Battery Cable Length

[Fosse]

If I am creating matching battery cables for two sets of 24 volt batteries in order to parallel them would it be good to reduce the negative cable the amount of the Daly BMS #2 AWG wires connected to the battery and to the load or is it just important to have both sets of negative cables equal length?

I have 2 foot Positive cables.

 

[DJSmiley]

Id suggest equal length.

If you're set them parallel cells by connecting + and -, and pulling the load from 1 battery, it makes a difference. (but thats not best practice)
Different length cause different voltage drop, thus uneven discharging. It depends on the current, if you're pulling 1A it's not noticable, but the more current you're pulling, the bigger the voltage drop (and so the inbalance)

Remember for a decent equal discharge it's best practice to wire them 'crosswired', setting the sets in parallel, and connecting the + from the first set, and getting the - from the second set. That would equal out any cable difference, but I still prefer equal length wiring.
I just like it that way

 

[TomC4306]

But @DJSmiley that doesn't really answer the question...
Deduct the length of the bms wires or not?
I suppose by saying equal length you mean yes, take those bms wires into account?

 

[Fosse]

"take those bms wires into account?"

Yes that is what I was wondering. Or would equal length negative cables to each battery set satisfy voltage drop issues.

Would messuring the 3-4 inches of each cable coming out of the BMS to and from the battery and removing that much from the negative cables be closer to perfect?

 

[Fosse]

But @DJSmiley that doesn't really answer the question...
Deduct the length of the bms wires or not?
I suppose by saying equal length you mean yes, take those bms wires into account?

Id suggest equal length.

If you're set them parallel cells by connecting + and -, and pulling the load from 1 battery, it makes a difference. (but thats not best practice)
Different length cause different voltage drop, thus uneven discharging. It depends on the current, if you're pulling 1A it's not noticable, but the more current you're pulling, the bigger the voltage drop (and so the inbalance)

Remember for a decent equal discharge it's best practice to wire them 'crosswired', setting the sets in parallel, and connecting the + from the first set, and getting the - from the second set. That would equal out any cable difference, but I still prefer equal length wiring.
I just like it that way

2 sets of 8 x 280ah batteries each with a Daly BMS hence 2 positive cables going to a common Bus Bar 3/8" Power Distribution Block and same for negatives cables. If that paints a better picture.

 

[Woody]

If I read and understand your question, we’re talking about a 2ft run per cable. The delta in them is insignificant. You’ll have more resistance/voltage loses at the terminals/lugs. If you’re super concerned about adding any minor loss due to cable length, the go up one diameter size on the cable.

 

[Alphacarina]

With short cables - 2 feet is certainly short - and adequately sized cable - 2 AWG should be fine, I wouldn't worry at all about the negatives being a few inches longer than the positives. I'm sure some fancy measuring equipment could tell the difference in voltage drops at max load, but you'll never notice any differences in the way your system performs

Don

 

[Whinny]

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....

 

[DanBo]

Wow...this takes me back. Cables of equal length. Holy crow. Electricity travels pretty close to the speed of light which is fast. I recall all the old arguments in stereo forums over equal lengths of stereo speaker cable. Somewhere I must still have the ad for the special type of speaker cable not only delivered in matching pairs of lengths equal down to the mm, but arrows on the cable to ensure the electrons travelled in the right direction.

 

[Hedges]

Length of one positive cable from battery to inverter, and one negative cable from battery to inverter, don't have to be same length.
Cable connecting two battery positives should be identical to cable connecting two battery negatives. And connect inverter cables to diagonally opposite corners.

Electrons travel at a snail's pace. EM waves travel in the wire near the speed of light (sometimes half that speed, if E-field is in a dielectric with dielectric constant 4.0, like PCB.)
It isn't time of flight we're matching, it is voltage drop, V = I x R

 

[DanBo]

https://diysolarforum.com/threads/b...we're matching, it is voltage drop, V = I x R... Surely the voltage drop between point 1 and point 2 is/must be exactly the same. If one cable is slightly longer than the other, it's resistance will be correspondingly slightly higher, the current slightly lower....but I think physics demands that the voltage drop is identical in each cable.

 

[bladetoyshop]

Actually, audio, like RF, is AC (alternating current) so it's more critical for phasing for the length of leads to be as close to identical as possible. DC is less critical.

 

[DerpsyDoodler]

Actually, audio, like RF, is AC (alternating current) so it's more critical for phasing for the length of leads to be as close to identical as possible. DC is less critical.

For the sake of paralleling batteries, it is critical. for a positive and negative lead to a single device, not critical.

 

[Fosse]

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?

 

[bladetoyshop]

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?

Correct. Having the charge current balanced is the key here.

 

[DerpsyDoodler]

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?

No. What's important is that the cables (or bars) parallel-connecting the batteries are all the same length.

 

[Steve_S]

I've always have & will recommend equal length wires from Battery Terminals to DC Bus Bars and equal length right across to the Inverter with the DC Cables and binding the +&- wires to reduce eliminate EMF/RFI noise.

At this moment I run with 4 Packs in Parallel and just completing Pack 5 for the final version with 1190AH. I also have a "Long" cable run of 14' which applies to the wire from Inverter/Charger to Each Battery Pack, as such the noise was quite prevalent until I zippy tied the DC Wires together.

Diagram of 4 24V LFP Bank-1 & 24V FLA Bank-2 (Heavy Rolls Lead.

 

[Hedges]

binding the +&- wires to reduce eliminate EMF/RFI noise.

... as such the noise was quite prevalent until I zippy tied the DC Wires together.

That reduces loop area so a poorer performing antenna, also reduces inductance.
There must be some higher frequency content due to sharp waveform edges. Do you have an oscilloscope, spectrum analyzer, or other way to measure them?

Capacitance with high enough frequency response could supply some of that current locally.
Would like to do a "pi" filter (C-L-C) but the "L" is a bit difficult.

An inductor core to provide impedance blocking differential current has to handle magnetic field from DC current. Common mode, both + and - wires through a single ferrite can have a small core because they cancel, but will pass differential mode. Still, individual ferrites on each wire may do something, may not saturate completely. One of the guys at work did that to pass EMI test, but I don't know how much emissions were reduced.

Air core coils of wire would not magnetically saturate. They would radiate effectively so would need a shield can around them.

I think of battery bank having a loop area as well. Maybe extra length of cable, snaking back along the path where batteries are stitched together. I used capacitor with self-resonance at problem frequency across battery terminals of a hand-held device (in that case it was susceptibility to RF pickup rather than emission.) But a pi filter at inverter would reduce RF signal reaching battery.

 

[Woody]

Here’s a reference to help simplify this ongoing discussion.

Diameter is important not length for these short runs. Please do the calculations and you’ll confirm this for yourself.

https://www.solaris-shop.com/content/American Wire Gauge Conductor Size Table.pdf

Again this short of a wire length isn’t a significant concern. Your terminal connections, quality of lug crimps, and oxidation are areas to focus your attention in order to minimizing poor connections aka resistance.


Regarding RF noise, shields up! You can twist cables, add toroids, grounding, shield cables, faraday cage around everything, etc but your entire alternative power system is comprised of RF noise generating devices and radiating antennas. Talk with a HAM radio operator and they’ll give you the skinny on how bad these devices are and suggestions on how to reduce the noise. But this is the original off topic.

 

[Alphacarina]

No. What's important is that the cables (or bars) parallel-connecting the batteries are all the same length.

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