Combining a number of smaller gauge wire to get to ampacity requirement

Gould

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I have an issue in that my cells came with 6mm welded terminals instead of the requested 8mm. The smallest terminal home on a 4/0 lug that I can find is 3/8” - this is two big for the area at the base of the terminal.
Seeing that my BMS uses multiple smaller gauge cable I figure it should be no problem doing the same from cell to the main battery terminal. If this is true is there a reference available to show what combination of smaller gauge is required to get to the desired ampacity?
 

Gould

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Brilliant, thank you.

Now a question; the Overkill BMS comes with two configurations, 3 8 AWG pre-wired or 2 lugs. 3 8 AWG is the equivalent of 3 AWG. By the charts I’m able to find online, 3 AWG is rated at 115 amps. The Overkill states 120 amps continuous. Should a person be concerned with that? I have the lugs and have been planning 2 6 AWG which would give me the same equivalent of 3 AWG.
 

smoothJoey

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I have an issue in that my cells came with 6mm welded terminals instead of the requested 8mm. The smallest terminal home on a 4/0 lug that I can find is 3/8” - this is two big for the area at the base of the terminal.
Seeing that my BMS uses multiple smaller gauge cable I figure it should be no problem doing the same from cell to the main battery terminal. If this is true is there a reference available to show what combination of smaller gauge is required to get to the desired ampacity?
These have a 4/0 awg 1/4" option.
1/4" is close enough to m6.
 
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newbostonconst

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This has been covered multiple times in other threads and really shouldn't be done and is actually again NEC code in the US if you want to search.

But have at it if you like....
 

Gould

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Sorry, what are you referring to; the combining of smaller gauge is not to code? All of the BMS devices I’ve come across do this?
 

HRTKD

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Brilliant, thank you.

Now a question; the Overkill BMS comes with two configurations, 3 8 AWG pre-wired or 2 lugs. 3 8 AWG is the equivalent of 3 AWG. By the charts I’m able to find online, 3 AWG is rated at 115 amps. The Overkill states 120 amps continuous. Should a person be concerned with that? I have the lugs and have been planning 2 6 AWG which would give me the same equivalent of 3 AWG.

I kept my three wires as short as I could. I figured a 3' length, that includes the wires on both sides of the BMS. With that length, there is less than .77% voltage drop. Good enough for me.

If I were to do it again, I would choose the 2 terminals on the BMS instead of the soldered wires. I think it would be easier to deal with the terminals.
 

Gould

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I have the bolt version of the BMS with runs no longer than 12” to a bus bar, from there a single 4/0 to my battery terminals. Looks like the BMS will be the weakest link here. Perhaps I should look at 2 4 AWG vs 6. I suppose there’s nothing stopping me from using a 4/0 cable connected to a single bolt on the BMS?
 
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HRTKD

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The heavier the cable, the less flexible it is and the more it has to be supported/secured. Just throwing that out there. Use the right size cable for the job.
 

Gould

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Here’s a shot of the build, I just finished up the positive wiring on one battery. It definitely looks weak for a 120 amp BMS. I’ll use this setup for capacity testing (1500w load) and move to a single 1 AWG - the BMS has two lugs, I’m assuming using 1 will be fine?
 

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HRTKD

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Why didn't you run the positive wires from the battery over to the Class T fuse?

If the BMS has two terminals, I would use both. One option I saw recently was to install a bus bar (copper bar) between the two BMS terminals and then run a single large cable from that to the battery.
 

Gould

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I forget how observant this group is! This is temporary while I wait for my T fuse. It’s been incredibly difficult for me to source one and with my cells topped up after balancing, I want to get them discharged.

I like the idea of a bus bar bridging the BMS terminals. The larger the gauge the more difficult it is to tie multiple lugs onto a single post.

EDIT - to be clearer on the fuse, it will be incorporated between the batteries and the single terminals that are yet to be installed. I added the bus bar because I wanted to avoid the stacking of multiple lugs on the single fuse terminal. Hope that makes sense.
 
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Gould

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Running a capacity test with a 100 amp load and the cables are Luke warm. I’ll still swap them out for a heavier gauge (likely 1/0) but at least I know I’m good for now.

Real subjective question I know and please, not my intent for this to turn into a drawn out debate just looking for an order of magnitude. So, regarding compression, should the cells be tight against each other and difficult to move when properly compressed?
 

HRTKD

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Real subjective question I know and please, not my intent for this to turn into a drawn out debate just looking for an order of magnitude. So, regarding compression, should the cells be tight against each other and difficult to move when properly compressed?

Yes. I can pick up my battery by the threaded rod and carry it. The cells aren't going anywhere. The amount of torque to put on the threaded rod isn't much, like 10 PSI. Even at that low torque, the batteries are snugged up quite well. I've had zero movement in my cells, even over the rough roads I take my trailer on.
 
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