Max length of parallel group/busbar questions

OffGridHawaii

OffGridHawaii

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Jun 14, 2020
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Aloha,

I am building a 14s172p powerwall with the standard 4x5 cell holders. The busbar designs of packs that are wider than 4 cells seem too complicated for me to fabricate. I like how the 4 cell wide parallel groups allow for the busbar to run between the cells making the connection to each cell even. I have 2 questions:

1. Are there any problems with me forging along making 4 cell wide by 43 cell long parallel groups? roughly 3 feet in length.I will have them secured to a shelf. I realize there will be voltage drop along the length, but I'd like to compensate that with proper busbar sizing.

2. I purchased 8 gauge bare copper wire to make my busbars, I decided to do this rather than twist smaller wires. Will 8 gauge bare copper wire be sufficient for parallel groups of this size?My powerwall will be charged by 10,400w of solar, and will supply an 8,000w inverter as the load. I have seen the wire gauge/length/amp tables but would still like feedback given all the variables I'm working with.

Thanks! I appreciate your feedback.
 
OffGridHawaii said:
1. Are there any problems with me forging along making 4 cell wide by 43 cell long parallel groups? roughly 3 feet in length.I will have them secured to a shelf. I realize there will be voltage drop along the length, but I'd like to compensate that with proper busbar sizing.
Click to expand...
Your SolArk could max out at8000w/52v = 154amps. 154a/172cells = 896ma/cell. Let's say you run 1/3-1/2 thatfor 99% of the time, you're down to 296-448ma/cell.
Your PV may generate104002/52v = 200amps. 200a/172cells = 1163ma/cell. Let's say you run 80% of 10.4kw on regular basis, that's 930ma/cell.

If weassume you have + and - at opposite ends of your packs - then no, this won't be anissue if the buss is big enough....


OffGridHawaii said:
2. I purchased 8 gauge bare copper wire to make my busbars, I decided to do this rather than twist smaller wires. Will 8 gauge bare copper wire be sufficient for parallel groups of this size?My powerwall will be charged by 10,400w of solar, and will supply an 8,000w inverter as the load. I have seen the wire gauge/length/amp tables but would still like feedback given all the variables I'm working with.
Click to expand...
8awg (solid copper as apposed to twisted strands) is a bit small in my mind. I run6awg twisted copper (https://www.wireandcableyourway.com/6-awg-soft-drawn-stranded-bare-copper-wire.html)busbars with a max of 500ma/cell charge + discharge design. The base packs (100-120p)are 6awg buss wireinter-connected with 4/0 welding wire to the shunt-trip/control box. I think you'll find most DIY'ers would recommend larger bussbars than 8awg for your size pack. You could twist 2 of your 8awgs togetherto get 5awg effective - which would be OK in my opinion.

I'm sure you'll get additional comments but from a hands-on perspective the things I observe is that you don't want to skimp on busbars. Yes, smaller wire may work and voltage drop is not a significant issue at 500ma/cell - but why stress the system and what happens if you start kicking up to maximums. And as many of us have found, you'll only want to expand your system in the future years. For example, I started with a single AIMS 12,000w inverter and now I'm running 2 of them for 24,000w as I added a whole househeatpump to the system. It was good that I started with thicker busbars :)
 
Have you considered building 2 batteries of 14s86p, and connecting them in parallel on a master-busbar? Most people seem to build packs around the 80p~120p size, and it's a good size for a number of reasons, including smaller physical size & weight for easier handling, and smaller bus bar and connector size requirements.

And with a bit of planning, you could easily add more parallel batteries in the future, take one offline for maintenance, etc.
I have currently 6 batteries connected in parallel, with 4 more being planned.
 
OffGridInTheCity said:
OffGridHawaii said:
1. Are there any problems with me forging along making 4 cell wide by 43 cell long parallel groups? roughly 3 feet in length.I will have them secured to a shelf. I realize there will be voltage drop along the length, but I'd like to compensate that with proper busbar sizing.
Click to expand...
Your SolArk could max out at8000w/52v = 154amps. 154a/172cells = 896ma/cell. Let's say you run 1/3-1/2 thatfor 99% of the time, you're down to 296-448ma/cell.
Your PV may generate104002/52v = 200amps. 200a/172cells = 1163ma/cell. Let's say you run 80% of 10.4kw on regular basis, that's 930ma/cell.

