Surface resistivity & volume resistivity?

Buk___

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I have the data sheet(pdf) for some Kemtron conductive foam.

The specs give two figures for the foams resistivity:

Surface resistivity: 0.05Ω/sq
Volume resistivity: 0.015Ω/sq

I was trying to work out what energy would be consumed if this was used to connect to a 18650 battery, but the units; Ω/sq, is something I've never encountered before; and the wikipedia article doesn't clarify things much.

To turn this into an example to which I might understand the answer, if I draw say 5A at 3.7 volts through a pad of 1.5mm thick conductive foam attached to the 254mm^2 base of the cell; how much energy will be lost to heating the foam?

Thanks for any insights you can give me.
 
You'd need to find out from the manufacturer what unit of measurement they intended to (but did not) put there. Given their other measurement on the same line is in mm, then it's likely the unit is metric, and probably mm, but you'd have to ask. If none of their data sheets use a unit there, then that manufacturer probably uses the same unit for them all, but only documents it externally to the datasheets, perhaps in their testing-process information?

"sq" is usually used for "square" as in "square mm" or "square inch", in linear (vs volumetric which would used "cubed") measurements, and without knowing what unit, you can't calculate anything.


Regarding using this to run current thru...it's just like any of the "conductive" glues--it's not meant to carry any significant current. There's been a few threads about those glues, epoxies, etc.
 
amberwolf said:
You'd need to find out from the manufacturer what unit of measurement they intended to (but did not) put there. Given their other measurement on the same line is in mm, then it's likely the unit is metric, and probably mm, but you'd have to ask. If none of their data sheets use a unit there, then that manufacturer probably uses the same unit for them all, but only documents it externally to the datasheets, perhaps in their testing-process information?

If you follow the wikipedia link, it "explains" that the 'Ω/sq' units is a real thing:

Units[edit]
Sheet resistance is a special case of resistivity for a uniform sheet thickness. Commonly, resistivity (also known as bulk resistance, specific electrical resistance, or volume resistivity) is in units of Ω·m, which is more completely stated in units of Ω·m2/m (Ω·area/length). When divided by the sheet thickness (m), the units are Ω·m·(m/m)/m = Ω. The term "(m/m)" cancels, but represents a special "square" situation yielding an answer in ohms. An alternative, common unit is "ohms square" (denoted " {\displaystyle \Omega \Box } {\displaystyle \Omega \Box }") or "ohms per square" (denoted "Ω/sq" or " {\displaystyle \Omega /\Box } \Omega /\Box "), which is dimensionally equal to an ohm, but is exclusively used for sheet resistance. This is an advantage, because sheet resistance of 1 Ω could be taken out of context and misinterpreted as bulk resistance of 1 ohm, whereas sheet resistance of 1 Ω/sq cannot thus be misinterpreted.

The bit I've highlighted above made me laugh. It seems to be akin to one of those "dimensionless quantities" like Reynold's numbers and the fine structure constant; but different.

I'll drop them an email later.
 
Ah--when I clicked the link, a blank page opened, no article, so I didn't see that.

Either way, you still have to know which unit they're actually using, but you could "assume" it's mm since the other unit on the same line is mm. :/
 
A property of sheet conductors is the resistance is independent of the distance between points. In your example, if the foam is sandwiched between the cell and a bus bar, you will have something close to the volume number.
 
fechter said:
In your example, if the foam is sandwiched between the cell and a bus bar, you will have something close to the volume number.

So basically, less heat generated by the foam connector, than the internal resistance of the cell?
 
amberwolf said:
Ah--when I clicked the link, a blank page opened, no article, so I didn't see that.

Either way, you still have to know which unit they're actually using, but you could "assume" it's mm since the other unit on the same line is mm. :/

According to the "Sheet resistance" section of this (I cannot link to it directly), and assuming I'm understanding it correctly -- which isn't a given -- because width (in whatever units) and length (in the same units) are defined to be the same for Sheet Resistance, they cancel each other out, so the number is independent of the unit of distance used in its calculation.

Which kinda makes sense.
 
I think you can ignore the sheet resistivity as you're not working with the conductance along a sheet here, but through a block of material.

I also think the volume resistivity of 0.015 Ω/sq can be read as 0.015 Ω per cubic-metre. I.e. if you have a cube of the material 1m on all sides with two opposite faces as electrodes, the resistance through the block will be 0.015Ω. Half the thickness and it should be only 0.0075Ω.

Obviously your cell contact will be much, much smaller. 254mm^2 = 0.000254m^2. Assume your pad is 1m thick and resistance will be:

0.015/0.000254 = 59.06Ω

Reduce thickness to 1.5mm (0.0015m): 59 x 0.0015 = 0.089Ω

*I think*
 
Punx0r said:
Reduce thickness to 1.5mm (0.0015m): 59 x 0.0015 = 0.089Ω

Yeah. That's about what I got.

Punx0r said:
*I think*

That too :)

At 2C discharge from 2600mAh cells, that's a bit under 0.5W. Times 2 for each end and 60 for a 12s5p pack and it's not negligible.
 
It might be worth testing. I've seen that stuff used in medical equipment (for RFI shielding).

Being a foam, the more you compress it, the lower the resistance will be. it can be used as a force sensing resistor.
 
fechter said:
It might be worth testing. I've seen that stuff used in medical equipment (for RFI shielding).

Being a foam, the more you compress it, the lower the resistance will be. it can be used as a force sensing resistor.

I've requested a sample. If the send one, I'll try it out.

My idea is to attach circles of the foam to the batteries using the conductive, self adhesive and then use that to connect to the copper surface of strips of pcb board to act as parallel connection bus bars and wrap the p-groups of cells with board top and bottom in heat shrink.

I'll make a small (3p) pack and attach it to my frame to supply my lights as a test to see how well it stands up to the vibration of use.
 
It will be interesting to see how it performs.

Cell connections with greater resistivity than the cell internal impedance makes everyone sad in pants, though :(
 
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