Wire Gauge Questions

Hey guys, sorry if this has been addressed before, I tried searching but didn't come up with much. Also I can't post a reply in the FAQ page I'm guessing by design. So thought I'd just make a wiring/terminal thread.

In any case, my specific question is that I have a 4S100P battery that I'm creating out of the standard repurposed 18650s. My design was to have a max of about 100A be drawn from the battery. So, for the overall negative and positive wires/terminals, I was going to go with a short length of 4AWG with Anderson Powerpole 120s (which can handle up to 120A obviously)...

But, for some reason, I always had it in my head that the connections between the cells in series would not need to have that big of wires or connectors. So the original plan was to use, say, 8 or 10AWG with maybe the smaller Anderson PP75s or even the PP45S. But, now that I think about it, I'm not sure why I assumed those connections didn't need to be rated for the same current that the overall battery can put out or if I saw it somewhere. I'm now starting to second guess myself and just wondered if anyone had any input or thoughts.

If the overall battery is designed to put out 100A do ALL of the wires/terminals need to be rated for that, including the inter-series connections between cells, or am I okay with just using cabling rated for about half that total current between the cells in series?

Those PP120s aren't exactly cheap and I already have quite a few PP75s as well as PP45s. But, of course, safety comes first so, if I need to, I can always get more 120s.

(I figure this thread can be used for any other weird, specific wiring questions as well)

Yes all in series need to cope with 100A. The 100A travels in the loop you could say

daromer said:

Yes all in series need to cope with 100A. The 100A travels in the loop you could say

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Okay, that's what I was starting to think, thank you. Not sure why I assumed incorrectly at first but at least someone knows what they're doing! Thanks again buddy.

+ 1 to daromer's comments, yes all of the series path needs to be rated for the current.

Maybe you could use lugs & bolts for inter pack connections (insulate with some heatshrink) for a lower cost & just have the anderson's on the output.
Lots of pack builders are doing that

Redpacket said:

+ 1 to daromer's comments, yes all of the series path needs to be rated for the current.

Maybe you could use lugs & bolts for inter pack connections (insulate with some heatshrink) for a lower cost & just have the anderson's on the output.
Lots of pack builders are doing that

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Haha, I actually thought about doing this today trying to come up with a good method to use what I have instead of buying new. Good call, glad there's so many people on here who understand not only the technical requirements but practicality as well.

You know, for a free time hobby and just-for-fun learning experiment, it's weird how I often I find myself rationalizing new tool or consumable purchases. Gotta learn to break that habit. But damnit, I really need that new CNC router so I can mill out cell holders, etc. etc.

OhmGrown said:

But damnit, I really need that new CNC router so I can mill out cell holders, etc. etc.

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but of course! now you're talking
Welcome the the "addicted to tech anonymous" inner circle

I don't know what you're planning to power, but I think most people would look at an AWG4 cable and decide to upgrade to a 24V or 48V system.
With a 24V system you only need 1/2 the current (AWG4 -> AWG7).
48V and you only need 1/4 the current (AWG4 -> AWG10).
Inverters and charge controllers might cost more, but they're often more efficient and you save heaps of money on cabling.
And upgrades in the future will probably be easier, too.

ajw22 said:

I don't know what you're planning to power, but I think most people would look at an AWG4 cable and decide to upgrade to a 24V or 48V system.
With a 24V system you only need 1/2 the current (AWG4 -> AWG7).
48V and you only need 1/4 the current (AWG4 -> AWG10).
Inverters and charge controllers might cost more, but they're often more efficient and you save heaps of money on cabling.
And upgrades in the future will probably be easier, too.

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Yes, when I move on to making an actual powerwall or a battery bank that is stationary, I'll definitely be going with either 24V or 48V. But on this first "test" project to work all out the kinks and get some experience under my belt, I'm actually making a type of mobile powered work station for the techs at the AV installation company I co-own. So space and weight is the biggest limiting factor for this one, meaning, unfortunately, I must go with a 12V pack.

