Cable position and length, Positive vs Negative

[1361]

First and foremost, I am in awe of the knowledge and expertise here. I have a hard time wrapping my head around many of the discussions here. I guess I'm a bit of an uneducated simpleton.
I'm putting the finishing touches on designing my inverter install. I will be using 4 SOK batteries. I have three now, waiting on the fourth. What is unique about these batteries is the terminal position. My install will be two wide and two long on the batteries. They'll be under the bed in my travel trailer. There's not much room there and I will be filling the space with batteries. I plan on diagonally wiring them, positive on one side of the bank, negative on the other (one of the things I've learned here). The terminal position will require unequal lengths of battery cable between these four batteries in parallel. I plan on using 4/0 wire. It will also require me to have the pos. and neg. cables crossing over one another at least once. Or, even greater unequal lengths between the pos. and neg. with no crossing over of the cables. Is one method preferred over the other?

I've also read that it is important to keep the length (pos. and neg.) between the batts and the inverter (Magnum 2812) equal. I will be adding buss bars, fuses, switches in this run. My understanding is it's to keep the resistance the same on both pos. and neg. sides. However, each side will have different accessories in line. An example is I'm planning to put a 400 amp class T fuse both first next to the batteries, and last just before going into the inverter. There will be a buss bar in the middle connecting all my other 12v RV wiring. Plus a switch just before the fuse to the inverter. As well as an empty spot to connect any future solar I might add later. The neg. will only have a shunt, switch and buss bar. How important is it to try and calculate these lengths to get them equal?

If the batteries are mounted in opposite orientation, I could zig zag the pos. cables down the middle. then run the negs. in a "U" shape along the outside edges. If the batteries are mounted in the same orientation, I would have to lay a pos. and neg. cable over each other. Again, is one method preferred over the other?

 

[FilterGuy]

First and foremost, I am in awe of the knowledge and expertise here. I have a hard time wrapping my head around many of the discussions here. I guess I'm a bit of an uneducated simpleton.
I'm putting the finishing touches on designing my inverter install. I will be using 4 SOK batteries. I have three now, waiting on the fourth. What is unique about these batteries is the terminal position. My install will be two wide and two long on the batteries. They'll be under the bed in my travel trailer. There's not much room there and I will be filling the space with batteries. I plan on diagonally wiring them, positive on one side of the bank, negative on the other (one of the things I've learned here). The terminal position will require unequal lengths of battery cable between these four batteries in parallel. I plan on using 4/0 wire. It will also require me to have the pos. and neg. cables crossing over one another at least once. Or, even greater unequal lengths between the pos. and neg. with no crossing over of the cables. Is one method preferred over the other?

I've also read that it is important to keep the length (pos. and neg.) between the batts and the inverter (Magnum 2812) equal. I will be adding buss bars, fuses, switches in this run. My understanding is it's to keep the resistance the same on both pos. and neg. sides. However, each side will have different accessories in line. An example is I'm planning to put a 400 amp class T fuse both first next to the batteries, and last just before going into the inverter. There will be a buss bar in the middle connecting all my other 12v RV wiring. Plus a switch just before the fuse to the inverter. As well as an empty spot to connect any future solar I might add later. The neg. will only have a shunt, switch and buss bar. How important is it to try and calculate these lengths to get them equal?

If the batteries are mounted in opposite orientation, I could zig zag the pos. cables down the middle. then run the negs. in a "U" shape along the outside edges. If the batteries are mounted in the same orientation, I would have to lay a pos. and neg. cable over each other. Again, is one method preferred over the other?

There are a couple of ways to do this but first, be aware that with more than two batteries, plain 'diagonal' is better than nothing but less than ideal.

The brute force way to deal with it is to just bring all of the connections to a central point. As long as all the negatives are the same length and all the positives are the same length, things will be balanced.


A slightly more elegant way of doing it would be combination of diaganal and brute force like this:


A third way is what I call double diaganal. You set up two sets of two in diagonal, and then diagonal those

This is ballanced, but it puts three large lugs on a single battery terminal and I don't like to do that.

 

[SkippyJonJones]

I'm interested in the SOK batteries myself. I also wondered about wiring those terminals. Why would SOK align their terminals this way?

 

[time2roll]

Since you have a bus I would use equal length separate wire from each battery. 4/0 to the inverter sounds right but #2 seems more manageable for the batteries to the bus.

Magnum probably says 400 amps and still I would be 300 Class-T as there is plenty of headroom in a class-T for surge performance. Or consider 100 amp fuses on each battery and skip the main fuse to the inverter. Separate fuses is probably better if you downsize the battery cables to #2.

I also would skip the inverter cut-off switch and relent to removing the cable for service. My inverter has not been disconnected for several years. Switch adds one more complication to me.

