Doubling up cables

[Texas-Mark]

So, now you think - great I will double up all my wires and they will be cooler. But, now look at the resistance

And, just like with batteries with uneven internal impedance there will be a small but noticable difference in current carried..

4/0 wire has 0.0314Ω per thousand foot
4 awg has 0.0792Ω per thousand foot

so - 2 x 4awg has 0.0396Ω per thousand foot.

Now do the math for 2ft of each setup carrying 150amps. Both class T JLLN style.

Based on your resistance figures, two feet at 150 amps:

4/0 awg = 0.0000628 = 0.00942 volt drop = 1.413 watts dissipated

2 x 4 awg = 0.0000792 ohms = 0.00942 volt drop = 1.414 watts dissipated

 

[robbob2112]

Based on your resistance figures, two feet at 150 amps:

4/0 awg = 0.0000628 = 0.00942 volt drop = 1.413 watts dissipated

2 x 4 awg = 0.0000792 ohms = 0.00942 volt drop = 1.414 watts dissipated

Right and I said

"The wire length difference of 2" didn't really make a difference, but having 2 wires did."

It is what happens when one fuse blows without the other. Pretty easy to happen if one cable shorts right after the fuse. The current in the other cable will jump for a second but not nearly as quick as the cable shorted then it will drop back to whatever double was.

 

[Texas-Mark]

"The wire length difference of 2" didn't really make a difference, but having 2 wires did."

Not sure if I am following. .001 watt is beyond negligible.

Also not following the fuse concerns. Why would you have two fuses in the first place?

 

[HarryN]

The main challenge with using point to point wiring like that with Li batteries is the need to re-set the BMS sometimes.

The normal way to re-set the internal BMS on most batteries is to disconnect it from the rest of the system. When it is wired point to point, this can be a pain.

That is why I do it this way.

 

[ThaiTaffy]

Btw the maths seems wrong voltage drop on 4awg at 12v and 2ft is 1.33% so 0.1596v at AWG4/0 it's 0.28% or 0.0336 I'm getting closer to 5w lost on the 4/0 no idea how to calculate the twin awg4

 

[robbob2112]

Not sure if I am following. .001 watt is beyond negligible.

Also not following the fuse concerns. Why would you have two fuses in the first place?

It isn't about the power - it is about the uneven loading - added percent difference between wires as well. One inch difference is tiny and two is just a bit more. But three inches is 10% imbalance and so on. I can easily see getting a couple of inches out if someone isn't careful.

Someone that just reads to double the wires without caring about the lengths involved could easily go a foot out if the cable routed good.

Added columns to calculate the wattage lost between the two of them - you have to sum them up after calculating independantly because they are carrying different current.

Edit - I am using 12.8 as the voltage of a LFP battery

 

[Texas-Mark]

Btw the maths seems wrong voltage drop on 4awg at 12v and 2ft

The math was based on the resistance given and the current given.

no idea how to calculate the twin awg4

Simple for two parallel equal resistances, 1/2 of one. I.e. 100 ohm in parallel with 100 ohm = 50 ohm.

 

[HarryN]

Btw the maths seems wrong voltage drop on 4awg at 12v and 2ft is 1.33% so 0.1596v at AWG4/0 it's 0.28% or 0.0336 I'm getting closer to 5w lost on the 4/0 no idea how to calculate the twin awg4

https://www.allaboutcircuits.com/tools/parallel-resistance-calculator/

 

[Fd9031]

Looked again and apparently I have another comment -

Each battery should have a MRBF fuse on the positive post for 20% over what the BMS will put out.

I really really dislike ANL fuses - they generate a lot of wasted heat - they are great in a car stereo install, but not in a battery install. In car audio the amp is constantly changing the current drawn with a boom - boom - boom... the average of the booms over time is MUCH lower than the amount of current you fuse for so ANL doesn't really heat up so much.

With batteries and inverters you are also changing, but the constant load is more even and higher than an audio amplifier. So you loose a lot of energy to heat.

I would suggest replacing the ANL with a class T of the appropriate size for the inverter.

What is a class T please

 

[robbob2112]

A class T fuse - they are DC fuses that are fast acting and have very high AIC (amp interupting capacity) - so if there is a short at the inverter it blows and the resulting electrical arc will get put out before there is a problem. With low AIC fuses they can blow and a plasma arc will continue until bad things happen.

 

[Bananaman321]

The only real concern is that the same amount of current flows through both batteries the jumper cables between the 2 batteries are the same length so the same amount of current will flow between both batteries

I believe this is the big concern here

I also believe we all can agree that 2 identical conductors of differententh will carry current directly correlating with their length respectively

That being said and after a closer examination of the OP's photo wire length does seem a little excessively imbalanced possibly to the extent of 25% which would lead to the shorter conductor carrying 25% more load

If the wire is of adequate size and fuse appropriately I don't see that this would be a problem
But for the sake of everyone here let's just shorten The Wire
It's only 2 Lugs

And then have a Beer

 

[Texas-Mark]

That being said and after a closer examination of the OP's photo wire length does seem a little excessively imbalanced possibly to the extent of 25% which would lead to the shorter conductor carrying 25% more load

You are joking right?

 

[Bananaman321]

You are joking right?

If I had a DC clamp meter I would replicate the Ops setup for proof of concept

This will have to do: one jumper wire is 1.5x longer than the other with 20a combined load wire is 14ga solid.
The resulting load is exactly what we would expect

I made wire length extreme deliberately. It was powering my house through Gen port on 18kpv from 2 eg4-3000ehv-48 joined p1-p2 240v open neutral to 18vpv

 

[Bananaman321]

If I had a DC clamp meter I would replicate the Ops setup for proof of concept

This will have to do: one jumper wire is 1.5x longer than the other with 20a combined load wire is 14ga solid.
The resulting load is exactly what we would expect
View attachment 228818View attachment 228819View attachment 228820View attachment 228821
I made wire length extreme deliberately. It was powering my house through Gen port on 18kpv from 2 eg4-3000ehv-48 joined p1-p2 240v open neutral to 18vpv

I can perform this exercise with equal length conductors of different guage wire if you'd like