Battery Cable Length

[DerpsyDoodler]

If the current in the cables is high, near the max ampacity of the cable, then keeping them all pretty darned close in length is more important. But if the current is relatively low compared to what the cable can handle, it's not so important. If the voltage drop in unequal length cables has a couple zeroes after the decimal point, the batteries would charge and discharge so closely together that you'd never have any problems

I would try to stay pretty close with all cables when paralleling batteries . . . . but I wouldn't be measuring with a micrometer - An inch or two isn't that big a deal, IMO . . . . assuming adequately sized cables - Which for me is a wire gauge or two above what's minimally called for

Don

So when your application calls for 4/0, you go with a mcm equiv of 2 sizes larger? I'd just assume go with what ampacity dictates and keep the parallel cables of equal length. Not that your approach won't work. It's all about resistance. Those 2 sizes larger just aren't necessary if you keep the parallel cables/bars similar lengths. Maybe an inch is fine, but when I'm cutting cable, I cut the first piece to length then use it as a jig. Unless you have really bad eye-sight, you'll get it within a 1/2" or less.

 

[Woody]

Please observe how small the resistance is. It’s noted per 1000ft.

The voltage drop equation is:
Voltage Drop = Resistance x Current

The resistance (ohms) is for the the difference in length between the power runs aka cable length.

Take the time to do the calculations and see that we are talking a very small fraction of a volt.



For system where the voltage drop is 1 or more volts, it’s an issue that can be addressed by 1) going up in diameter or 2) redesign your system to use a higher voltage. Typical issue in 12v systems using 4/0 cables pulling hundreds of amps. Switching to a 24v design drops the current in half. Same with 24v system going to 48v. Current drops in half for the same power.

 

[Alphacarina]

So when your application calls for 4/0, you go with a mcm equiv of 2 sizes larger? I'd just assume go with what ampacity dictates and keep the parallel cables of equal length. Not that your approach won't work. It's all about resistance.

Personally, I've never had any project requiring 4/0 cable or anything even close. That's lots of copper! - To me, that would be a sign that the system design voltage is just too low. For example, if I was designing an RV system to run air conditioners with solar power, I wouldn't even consider 12 volts, nor even 24 volts - I would go straight to 48 volts and it would simplify so many things, especially if a lower voltage meant paralleling any batteries. Even with properly sized cables of perfectly equal lengths, when you're running hundreds of DC amps things tend to get hot whenever there is any compromised connection

Don

 

[DerpsyDoodler]

Personally, I've never had any project requiring 4/0 cable or anything even close. That's lots of copper! - To me, that would be a sign that the system design voltage is just too low. For example, if I was designing an RV system to run air conditioners with solar power, I wouldn't even consider 12 volts, nor even 24 volts - I would go straight to 48 volts and it would simplify so many things, especially if a lower voltage meant paralleling any batteries. Even with properly sized cables of perfectly equal lengths, when you're running hundreds of DC amps things tend to get hot whenever there is any compromised connection

Don

24v 4000 watt low frequency inverter with high surge capability. It is what my samlex evo inverter calls for with a 350a fuse. Quite within spec. When my A/C is running during the day, with lights on and a few devices charging/laptop plugged in, the draw off my battery bank is ~45amps. I can run my induction stove without turning off the A/C. I will have no problems powering my 40' coach.

Being a novice, I was not comfortable working with a 48v battery. 24v is more than enough to command my respect. Nervous enough about the 100v strings of solar panels I'm installing on the roof. The 5 year plan would
be to upgrade to a 48v bank and inverter(and what do you know, reuse all that beautiful copper as well as the DIY battery cells). Quite happy with the investment I've made.

 

[BobCollins]

All parallel cables should be the same length, series cables not so much-shorter is better. If your positives are the same as each other and the negatives the same as each other you are good....

I have been trying to find the source of this nonsense. Do you have a link of reference to where you "learned" this?

The Internet can provide useful information, but many times it is often repeated nonsense.

 

[BobCollins]

So in summary: When paralleling batteries as long as all positive cables going to the charge controller are the same length and the negative cables going to the charge controller are the same length we are on the right track. Correct?

Incorrect. Total BS.

 

[Whinny]

The context was paralleling batteries. Any conductor has resistance, as conductors heat due to high current this resistance increases. So the battery in the higher resistance circuit will contribute less current to the load, the low resistance battery will contribute more and that leads to uneven discharge.
And no, I didn't read that on the internet....