 

[1361]

There are a couple of ways to do this but first, be aware that with more than two batteries, plain 'diagonal' is better than nothing but less than ideal.

The brute force way to deal with it is to just bring all of the connections to a central point. As long as all the negatives are the same length and all the positives are the same length, things will be balanced.

View attachment 60890
A slightly more elegant way of doing it would be combination of diaganal and brute force like this:
View attachment 60892

A third way is what I call double diaganal. You set up two sets of two in diagonal, and then diagonal those
View attachment 60893
This is ballanced, but it puts three large lugs on a single battery terminal and I don't like to do that.

Thanks for these diagrams. It makes it much easier for me to understand. As I ponder how to implement each of these into my specific installation, cable management takes on a whole new meaning. In each of these, I would need to increase the length of the cables between the batteries to keep the lengths equal. Added length creates an excess of cables in my small cramped space. It would require a "rat's nest" like installation as well as potential for shorting if insulation ever wore through. Are the advantages of a completely balanced setup significant enough to justify a more complex install? Is there a way to quantify my battery life decrease from a simple diagonal vs diagonal/brute force configuration? The batteries are the largest expense in my setup. I certainly want to maintain the longevity and efficiency of them. Perhaps I need to further modify the compartment under the bed to accommodate a proper installation.

Thank you for your time and help.

 

[FilterGuy]

Is there a way to quantify my battery life decrease from a simple diagonal vs diagonal/brute force configuration?

I do not know how to do that.

The uneven wear problem was very well documented (and very real) on lead acid batteries. There is far less data on it for LiFePO4. The huge cycle life of LiFePO4 makes it a much longer term problem and therefor much harder to measure.

 

[FilterGuy]

Will you be living in the RV? If not, perhaps the uneven wear doesn't matter.

Lets assume you are using the RV 30 days a year. If you do a full charge cycle every day that would be 30 charge cycles per year.... If we assume a cycle life of 3000 for the batteries, that means you have 100 years. Now lets assume the uneven wear cuts that in half. Now we are at 50 years.
Calendar life will kill the batteries long before that.

 

[1361]

That is a very good point. Another aspect of that thinking is the RV will likely be traded off to someone else by then as well. This project may or may not stay with it when it goes.
Thanks for all the help.

 

[FilterGuy]

That is a very good point. Another aspect of that thinking is the RV will likely be traded off to someone else by then as well. This project may or may not stay with it when it goes.
Thanks for all the help.

Yup..... Do the best you can in the space you have and move on. There are plenty of more important things to worry about in life.

 

[1361]

Sorry to beat this dead horse up. I've been thinking about this as I'm waiting on the last few things to arrive. My 400 amp busbars have only four posts. If I connected up all the batteries to it, I'd have to double up the lugs on at least one post. Is it OK to stack high amp lugs? If this is acceptable practice, could I also use these heavy duty posts I have sitting around? There is room for four lugs from each battery plus the lug feeding everything else. That would allow me to mount my busbar in the area of the inverter, fuses, ect.

 

[FilterGuy]

Sorry to beat this dead horse up. I've been thinking about this as I'm waiting on the last few things to arrive. My 400 amp busbars have only four posts. If I connected up all the batteries to it, I'd have to double up the lugs on at least one post. Is it OK to stack high amp lugs? If this is acceptable practice, could I also use these heavy duty posts I have sitting around? There is room for four lugs from each battery plus the lug feeding everything else. That would allow me to mount my busbar in the area of the inverter, fuses, ect.

I don't like to stack two high amp lugs but will occasionally do it. I never stack 3 or more.
When I do stack two, I make sure the bottom one has a nice flat surface on the top and bottom so there is good contact on both sides.

Have you considered making your own busbars?

DIY Heavy duty distribution bus bar

To get the paper, click on the orange button at the top of this page. These are instructions for building two style of your own heavy duty bus bars. Bus Bars with soldered in Posts. Bus Bars with Holes for nuts and bolts

diysolarforum.com

 

[time2roll]

Sorry to beat this dead horse up. I've been thinking about this as I'm waiting on the last few things to arrive. My 400 amp busbars have only four posts. If I connected up all the batteries to it, I'd have to double up the lugs on at least one post. Is it OK to stack high amp lugs? If this is acceptable practice, could I also use these heavy duty posts I have sitting around? There is room for four lugs from each battery plus the lug feeding everything else. That would allow me to mount my busbar in the area of the inverter, fuses, ect.

I think I have read some bars are rated four lugs. I don't think stacking is an issue as long as they fit square flat and flush for good contact.

Those posts are specifically made to stack multiple lugs. Best to have similar rated lugs on the post. That is don't sandwich a 10a ring terminal with a 250a lug.