If weassume you have + and - at opposite ends of your packs - then no, this won't be anissue if the buss is big enough....


OffGridHawaii said:
2. I purchased 8 gauge bare copper wire to make my busbars, I decided to do this rather than twist smaller wires. Will 8 gauge bare copper wire be sufficient for parallel groups of this size?My powerwall will be charged by 10,400w of solar, and will supply an 8,000w inverter as the load. I have seen the wire gauge/length/amp tables but would still like feedback given all the variables I'm working with.
Click to expand...
8awg (solid copper as apposed to twisted strands) is a bit small in my mind. I run6awg twisted copper (https://www.wireandcableyourway.com/6-awg-soft-drawn-stranded-bare-copper-wire.html)busbars with a max of 500ma/cell charge + discharge design. The base packs (100-120p)are 6awg buss wireinter-connected with 4/0 welding wire to the shunt-trip/control box. I think you'll find most DIY'ers would recommend larger bussbars than 8awg for your size pack. You could twist 2 of your 8awgs togetherto get 5awg effective - which would be OK in my opinion.

I'm sure you'll get additional comments but from a hands-on perspective the things I observe is that you don't want to skimp on busbars. Yes, smaller wire may work and voltage drop is not a significant issue at 500ma/cell - but why stress the system and what happens if you start kicking up to maximums. And as many of us have found, you'll only want to expand your system in the future years. For example, I started with a single AIMS 12,000w inverter and now I'm running 2 of them for 24,000w as I added a whole househeatpump to the system. It was good that I started with thicker busbars :)
Click to expand...
Interesting, thank you for your help with some of my calculations. I will twist two strands of 8 gauge which has roughly the same cross sectional area combinedas 3-4 gauge wire.

Also,

1. What is the benefit of having the + and - on opposing ends?

2. What gauge wire should I use to connect my parallel groups in series? (The wire gauge connecting each group + to -). I am connecting the + and - of the entire battery to my inverter with 2/0AWG inverter cables, but I'm curious of the connections within the battery.
 
OffGridHawaii said:
1. What is the benefit of having the + and - on opposing ends?
Click to expand...
There are 2 benefits
- Its safer to handle the packs. When the + and - are on the same end, you can easily drop something or pass something across the terminals and cause a short. When they are on opposite ends, an accidental short event is greatly reduced.
- It causes the current to flow thru the pack from end to end as apposed to having + and - at one end which will draw current heavier from one end of the pack stressing those cells over the 'remote' cells. However, I've been advised by a trusted EE engineer that unless you get into significant amps (e.g. much higher than 500ma/cell) its not a huge consideration.


OffGridHawaii said:
2. What gauge wire should I use to connect my parallel groups in series? (The wire gauge connecting each group + to -).I am connecting the + and - of the entire battery to my inverter with 2/0AWG inverter cables, but I'm curious of the connections within the battery.
Click to expand...
In my own case of 100-120p I have 4 x 6awg wires on each side (+ and -) which combined are 53mm^2 or 1/0 guage which is good for 200a range. The packs are connected inseriesthru flattened 3/8" copper pipe which exceeds the 1/0 guage of the combined buswires.

image_fwdsaa.jpg


So I would say that if you have at least4 x buss-wires ( of yourpaired/twisted 8awg(s) )on each side of your 172p pack that come together in a common lug or buss and then bolt those lugs/buss together in series ( or use your 2/0 wire ) then you'll be in great shape for 200a to easily flow thru the series of packs that make up yourbattery.

200a*52v = 10,400w is an appropriate range for the charge and discharge parameters of your system and consistent with the2/0 wire you plan to use to hookyour battery to the inverter and charge controller assuming they are nearby (e.g <15ft or so).
 
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