 

[Hedges]

I have been trying to find the source of this nonsense. Do you have a link of reference to where you "learned" this?

The Internet can provide useful information, but many times it is often repeated nonsense.

Incorrect. Total BS.

??

Please explain, in engineering terms, why you call this "nonsense" and "BS"

 

[Fosse]

From Wiring-Unlimited-EN.pdf

3.3 Parallel battery bank wiring

 

[Wibla]

When connecting multiple lithium battery packs together in parallel, wire length from the bus bar to the individual battery actually matters because they have very low internal resistance. The effect isn't very big, but will accumulate to meaningful SOC changes and uneven battery cycle wear over time, and will lead to issues down the road.
Lead acid with its higher internal resistance is less picky about this.

Incorrect. Total BS.

Your posts are, yes.

 

[BobCollins]

The context was paralleling batteries. Any conductor has resistance, as conductors heat due to high current this resistance increases. So the battery in the higher resistance circuit will contribute less current to the load, the low resistance battery will contribute more and that leads to uneven discharge.
And no, I didn't read that on the internet....

The discussion with respect to parallel batteries in Victron's Wiring note refers to the difference between the cable lengths to the individual batteries, suggesting that the differential resistance may matter there. However, with more than two batteries, the common "diagonal" method still allows a small differential between the batteries. And the "impressive" wire sizes many people use would likely make all this insignificant. Please look at the Wiring document again.

However, what I was addressing was the nonsense of matching the positive and negative wire lengths to other devices, such as to a charge controller or an inverter. I watched a popular YouTuber stuff an extra coil of black (probably 4/0!) inline with his inverter attempting to "balance" the wire lenghts. What he accomplished was a little (very little) more resistance between the battery bank and the inverter.

 

[Fosse]

However, what I was addressing was the nonsense of matching the positive and negative wire lengths to other devices, such as to a charge controller or an inverter

I think you're in the wrong thread.

 

[BobCollins]

I think you're in the wrong thread.

From your earlier comment, in this thread:

So in summary: When paralleling batteries as long as all positive cables going to the charge controller are the same length and the negative cables going to the charge controller are the same length we are on the right track. Correct?

And from Whinny's comment:

All parallel cables should be the same length, series cables not so much-shorter is better. If your positives are the same as each other and the negatives the same as each other you are good....

 

[Fosse]

and the title of the thread is "Battery Cable Length". Not Inverter Cable Length

... and yes Batteries are connected to charge controllers.

Everything I read states to keep your interconnect cables the same length. Well except yours "Incorrect. Total BS. ". So I guess that settles it.

 

[BobCollins]

and the title of the thread is "Battery Cable Length". Not Inverter Cable Length

... and yes Batteries are connected to charge controllers.

Everything I read states to keep your interconnect cables the same length. Well except yours "Incorrect. Total BS. ". So I guess that settles it.

And the Internet vote wins again!

 

[Fosse]

And the Internet vote wins again!

Incorrect. Total BS. Okay you go!

 

[Zil]

Basically. Each equivalent positive cable should be the same as each other. Same with negatives. But negatives do not need be the same length as the positives. The same wire gauge is important.

 

[Hedges]

Basically. Each equivalent positive cable should be the same as each other. Same with negatives. But negatives do not need be the same length as the positives. The same wire gauge is important.

Correct.
Same length if in parallel, both ends connected together, cable in series with battery, then connected in parallel.

With batteries in parallel, we want identical resistance so current splits evenly. (Same for AC wiring of my pair of inverters feeding current from grid side through relay to island side.)

Coming from two SCC to a battery bank, or going from battery bank to two inverters, exact match isn't critical. SCC operating in CV mode would split current unevenly if different wiring resistance. Inverters paralleled on their AC output would see different DC voltage if different battery wiring resistance and pull different current as needed.

With two or more batteries (or series strings of cells or lower voltage batteries) which are then wired in parallel, we would usually make all positive same length, and all negative same length. But what matters is length of (positive + negative) of one equals length of (positive + negative) of the other. When two 12V batteries are connected in parallel (use same length cable), diagonal connection to charge and loads matches total length and resistance for each, even though one has a longer positive cable while other has shorter positive cable. With four batteries in parallel, perfectly balanced wiring can be done with an extension of the diagonal concept. Or, with exactly matched cable lengths to a busbar.