I could probably cut down on the capacity and get up to 24V in the same space I have available (sticking everything in one of those tough pelican cases) but the techs work for so long in the field that I think I'd rather go with brute force duration over efficiency. But, yes, I can't wait to make a larger scale battery and start using the nice 24V inverters.

Why do you have to go with 12V if space is limited? I think you have not realy grasp the concept of current, voltage, power, Ah and work (energy in Wh, kWh). The energy (capacity in Wh) will stay the same, if you go with 12V or 24V. With 24V you have double the voltage and only half the Ah.

Herbi said:

Why do you have to go with 12V if space is limited? I think you have not realy grasp the concept of current, voltage, power, Ah and work (energy in Wh, kWh). The energy (capacity in Wh) will stay the same, if you go with 12V or 24V. With 24V you have double the voltage and only half the Ah.

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Specifically because the inverter I'm using is 12V & the right size, whereas the 24V inverters I've found are much bigger. Also with the layout I have, I can only fit 4 strings of 100 cells in my enclosure. I understand I could break that down to 7 strings of 56 cells but that changes drastically the layout of the cell packs and the limited space I have for wiring. Everything right now just barely fits perfect with a 12V application and I also have the added benefit of using a lot of components originally designed for the auto industry without having to get a large step down converter that can handle the amps I need.

But, you're probably right, I likely don't know or understand everything I should about electronics or batteries. That's why I'm here posting questions. Someday though I hope to be just like you with everything down pat!

OhmGrown said:

Specifically because the inverter I'm using is 12V & the right size, whereas the 24V inverters I've found are much bigger. Also with the layout I have, I can only fit 4 strings of 100 cells in my enclosure. I understand I could break that down to 7 strings of 56 cells but that changes drastically the layout of the cell packs and the limited space I have for wiring. Everything right now just barely fits perfect with a 12V application and I also have the added benefit of using a lot of components originally designed for the auto industry without having to get a large step down converter that can handle the amps I need.

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Then I would recommend to go with LiFePO4 cells. Keeping the cells balanced and operating within the voltage range of the inverter is drastically simplified.

Based on this chart https://www.powerstream.com/Wire_Size.htm - the 2/0 (i.e. 00 gauge) chassis wiring is listed at 283a. These charts vary here and there. As a real world example, I use 2/0 AWG welding wire (i.e. flexible, multistrand copper) with the longest stretch is about 6ft (from battery to control box). I can attest that it get's 'slightly warm' to the touch at the 100a to 120a level (several hrs/day) in a 27C ambient air environment.

OffGridInTheCity said:

Based on this chart https://www.powerstream.com/Wire_Size.htm - the 2/0 (i.e. 00 gauge) chassis wiring is listed at 283a. These charts vary here and there. As a real world example, I use 2/0 AWG welding wire (i.e. flexible, multistrand copper) with the longest stretch is about 6ft (from battery to control box). I can attest that it get's 'slightly warm' to the touch at the 100a to 120a level (several hrs/day) in a 27C ambient air environment.

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Sweet, I've found quite a few of those charts online and, you're right, they all do seem vary fairly significantly between each one. I'll probably use this one moving forward since it seems to be the most conservative though.

OhmGrown said:

OffGridInTheCity said:

Based on this chart - the 2/0 (i.e. 00 gauge) chassis wiring is listed at 283a. These charts vary here and there. As a real world example, I use 2/0 AWG welding wire (i.e. flexible, multistrand copper) with the longest stretch is about 6ft (from battery to control box). I can attest that it get's 'slightly warm' to the touch at the 100a to 120a level (several hrs/day) in a 27C ambient air environment.

Click to expand...

Sweet, I've found quite a few of those charts online and, you're right, they all do seem vary fairly significantly between each one. I'll probably use this one moving forward since it seems to be the most conservative though.

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Those charts all you a really long(for this use) length for the minimum length, I think the cables will handle more current when they are not 4ft long or etc.