Running dedicated alt wire?

[Mr DeeBeez]

I have been having a lot of voltage difference between my front battery, and my back batteries. I have an xs power D3400 up front, and two xp3000's in the back. I have a mechman 240a alt (Soon to be two 240a alts) and two runs of power, two runs of ground from front batt to back batts. 

I have a volt meter to my front battery, and an smd vm1 on the back batteries. When I full tilt, my voltage differs a lot between the front and back. I was thinking this was because the power and ground cables were sky high cca that has been in there for about a year. I changed all the runs to 2/0 welding cable, and still have that issue. My back batteries will be in the low 13's, while my front battery is still at 14.7-14.9.

Should i run a dedicated alt wire to the back batteries to help this problem? Or should I run a ground from my back batteries to the frame in the rear? Dont think I should have such a difference in voltage with 4 total runs of 2/0 welding. 

Amp is a sq 3500 @ half ohm

Any input is greatly appreciated

[ShadeTreeMechanic]

What you are seeing is the voltage drop across the front to back cables. This is why front and back volt meters are so important. Has this always been an issue or did it just happen? If this has always been an issue, then grounding to the chassis would help and if it still does it, then keep adding positive runs and negative runs until the voltage of the rear is the same as the front and then add one more.

Edit: before someone else says it, check the grounds. Lol

[audiofanaticz]

Just adding more runs from your front battery will fix it, as long as you do have alt to front battery wire.
However keep the runs all the same length, if one is longer than the other, the longer wire wont be used to its full potential due to it having more resistance.
A couple inches wont make much of a difference, but once you get long runs of wire that are 1+ foot longer than the rest it you will see a lesser draw if you where to clamp wires.

Also as Shadetree said, chassis grounds in front and rear are a MUST if you dont have them. This may make a world of difference since the chassis has a lower resistance than the actual ofc (hopefully) wire that your using.
Its been tested and proved, however when you get into big boy power the additional negative runs will help as well.

OCD and equal lengths is a pita... and Now I want to rewire once again. LOL

[Mr DeeBeez]

I figured two power and two ground runs were already overkill. I'm only clamping 2700 watts. Should not be more amperage than the wires will carry. I do have front grounded to chassis, but not the back. I will add back grounds and see if that helps. 

 

And yes this has always been an issue. 

[audiofanaticz]

It doesn't come down to the amperage that the wire carries, but the resistance of the wire.

[ShadeTreeMechanic]

Yeah the power that a cable can carry is always at a certain voltage drop. Some cables are rated at a ( gasp) 6 volt drop. Keep adding until you are happy and then one more.

[audiofanaticz]

Do you have a DC clamp meter handy that you can go out and clamp both your amps positive dc inputs on a full volume 7-10 second test tone?
And do you know about how long your wires are?
Also an accurate front voltage during the burp with a dmm, and the rear voltage during the burp off the smd meter would be fine.

If you do, I can calculate a decent rough estimate and how much wire you would need to add to see a decent rear voltage.

Clamped output power of the amp is meaningless even if it is only "clamping" 2700 watts, because no way that thing is only pulling 270ish amps continuously when full tilt being wired at a half.
I could be wrong judging off my Four 4500s wired at .35 that see around 880 amps draw each.

[reedal]

22 minutes ago, audiofanaticz said:

It doesn't come down to the amperage that the wire carries, but the resistance of the wire.

But as resistance goes up, amperage goes down due to them being inverse of each other. So your statement is kind of null.

[Mr DeeBeez]

the runs are roughly 16'

and yes i have a dc clamp meter. I will go check right now the amp draw at each of the inputs. as well as voltage during the burp.

 

[ShadeTreeMechanic]

 

38 minutes ago, reedal said:

But as resistance goes up, amperage goes down due to them being inverse of each other. So your statement is kind of null.

But the wire is rated in current capacity. It is at a certain voltage drop but they don't always tell you that. Any resistance is voltage drop. That is what we are trying to minimize. Throw the current rating out the window.

Edit: in the end, you want the rear batteries to act like they are beside the front battery, connected by a bus bar.