end bolted on to connect with. That knocked the neg R way down, and made the amps from the converter to the batteries go up quite a bit.

So next I ran a 25 ft #1AWG cu-al jumper cable set from the battery lugs on the fuse panel by the converter inside, out the door, and up to the battery bank and clamped on. This really knocked down the R on the positive side!

With that all set up for both paths, I was able to get 56 amps from my 7355 13.8v converter to the batteries (as seen on the Trimetric)

So yes, you can make improvements and measure the results so you don't overdo it-- On that, once you get the R down to where the converter is doing its max amps, there is no point reducing the R further, since the converter can't do any more amps than its max.

As it happens, I didn't take advantage of all that! I got different chargers mounted up front close to he batteries instead. The 7355 is still there as before.

[SoundGuy]

enblethen wrote:
You must consider the length of the conductors and their resistance. The positive would be copper with more resistance then a welded frame. The welded frame will have no voltage drop that you would need to consider into the calculations. Blue Sea is most likely figuring a boat where both the positive and negative are using copper conductors.

Good point. However, the entire negative run from battery to converter wouldn't be just the frame of the trailer ... from my negative battery post to where it connects to the frame the run consists of ~ 4' of 6 gauge. The converter is located at the far rear of the trailer so the positive cable which is also 6 gauge would have to be at least 25', if not longer, while the negative run from the converter to the trailer frame is likely also several feet of 6 gauge. No wonder my WFCO can't deliver it's rated 14.4 volts at the battery. :M

Mounting any replacement charger as close as it can be to the battery location is the best solution, along with using as heavy a gauge of cables as is practical.

[enblethen]

You must consider the length of the conductors and their resistance.
The positive would be copper with more resistance then a welded frame. The welded frame will have no voltage drop that you would need to consider into the calculations.
Blue Sea is most likely figuring a boat where both the positive and negative are using copper conductors.

[SoundGuy]

johnm1 wrote:
Do I count both the pos and neg in the cable length? As in, 25ft one way is counted as 50ft in the calculations?

From that Blue Sea link I provided earlier ...

"Find circuit LENGTH IN FEET along the left side of the chart. Note that the total length of the circuit is the roundtrip distance from power source (usually the battery) to the product and back."

That's a mighty l-o-n-g run that for high current draw is going to take substantial cable gauge in order to achieve a minimal 3% loss. :E That's why I left my original converter in place, turned it off, and mounted my replacement charger in the trailer's front pass through cargo compartment, not far from the battery location ... minimal loss even though I used just 4 gauge cable. 🙂

[johnm1]

OP here ...

Thanks all for replying! 1st, my apologies. My crimp count was off.

I do not have 14 crimps as I stated. I have 10 crimps and 2 bare connection pts at the Boondocker. I mentally counted 6 at the battery when I wrote this, not the 4 that it is and I also counted the bare wire pts at the Boondocker.

Here's what I have:
4 - Battery pts (2x6v)
2 - Pos cable circuit breaker on the frame
2 - Battery cut off switch (factory installed)
2 - Neg cable screwed to frame (with #10? self tapping screws)
2 - Bare wire taps into Boondocker.

Seems like I can't get any less than this. However, I can do better with the wiring and the connection pts.

I don't think I have room to move the Boondocker closer to the batteries as it would interfere with the only outside storage compartment that I have and it's small enough to begin with.

Do I count both the pos and neg in the cable length? As in, 25ft one way is counted as 50ft in the calculations?

I don't expect the truck to recharge the batteries unless it's a long day's driving and I don't even know what voltage the dual alt configuration of the 2013 GMC Duramax puts out.

Thanks again,
John

[SCVJeff]

The gauge has clearly been settled, but what stands out is the 14 crimp connections between two places that should be hard wired, or have a master switch in between at best. What are all these stops along the way ?

[theoldwizard1]

Just to add a couple of things

• Use "marine" grade wire. It will not corrode
• Use "marine" (dual wall) on all crimps
• If you really need that many connection point on the vehicle 12V "bus" consider installing a "bus bar"

If you are assuming that you vehicle alternator is going to recharge your house bank while driving to your next campsite you may be disappointed. Most "modern" (>2000) vehicle charging systems will NOT put out enough voltage (>14V) to recharge your house bank.

There are a few DC-DC battery chargers that will step up low voltage (10VDC) to the require voltage.

[Matt_Colie]

John,

You have done a lot of good stuff and it sounds like you did it right.

If your new Boondocker is at the power center (25' away), I kind of hate to tell you that you are still in the weeds. With 50' of loss between the converter and the house bank, you will play hell recovering the bank. There is just no way around this. Even if you used silver plated buss bar, the loss would be still be too much.

Move the converter. Put it as close to the house bank as you can without having it exposed to the fumes that come off the bank when it is charging. The AC cable that runs this can be 14AWG (12 would be better) and it is much easier to deal with than 00 copper - even as welding cable.

The run to the distribution panel is nothing. That can just make the light dim when the water pump runs..

Before the depression, I did a great deal of work on people's boats. I had this same problem with my coach that I used then and you bet that I fixed it as soon as I figured it out. Now my converter is about 3' from the house bank.

Matt

[pianotuna]

Hi,

You may not need both generators for battery charging.

If you have the adjustable voltage boondocker, then check voltage at the battery bank and adjust as needed.

[SoundGuy]

Here's a Blue Sea DC Wire Gauge Chart I used when installing my inverter. Chances are your factory wiring is no better than 6 gauge from battery to converter, a situation made even worse in my case as my converter is located at the far rear of the trailer :E ... no wonder my WFCO converter can never get beyond 13.6 volts. :R It works well enough for general use but I've since installed a stand alone charger within feet of the battery that can do an honest 14.4 volts on the battery through 4 gauge cable ... the battery clearly likes it and so do I. :W