Need help understanding inter-battery connections (Ampacity)

coreyzev

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Building a diy bank using BH's pre-built packs, 16 sets of 3 cells on 1 BMS. Making a 48v 300ah battery.

I would like help understanding the ampacity of the connections between the batteries, both parallel within the sets, and in series across the sets.


Trying to understand how much material/wire I need for each connection.

This is a mockup of my build. Currently planning on using doubled up 3/4" x 1/8" aluminum flat bar (from lowes) for the parallel sets. And connecting then using 1 bolt through both the positive and the negative for the series sets on the doubled up flat bar. Really trying to conserve space.

(RE concerns about shorts: I will be heat shrinking the alu bars entirely, and wrapping the bolts w/ electrical tape.

Pics to try and show better, cause I suck at explaining it:

1634063773351.png1634063811424.png

this picture shows how i want to connect them in series. (above you can see how i change levels or sides)
1634063847986.png

Thank you.

Corey



Edit: I'll be using the Victron 48/3000VA 120v inverter. But I'm more interested in the general rules of how current flows through a series parallel build like I'm making than someone giving me the answer to my specific problem.
 
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Shale MacGregor

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Shale MacGregor

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You are doing 16 in series, so 100 amps max discharge becomes, 100 amps. You are doing 3 in parallel so 100 amps becomes 300 amps. Across 3/4 1/8th ? Those bars are going to get mighty toasty. Need to boil some water?

If you doubled up the busbars you would hit really close to the 300 amp rating, but that doesn't account for through-holes etc.
 

coreyzev

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You are doing 16 in series, so 100 amps max discharge becomes, 100 amps. You are doing 3 in parallel so 100 amps becomes 300 amps. Across 3/4 1/8th ? Those bars are going to get mighty toasty. Need to boil some water?

If you doubled up the busbars you would hit really close to the 300 amp rating, but that doesn't account for through-holes etc.
Thank you. For the info. And the link for @FilterGuy (see above).

So, in theory yes it could pull that much. I didn't mention the inverter, I'll update the post. I didn't cause mostly I was trying to understand the physics and proportions of how the current flows. And would still love a better explanation of that.

The inverter is the Victron 48/3000 120v. Nominal it'll pull about 62 amps, peak it'll pull about 113amps.
 

coreyzev

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I'm more interested in the general rules of how current flows through a series parallel build like I'm making than someone giving me the answer to my specific problem.

Someone suggested that the load is actually shared between the 3 in parallel, each only getting 1/3 of the current. Which is opposite of what you just said, @Shale MacGregor.
 

Shale MacGregor

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I thought you were trying to understand the ampacity across the busbars, not the load on each cell. If you put the cells in parallel they share the load equally across each cell, but if all 3 are attached to the same busbar, 3(x/3) = x no matter which way you slice it.

If you are trying to visualize it draw it out in a way that makes sense to you, 1 line = 10 amps, draw 2 lines from each battery, they can only start at the positive or negative of an array of cells, so the amps passing through the conductors are a function of how many cells in parallel there are.

If I line 16 batteries up in series, the voltage is additive, the amps are not... if I line up 16 batteries in parallel and attach them with a busbar the amps are additive the load is spread out evenly (though you do have to pay attention to where the load is connected to the conductor because the resistance of the busbar will come into play and the batteries will not get 100% evenly drawn down).

If I am top balancing a row of 32 batteries, with batteries labeled 1 through 32, I attach my charger to the positive on 1 and the negative on 32 because of this, to average the distance the current has to travel on avg through each battery.
 

FilterGuy

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Oops.... I somehow did not get the text into the previous post. As you can see in the diagram, If there is 'I' current through each pack, then there is 3xI between each group of packs.

I can't quite tell from the pics how the busbars will be connected so I can't say how the current will flow through the various busbars.
 

coreyzev

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I thought you were trying to understand the ampacity across the busbars, not the load on each cell.
Both I guess... I'm trying to figure it all out. I've heard both versions of 300a and sharing the load.

@FilterGuy

So my plan is to not have a single wire between the bus bars. My goal is to actual just have one thick bus bar connecting both groups in series. So one bus bar with three holes. On one side is the black from cells 1-3, each cell with its connector bolted into the bar, on the other side of the bar is red from cells 4-6, connected via the same bolts. Plan for the bus bar is currently aluminum 3/4" x 1/4" (by doubling up my 1/8th bars)

So yeah figuring out the ampacity is probably my main goal.
 

FilterGuy

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Both I guess... I'm trying to figure it all out. I've heard both versions of 300a and sharing the load.

@FilterGuy

So my plan is to not have a single wire between the bus bars. My goal is to actual just have one thick bus bar connecting both groups in series. So one bus bar with three holes. On one side is the black from cells 1-3, each cell with its connector bolted into the bar, on the other side of the bar is red from cells 4-6, connected via the same bolts. Plan for the bus bar is currently aluminum 3/4" x 1/4" (by doubling up my 1/8th bars)

So yeah figuring out the ampacity is probably my main goal.
If I follow you correctly, it will be kinda like this:
1634081820962.png
For the most part, the busbars between the packs will not be carrying much current. The busbars themselves will only carry enough current to balance the voltage. For these busbars, you should plan on 1/3 of the total current (Plus some fudge factor of your choosing).

Only at the ends where the arrangement 'turns the corner' will you have larger currents through the busbars.

I am curious, Why are you planning on aluminum instead of copper?
 

coreyzev

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You got it! Thank you. great visualization, i should've done that from the start. I was looking forward to your answer, as it was.

