Equal Cable Lengths

[Brewster 2]

Apologize to bring this up again but my potential small backup system with only one 75wh flooded to start with, didn't seem to apply to the previously asked questions here.
Due to many factors I'm building a low budget little back up, just to support a couple of lap tops and a Waco 50. I have the 75 ah battery that may be increased to a 100, or even 2x100ah's at a later date.
In this case is equal cable lengths from battery to inverter still important?

2 x 75w panels (also already have), 30 amp PWM charge controller, 1000w inverter.

 

[1234enough]

Each battery in a multi battery set up should have equal lengths of cabling, a single battery set up cabling can have different length of positive / negative cables. But then if you add another battery you should ensure the cable lengths/ size is equal to the new battery as the original.
This is to ensure each battery cables resistance is equal to ensure equal discharge / charge.

 

[littleharbor2]

With just two batteries all you need to do is put them both in parallel and run the positive charging/load cables onto one battery and the negative charging/load cables onto the opposite, diagonal corner of the other battery. Equal cable length isn't critical in this scenario.

 

[Rednecktek]

One thing most people don't tell you about this is that your Positives need to be the same length, and your Negatives need to be the same length, but your Positives and Negatives DON'T need to be the same.

So as an example, let's say the furthest length you'll need to reach your Positive terminals is 16in, and your furthest Negative terminal is 30in. All your Positives should be 16in and all your Negatives should be 30in, but you DON'T need 30in Positive cables.

 

[Brewster 2]

Each battery in a multi battery set up should have equal lengths of cabling, a single battery set up cabling can have different length of positive / negative cables. But then if you add another battery you should ensure the cable lengths/ size is equal to the new battery as the original.
This is to ensure each battery cables resistance is equal to ensure equal discharge / charge.

Ah thanks...the penny dropped.

 

[Brewster 2]

One thing most people don't tell you about this is that your Positives need to be the same length, and your Negatives need to be the same length, but your Positives and Negatives DON'T need to be the same.

So as an example, let's say the furthest length you'll need to reach your Positive terminals is 16in, and your furthest Negative terminal is 30in. All your Positives should be 16in and all your Negatives should be 30in, but you DON'T need 30in Positive cables.

Well I wouldn't have known that from what I've read, very enlightening, thanks.

 

[Brewster 2]

With just two batteries all you need to do is put them both in parallel and run the positive charging/load cables onto one battery and the negative charging/load cables onto the opposite, diagonal corner of the other battery. Equal cable length isn't critical in this scenario.

Thank you.

 

[Supervstech]

Exactly.
The purpose of equal length is to ensure all batteries in the bank have as close to the same resistance between themselves as possible.
Also, if you have a common buss bar setup, all negative cables from each battery to each buss bar must be equal.
That makes it easier to understand.

 

[Brewster 2]

Thank you.

 

[Steve_S]

I follow what Vicron, Schneider & other top manufacturers engineers recommend.

Battery Cable length from Batt Terminal to end "Device" (Inverter/AIO) should be of equal length and kept together (reduce & manage RFI/EMI). Wires should be of the same size (AWG), grade & quality as consistency is essential. This is from Lug to Lug length excluding devices like fuses/breakers/switches etc.
- Do note that every "item" in-line adds resistance and this can add up cumulatively and must be accounted for and settings adjusted accordingly.

Battery Packs in Parallel is where irregular wire lengths become very apparent and quickly...
Example: A typical system with 4 (Matched & Identical) packs in parallel with varying wire lengths will work but when charging for example, the battery with shortest wires will charge higher & finish sooner than the one with the longest wires who will come in last and never fully match to exactly equal ##.00 Voltage, That one will always have the lower SOC at the top and will discharge the last being a touch higher than the rest. There are other "common symptoms" that rear their ugly heads due to wire length & resistance issues.

Many people chase after BMS settings and attempting various calibrations, balancing tech and more while the root of the problem is rarely addressed. Many do not even realize that a FUSE adds quite a bit of resistance in-line with some being worse than others ! All of that needs to be accounted for.

