As secretly hoping to use our carbon fiber parts as a common ground for overall power distribution, I thought it would be good to actually measure how it is doing in a context of our other options. The plan is to measure the material resistance and voltage drop for copper wire, aluminium bar and our CF tape on 1 m of material.
Copper
Our first testing subject is 25A 2 Core Tinned DC Power Cable from Jaycar.
Running it through a nice cable calculator, we should be seeing following readings – expected resistance to be 5.8 micro-ohms.
Now to our actual measurement.
Well, we measured 0.3 ohm. That doesn’t seem to be correct as this would be 100 times higher value than what calculator shows. Voltage drop was 5.87 mV.
Aluminium
Our second testing subject is Metal Mate 20 x 1.6mm 1m Aluminium Flat Bar from Bunnings warehouse.
Running it through our nice cable calculator again, we should be seeing following readings – expected resistance to be 0.8281 micro-ohms.
Now to our actual measurement.
Well, we measured 0.4 ohm. That doesn’t seem to be correct as this would be again 50 times higher value than what calculator shows. Voltage drop was 0.6 mV. This value is lower than with copper as we have more much more cross-section area here (10 times more).
Carbon Fibre tape
Our last testing subject is 12k 200gsm 4″/10cm width Carbon Fiber Uni-directional Cloth UD Fabric Tap High strength Repair material Tensile strength 3400Mpa from AliExpress.
Unfortunately our calculator doesn’t know Carbon Fibre material, but as it didn’t help much for that copper and aluminium before we’ll just go ahead. Seb prepared aluminium clamps so we have something to connect to.
First reading came with 8 ohms resistance, what seems to be roughly 20 times higher value than for the others. As were trying to measure the voltage drop, we needed to put weights on those clamps to improve connectivity. Still, we measured quite high 1.2V drop just on that 1 meter!
So putting it all in a table:
| Material | Resistance [Ohms] | Voltage drop [mV] |
| Copper | 0.3 | 5.87 |
| Aluminium | 0.4 | 0.6 |
| Carbon Fibre | 8.0 | 1,250.0 |
Honestly I am not sure what to make out of this. Does this still qualify to be used as our common ground? I suppose someone with more insight into electricity needs to have a look. It is not a big deal if this is not ok, but we’ll have to count with additional weight for doubling our power cables.
Update from Richard 2023/1/23
You can connect it to GND, but you can’t use it for electrical GND for return currents. So you really want to be using:
P=I^2R
So if 2A flows P=32W
Update from Chris 2023/1/23
Resistance is different in different directions on carbon fiber as well – and almost none between layers. CF threads are “wet” with a chemical which helps the resin absorb – if you’re wealth enough to grow carbon nanotubes on the CF to use as the wetting agent, you then get excellent conductivity in all directions.
Or the opposite of that technology – when you don’t use nanotubes, conductivity is poor. Multi-Functional Carbon Fibre Composites using Carbon Nanotubes as an Alternative to Polymer Sizing – what’s the ground for? If it’s the high-current for the motors, perhaps some copper tape laid along with the fibers when you make the tubes like below?
There’s been also big comment from Adam, see in comments below.
Thank you guys!
Hey firstly thanks for publishing so much detail on the project. I’ve learned a lot just by following along and I’m really interested to see it keep progressing. Keep it up!
So I figure I should repay the favour a little. Firstly you mean milliohms (mΩ, 10^-3 ohms) not microohms (μΩ, 10^-6 ohms). But that’s a common error- I wish they’d used a latin letter for micro like all the other SI prefixes but oh well.
As you found measuring very small (and very large) resistances is tricky. A normal multimeter gets less and less accurate towards about 0.5 ohms and below that isn’t really useful at all.
But, you figured that out because the numbers didn’t match, and you also got most of the way to figuring out the way to do it- using the voltage drop. You pass a known current through your cable/carbon fibre and measure the voltage. And the easiest way to get a small known current is a known resistor + constant voltage source. You can use your lab power supply for the known current source too, but you’ll find they’re not very stable for low current limits. You can also use your multimeter to measure the current and make it more accurate. Just you have to swap the red lead to one of the current ports, put it in line instead of across the circuit, and make sure you won’t exceed the current limits printed on it (looks like 600mA on your multimeter and 10A on the high range). Otherwise you blow the fuse and they’re surprisingly expensive/hard to replace with the right ones. And you gotta remember to swap the lead back once you’re done or you’ll get wrong readings or blow the fuse when you try to measure something else. Been there, done that…
The whole process is described in more detail here: http://robotroom.com/Measuring-Low-Resistances.html
But you can probably avoid doing that entirely. For the copper and aluminium use your existing numbers. Observe that your millivolt values in both cases are about the same as what the calculator said your milliohms should be…. I.e. in Ohm’s law V=IR your I (current) is probably 1A. Also for the copper you can cheat and click the specification tab on the Jaycar site and they’ll quote the resistance per metre 😉
Your measurement of 8 ohms for the carbon is probably reasonably accurate, at least as a ceiling. The manual for the multimeter will specify the accuracy at different ranges but it should be pretty close. The main difficulty there, you again figured out, is getting good enough contact between the fibres and the multimeter probes. You could try dropping a bit of solder onto it but your tape probably has epoxy or polyethylene or PVC or some other coating/binder on it as well. Plastic fumes, fun! You could try crimping a ferrule or some sort of crimp terminal thing onto it if you have any lying around? The pressure on really high quality crimps is enough to cold weld metals so you might get a decent enough connection even with a coating/binder. At least to see if you get anything lower than 8 ohms, anyway.
If it is 8 ohms, it’s probably too high for a common ground… but it depends a lot on what you’re using it for. If it’s the motors, and it’s the return path for the main power, you’ll get a big voltage drop (V=IR again). And you’ll get resistive heating as a result- power in Watts P=VI, sub in V=IR, so P=I^2 x R. I had a quick search on old posts and found you had ~8A for each main motor. So you’d lose V=IR = 8 x 8 = 64V and you only have 14.8V to play with from the battery pack… not good. Even if you had a 64V battery pack you’d lose it all, P = I^2 x R = (2×8)^2 x 8 = 2 kilowatts due to heating!
If it’s just carrying low-speed digital signals you might get away with it. You’re probably looking at I2C or SPI which will go from around 50kHz to 10MHz or so. The lower end would probably work, the upper is a bit iffy. It depends on a lot more things then though. Capacitance and inductance are the main ones, because then they’ll form a filter or even resonate, and knock out/slow down/distort anything too fast. That’s probably directly proportional to length. Then there’s exact chips you’re using, interference from your other signals, blah blah. The two ends might also get “pushed apart” during high instantaneous loads- something like, the motors have a big fat cable with lots of instant current capacity and can drag the controller down really quick, but the low pass filter from series inductance/parallel capacitance would mean it’d propagate slowly along the digital line. One end would think it was sending a 1 and the other would see a 0, until the “new ground” propagated. So if you test it you’d want to try it in extreme conditions (all full throttle one way, then immediatelly all the other, that kind of thing) and watch for weird jittery intermittent behaviour, that’s what it will look like.
Hi Adam, wow, thank you – this is most appreciated! So to let you know, I already ran those results through my mates – Richard & Chris – and they came with not that detailed, but very similar same conclusions. I approved your comment and will leave it here for reference and further reading on this topic. Meanwhile I’ll be adding updates from Richard & Chris below the article together with the overall conclusion not to use CF for our returning currents – probably just for a common ground only. Your comment clearly states the same. 🙂
Please get in touch anytime, or contact us directly through our Discord channel.
Thanks again Jan
You’re very welcome!