Does it make any sense

[Hello Below]

I understand the potential benefit of balancing a battery bank by feeding positive and negative from opposite sides of the bank. But does it make any sense when batteries are separated by a fair distance.

I have 4 x 6v US Battery (US 2200 XC2) making up my house bank. Physically the 2 pairs reside about 3 feet from each other. Currently the positive and negative feeds are taken from the same pair of batteries (pair #1). I am about ready to wire in a Victron Charge Controller for a new solar panel and thought that I would take this opportunity to correct the wiring to meet standard practice. Currently the negative cable from the negative post of pair #1 to the -ve bus bar is about 1.5'. If (when??) I rerun that cable to the -ve bus bar from the negative post on pair #2 an additional 13' (round trip) of cable is necessary. I am not convinced that there is any merit in doing this.

 

[Johnb]

You need to state the wire size for a voltage drop calculation to be done.

 

[Hello Below]

You need to state the wire size for a voltage drop calculation to be done.

I will be using 2/0 cable. My concern is the total length of approx. 45' round trip to my Balmar 100A alternator with the "new" negative feed.

 

[Bill Roosa]

As long as your connections are good you can ignore all that stuff. Unfortunatly lots of folks don't maintenance their battery terminals at all or at least till the engine doesn't start so ....... I would note that when the high load is not on the bank the individual batteries are still connected and will equalize. So the only issue is when the admeral uses the hair dryer for more than 15 minutes.

 

[Hello Below]

So the only issue is when the admeral uses the hair dryer for more than 15 minutes.

But not with the Balmar trying to pump in lots of squiggly amps after lying at anchor for 3 days?

 

[Johnb]

2/0 cable has a resistance of 0.08 ohms per 1000 feet. With a charging current of 40 amps per example the voltage drop would be 30 thousandths of a volt over 10 feet of wire. (http://www.bulkwire.com/wireresistance.asp for a good calculator)
Where the world is connected to the combination makes no difference, as long as it is done right. The voltage drops on the individual pieces of wire machs nicht.

 

[Hello Below]

Thanks johnb for the great calculator, it's a keeper. I have used a number of these calculators in the past but none that give the results in such a logical layout.

So I calculate that there will be an increase drop of approximately 0.11v (0.804%) from the old wiring to the new, but for the closest pair of batteries only (Pair #1). The cable length for the furthest pair remains the same (Pair #2). This is with the assumption that the round trip cabling from the alternator to Pair #1 is currently 30' and will go to 50' and the alternator is putting out 70 amps. Of coarse the drop would be less as the current decreases (either into or out of the batteries). I would expect that overtime as the current decreases, the 2 pair would equalize regardless of distance between them (if I=0, voltage would have to be the same at the 2 battery pairs).

I agree that the battery pairs would be balanced with the relocated ground cable and the loss attributed to the increase length is minimal. However, over the past 3 years I have gone to great lengths to make best use of the FLA batteries (464 Ahr) as possible by swapping out halogen bulbs with LEDs, installing a higher output alternator with external regulator and ensuring that we are mindful of turning items off when not in use.

I guess the original question could have been: "What is the benefit of cross wiring battery pairs to ensure they remain in balance?" Is there maybe increased life of the batteries or some other physical/electrical phenomenea that I am missing?