Using copper flat bar to connect the batteries in a battery bank

[Zai]

Does anybody know what size copper bar would I need to get the same effect as 70mm battery cables?

I have always thought that the surface area of the strands were important, so it is not as simple as to calculate the area of a cross section of flat bar. Is this true?

Any other input as to using copper flat bar would be appreciated.

Thanks!

[GordMay]

See ➥ https://www.copper.org/applications/...us_table3.html

[Reefmagnet]

Skin effect where current travels along the surface of a conductor only becomes a consideration at high frequencies, so the same cross sectional area applies. Kind of. In reality you can use proportionally smaller barstock as it heats up less because it will have better heat dissipation because of its surface area to volume ratio and lack of insulation.

[mrm]

Seems like a good idea, however.. think of how much mechanical stress battery terminals will be subjected to because of thermal expansion and contraction of such solid bar, when batteries are firmly fixed. There is a reason solid copper interconnects used in LiFePO bank assemblies are shaped the way they are.

[GordMay]

Originally Posted by mrm

... think of how much mechanical stress battery terminals will be subjected to because of thermal expansion and contraction of such solid bar ...

Not much really.
The rate of thermal expansion for copper is a little less than .001" per inch of length per 100 degrees (F) delta T.

[Matt Johnson]

Originally Posted by GordMay

See ➥ https://www.copper.org/applications/...us_table3.html

Strange they don't list the voltage for this chart. It's for 60 Hz AC, and it should be listed as 120 volts if that's what it's for.

Matt

[rwidman]

Originally Posted by Zai

............. I have always thought that the surface area of the strands were important, so it is not as simple as to calculate the area of a cross section of flat bar. ............

What makes you think that? Did you see that on a boating forum?

A copper bar will be rigid and if there is any movement of the batteries, there will be stress on the terminals. If it was a good idea, boat manufacturers would be using copper bars. They are not.

Use cable like everyone else.

[GordMay]

Originally Posted by funjohnson

Strange they don't list the voltage for this chart. It's for 60 Hz AC, and it should be listed as 120 volts if that's what it's for.
Matt

The only part Voltage plays in conductor ratings is insulation, not ampacity.

[mrm]

Originally Posted by GordMay

Not much really.
The rate of thermal expansion for copper is a little less than .001" per inch of length per 100 degrees (F) delta T.

Gord, I know that the dimension change will be minuscule, nevertheless the forces involved will still be substantial. I will try to illustrate my thinking with a mental experiment. Let us imagine a piece of metal (copper) bar placed between two absolutely unyielding surfaces. When the temperature rises, the metal tries to expand, however surfaces it is trapped between do not yield, therefore that dimension remains unchanged. Am I wrong in thinking, that for that condition to be met, the stress the metal is subject to has to reach the yield strength of the metal? For copper this would be approx. 70 MPa. Am I correct calculating, that for a sample copper bar with a cross section of 0.04 x 0.01 m (4 x 1 cm) the force acting on terminals (should they not yield) would be 70 * 10^6 * 0.0004 = 28000 N ?

Edit: what Rwidman wrote above is probably a better way of expressing what I tried. Battery posts are not designed for stresses coming from attachment to rigid bars, be it from thermal expansion or more likely from battery movement.

[Cavalier]

Originally Posted by mrm

Gord, I know that the dimension change will be minuscule, nevertheless the forces involved will still be substantial. I will try to illustrate my thinking with a mental experiment. Let us imagine a piece of metal (copper) bar placed between two absolutely unyielding surfaces. When the temperature rises, the metal tries to expand, however surfaces it is trapped between do not yield, therefore that dimension remains unchanged. Am I wrong in thinking, that for that condition to be met, the stress the metal is subject to has to reach the yield strength of the metal? For copper this would be approx. 70 MPa. Am I correct calculating, that for a sample copper bar with a cross section of 0.04 x 0.01 m (4 x 1 cm) the force acting on terminals (should they not yield) would be 70 * 10^6 * 0.0004 = 28000 N ?



Edit: what Rwidman wrote above is probably a better way of expressing what I tried. Battery posts are not designed for stresses coming from attachment to rigid bars, be it from thermal expansion or more likely from battery movement.

The biggest issue is not going to be the linear thermal expansion of the bus bar, but the near inability to perfectly secure the batteries. Any motion of the boat will lead to differences in position between the terminals and the [yielding] plastic of the battery housing that is probably being used to clamp said battery in position. As mentioned above, the cyclic shear loads on the base of the terminals over time will lead to fatigue outside of the design parameters of the terminals themselves. One benefit of [curved] battery cables between terminals of different batteries is that they act as strain relievers for the terminals themselves.

Sent from mTalk

[Zai]

Wow!
Thanks for all of the useful information.
The reason I am considering this is because of the cramped quarters in the battery compartment.
The batteries are good and tight in place, but there is of course the likelihood that they will wiggle a bit from time to time.
If I go ahead this idea I think I will get some bars cut with high pressure water with a couple of s shapes in them. That should take care of the physical stress.

On a related matter, what\s the consensus on battery fuses?
Is it a consensus to use a fuse on each battery or will one suffice at the ground terminal?

[smac999]

batteries move. use wire and leave enough bend in it so they can move.

[Redpcx]

Insulated cable is always used to inter connect batteries. Bus bars are used to make a battery out of individual cells. It's practically impossible to align two batteries well enough to prevent stress on the battery terminals. Use flexible insulated cable. If you make bus bars, insulate them to prevent accidents. Certainly easier and probably cheaper to use cable, unless you have free copper stock.

You will never find a concensus of opinion on battery fuses. Some say fuse everything. Some say don't fuse the battery.

Don't fuse the ground. Don't fuse the ground. Do not fuse the ground.

[Matt Johnson]

Originally Posted by GordMay

The only part Voltage plays in conductor ratings is insulation, not ampacity.

? How does voltage not play into the resistance? Isn't that part of Ohm's Law?

Matt

[jeepbluetj]

Originally Posted by Redpcx

Insulated cable is always used to inter connect batteries. Bus bars are used to make a battery out of individual cells. It's practically impossible to align two batteries well enough to prevent stress on the battery terminals. Use flexible insulated cable. If you make bus bars, insulate them to prevent accidents. Certainly easier and probably cheaper to use cable, unless you have free copper stock.

You will never find a concensus of opinion on battery fuses. Some say fuse everything. Some say don't fuse the battery.

Don't fuse the ground. Don't fuse the ground. Do not fuse the ground.

ABYC says to fuse the batteries. I think fusing the batteries is a phenomenally good idea.

As usual, Maine Sail has a good article about it: Battery Bank Fusing Photo Gallery by Compass Marine How To at pbase.com