When you get to serious calibration to match everything up and account for the OFFSETS between charging & discharging and YES, the Voltage Offsets are DIFFERENT when charging OR discharging making things even more interesting.
A simple test for that:
- When charging, use a 2 decimal accurate DMM/DVOM and look at the voltage at the SCC or Inverter OUT Terminal to Batt and at the Batt Terminals (if component system, the SCC & Inverter/Charger will each have different values) and you will see a drop from Input Power to Batt Terminal.
- When discharging do the same steps as above and again you will see a difference between the device & batts
-- In both instances, the battery packs should be at the same voltage ##.00 +/- 0.02 on average IF all wires are the same with the same resistance.

Many Tier-1 products have OFFSET settings to adjust for this, often times it is only 1 offset and not for both charge or discharge offsets. This must be manually corrected in your settings with the Voltage @ Batt Terminal being the guidepost.

In closing, this is a previously contentious subject, as I said, I go with the designers & engineers from Tier-1 Companies as well as my personal experience over the years.

Hope it helps, Good Luck.
Steve

 

[Reed Cole]

I follow what Vicron, Schneider & other top manufacturers engineers recommend.

Battery Cable length from Batt Terminal to end "Device" (Inverter/AIO) should be of equal length and kept together (reduce & manage RFI/EMI). Wires should be of the same size (AWG), grade & quality as consistency is essential. This is from Lug to Lug length excluding devices like fuses/breakers/switches etc.
- Do note that every "item" in-line adds resistance and this can add up cumulatively and must be accounted for and settings adjusted accordingly.

Battery Packs in Parallel is where irregular wire lengths become very apparent and quickly...
Example: A typical system with 4 (Matched & Identical) packs in parallel with varying wire lengths will work but when charging for example, the battery with shortest wires will charge higher & finish sooner than the one with the longest wires who will come in last and never fully match to exactly equal ##.00 Voltage, That one will always have the lower SOC at the top and will discharge the last being a touch higher than the rest. There are other "common symptoms" that rear their ugly heads due to wire length & resistance issues.

Many people chase after BMS settings and attempting various calibrations, balancing tech and more while the root of the problem is rarely addressed. Many do not even realize that a FUSE adds quite a bit of resistance in-line with some being worse than others ! All of that needs to be accounted for.

When you get to serious calibration to match everything up and account for the OFFSETS between charging & discharging and YES, the Voltage Offsets are DIFFERENT when charging OR discharging making things even more interesting.
A simple test for that:
- When charging, use a 2 decimal accurate DMM/DVOM and look at the voltage at the SCC or Inverter OUT Terminal to Batt and at the Batt Terminals (if component system, the SCC & Inverter/Charger will each have different values) and you will see a drop from Input Power to Batt Terminal.
- When discharging do the same steps as above and again you will see a difference between the device & batts
-- In both instances, the battery packs should be at the same voltage ##.00 +/- 0.02 on average IF all wires are the same with the same resistance.

Many Tier-1 products have OFFSET settings to adjust for this, often times it is only 1 offset and not for both charge or discharge offsets. This must be manually corrected in your settings with the Voltage @ Batt Terminal being the guidepost.

In closing, this is a previously contentious subject, as I said, I go with the designers & engineers from Tier-1 Companies as well as my personal experience over the years.

Hope it helps, Good Luck.
Steve

I don't doubt all that, but they are describing about the simplest system you could imagine. I'm sure it'll work fine if they match cable lengths, no reason to go getting worked up about how resistance your fuses are adding here.

 

[Superhawk996]

How important is it to have equal wire lengths if the wire gauge is much larger than needed for the amperage? It seems that wire length would be 'invisible' to a tiny load through a huge wire, but I also know that electricity does strange things.

 

[Steve_S]

I don't doubt all that, but they are describing about the simplest system you could imagine. I'm sure it'll work fine if they match cable lengths, no reason to go getting worked up about how resistance your fuses are adding here.

It applies to a simple 1 Battery system with an AIO or a Multi-Battery Bank with paralleled inverter & SCC's.
Start by building it properly the first time and that will make additions easier (and cheaper) as time moves on.

 

[Supervstech]

How important is it to have equal wire lengths if the wire gauge is much larger than needed for the amperage? It seems that wire length would be 'invisible' to a tiny load through a huge wire, but I also know that electricity does strange things.

In parallel, wire length is pretty important.