Ok that's a bit reassuring then. The doubled up bars should be more than enough then. It's been hard to find consistent numbers on ampacity in aluminum.
Only at the ends where the arrangement 'turns the corner' will you have larger currents through the busbars.
I actually planned to "quadruple" the bus bars there. 2x going the full cross area, and the normal 2x at the cells. I'll probably just do 2x going across, and 1x at the cells.

And at the very ends I plan on using three 2AWG cables from each of the bolts to the shunt, and three to the cutoff switch, from those points on it'll be 1 cable.

I am curious, Why are you planning on aluminum instead of copper?

mostly accessibility, more than anything else. Lowes had it. I was trying to find something that was close by and ready. I was gonna buy even thicker bars, but they didnt have enough of what i needed. And I dont think there's anywhere near me to get the right size copper bars (that I know of, anyway). I certainly didnt want to make all those connections w/ 2AWG wire. ☠
 

FilterGuy

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mostly accessibility, more than anything else. Lowes had it. I was trying to find something that was close by and ready. I was gonna buy even thicker bars, but they didnt have enough of what i needed. And I dont think there's anywhere near me to get the right size copper bars (that I know of, anyway). I certainly didnt want to make all those connections w/ 2AWG wire. ☠
If you ever want copper, I find onlinemetals.com the best.
 

coreyzev

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If you ever want copper, I find onlinemetals.com the best.
Thanks... I better decide now, because It's going into a very compact space and will be absolute hell to change out in the future. McMaster Carr seems to be a good source too. (someone on fb group just reminded me it exists).

Everything about my system is going to be somewhat compromised, due to putting this much power in a 25' RV TT. Will definitely share more about the end result when I get there. As you can see above, I started mocking everything up in Fusion 360 to make sure it'll fit. If only I knew how flexible the wires I plan to get are. (will likely get welding cable, and if i'm feeling spendy, Polar Wire's Arctic Ultraflex).

I need to find a way to test the conductivity of the aluminum i got. Im certainly having doubts. Looks like "high conductive aluminum" on mcmaster carr is 2x as expensive as what I bought.
 
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Shale MacGregor

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Both I guess... I'm trying to figure it all out. I've heard both versions of 300a and sharing the load.

@FilterGuy

So my plan is to not have a single wire between the bus bars. My goal is to actual just have one thick bus bar connecting both groups in series. So one bus bar with three holes. On one side is the black from cells 1-3, each cell with its connector bolted into the bar, on the other side of the bar is red from cells 4-6, connected via the same bolts. Plan for the bus bar is currently aluminum 3/4" x 1/4" (by doubling up my 1/8th bars)

So yeah figuring out the ampacity is probably my main goal.
Put a metal aluminum bar between two stumps.

Have 3 big guys stand on top of the bar, then ask them to together, lift 300 lbs.

They will each be nice and share the load and lift 100lbs each, but the metal bar they are standing on still feels their weight + 300 lbs.

I kinda figured you wanted to know how to figure it out, as well as confirmation for when you figure it out. There are a lot of resources online but some of them are conflicting. One thing to remember when considering the ampacity of a conductor is what degree of rise you are able to allow as well. As conductors have more amps pushed through them they heat up, and that heat rating dictates wire gauge and type as well as busbar. This can become more of an issue as you look at tighter packs and enclosed areas because the rise means above ambient... once ambient goes up, so does the temperature of the conductor within that space.

Rule of thumbs when it comes to busbars can be helpful but they do not tell the whole story. Cross-sectional calculations can't account for surface area without knowing the actual dimensions and the rate of heat dissipation. Luckily with DC we don't have to worry as much as AC when it comes to things like skin effect, proximity inductance, etc.


But if you absolutely must use a generalized formula for aluminum, this is relatively safe

700 amps per square inch cross section...

1/8 * 3/4 * 700 = 65 amps

I made a few bars yesterday that were ~ 1/8 x 5 inches, my 55ah cells in 2p with 2c discharge can top out at 220 amps and the bar rates at 437.5 amps using that formula so plenty safe.

The more open area you have and airflow the further you can push the line, but if space becomes a factor I would go with copper every time.

**disclaimer** I barely know what I am talking about, listen to anyone else who disagrees with me.
 

coreyzev

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Put a metal aluminum bar between two stumps.

Have 3 big guys stand on top of the bar, then ask them to together, lift 300 lbs.

They will each be nice and share the load and lift 100lbs each, but the metal bar they are standing on still feels their weight + 300 lbs.

I kinda figured you wanted to know how to figure it out, as well as confirmation for when you figure it out. There are a lot of resources online but some of them are conflicting. One thing to remember when considering the ampacity of a conductor is what degree of rise you are able to allow as well. As conductors have more amps pushed through them they heat up, and that heat rating dictates wire gauge and type as well as busbar. This can become more of an issue as you look at tighter packs and enclosed areas because the rise means above ambient... once ambient goes up, so does the temperature of the conductor within that space.

Rule of thumbs when it comes to busbars can be helpful but they do not tell the whole story. Cross-sectional calculations can't account for surface area without knowing the actual dimensions and the rate of heat dissipation. Luckily with DC we don't have to worry as much as AC when it comes to things like skin effect, proximity inductance, etc.


But if you absolutely must use a generalized formula for aluminum, this is relatively safe

700 amps per square inch cross section...

1/8 * 3/4 * 700 = 65 amps

I made a few bars yesterday that were ~ 1/8 x 5 inches, my 55ah cells in 2p with 2c discharge can top out at 220 amps and the bar rates at 437.5 amps using that formula so plenty safe.

The more open area you have and airflow the further you can push the line, but if space becomes a factor I would go with copper every time.

**disclaimer** I barely know what I am talking about, listen to anyone else who disagrees with me.
Thank you so much for working so hard to help explain it to me. I really appreciate it.
 
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