This is exactly what I was referring to. I have used 4awg wire on robots since 2000 and have come across all sizes of supposed ‘4awg high strand count’ wire. If I am making a concerted effort to upsize my wire for current, heat and voltage drop reasons, I am going to use the largest stuff I can that still fits in the 4awg ring terminal (which has a significantly larger OD than the .22" SB50 ID). For reference and comparison, the 4awg Molex ring terminal we use on our batteries has a .280" minimum ID for the wire.
Good info to know but I’m not sure what else to say…
The wire we have matches the specs listed from PowerWerx for their 4 awg wire as well: https://powerwerx.com/welding-cable-epdm
I do agree though about not all wire being equal. The larger stuff I’ve seen listed as 4 awg and held has not been OFC or bare copper though, it’s been CCA - it’s lighter but it doesn’t conduct as well based on the measurements I’ve seen and what I’ve read.
I think your references and Nate’s are both great for teams to have access to and understand. Just helping to provide more information for people to leverage.
I can’t quite tell from your picture – did you just bend the battery terminals toward the center of the battery, or are you using pre-bent lugs?
These are the 90 degree lugs I was able to find for 4awg. I assume these are similar to what Jim is using. You can also use your DigiKey voucher for these if you haven’t done so already.
Marshall, Which diameter Nord-Lock Wedge lock washer did you guys use from Mcmaster? The 0.350" OD or the 0.430" OD?
You use, and specify, cable lugs with a 1/4" bolt hole. Is there an advantage over using using lugs with a #10 bolt hole that matches the battery terminals on the permitted batteries and the bolt sets that are provided by some battery makers?
The smaller hole would provide more mating surface (less resistance, less heat) between the lug and the battery and provide better gripping for the washers.
I believe purchased cables, which come assembled, use the 1/4" hole as well, but I think this is for the robot side of the power cables where the bolt posts are 1/4".
Alternatively, has anyone widened the holes in the battery terminals to 1/4" and used 1/4" bolt sets? Is this allowed?
I see the same thing. I have seen online specifications for 6 AWG wires and lugs ranging from .167 for single strand to .25 for the 1000+ strand that I use. I figure more strands leaves more air gaps around the strands, but when crimped properly the air gaps are eliminated provided a nearly solid wire under the crimp. Cutting some strands to fit in the .22 SB50 lugs slightly increases resistance over a length of less than 1/8". I don’t worry about this because it is less significant than using 50 AMP connectors on 75 AMP AWG 6 wire in front of a 120 AMP circuit breaker on a application that pulls over 200 AMPs for up to a few seconds! I would use #2 wire in SB120 connectors but this is less important than having a driver that is 5% better.
Yeah, we think so. In general, by increasing L/D ratio of the fastener, we decrease the stiffness of the bolted joint and make it more able to ride through a wider range of external load conditions without losing complete preload. Here is the lug we have used. We use #10-32 SHCS with nylon locknuts. Between the battery terminal and the cable lug we use 97527A005, an internal/external tooth lock washer with a slight belleville shape. It is the best washer I’ve seen.
We throw these away.
– Hex head screws encourage the user to try to break stuff
– Split lock washers are the devil’s lock washers
Love, love, LOVE the use of Nord lockwashers. I cannot speak highly enough of them.
I would encourage every team to section a few sample crimps to see that everything is working as expected. 4awg crimps should have 140lbf (UL) to 400lbf (Mil Spec) pullout strength, so a simple hand-tug will only catch the worst of the worst crimp attempts.
Great write-up, I am pleased to see more and more teams opening up the biggest electrical bottleneck in FRC!
Our crimp tool (very similar to you guys’, one of the hydraulic ones from Amazon) came with metric dies.
If we step die size up to the next largest size, it barely crimps at all and most definitely does not hold the wire. If we go down to a smaller size it just pancakes even more…
We put the end of the contact into a vice, tugged on the wire with as much force as we could and it seems to hold fine. Just wondering if this might result in any long term problems…?
Flashing from crimping is normal. Higher-end crimping tools have trimmers that cut this flashing off automatically. Just file the flashing down if it doesn’t fit in the SB housing and you’ll be fine.
So, two questions:
- Are SB120s actually legal to use on FRC robots? My impression was that only the SB50 was allowed. Did I miss something in the rules this year?
- What is the actual observed benefit for using larger gauge wire for FRC battery connections? I get that there’s less resistance, but does this result in longer runtimes or more efficient discharges, or something else? How significant is this benefit, has anyone measured it, or are the results mostly anecdotal?
- Yes, the rules simply specify an “SB” connector.
- Larger margin on brown-outs was the single biggest reason we switched to 2AWG main wiring last year, which improved ~0.4-0.7V under peak loading vs 6awg wiring.
Operating at higher voltages improves available power, run time, reduces motor heating, and results in generally more consistent performance over a match. Being worth it is up to your team to decide based on your priorities. We’re not going back to 6awg any time soon though.
Hey, I was wondering if someone could re-upload the whitepaper, it seems the script hasnt made it here yet.
You can use archive.org to pull it from the old Chief until Brandon gets the media stuff sorted out.
Checked this morning. It’s about 12% through
Edit: (20% now)
If anyone else is looking for it
This is my first reply under the new software, here goes…
First, I have to say this is fine White Paper, well done guys.
Second, I have to point out that copper clad aluminum wire is illegal for use on FRC robots under R60.
Yes SB120 connectors are legal, but as Allan pointed out above and as specified in the Robot Rules, many batteries and in particular those provided in the Spare Parts will be wired with the SB50.
As to wire sizing, each 3 steps in AWG wire size is approx. doubling or halving the wire conductive cross section. The step from #6 to #4 is less than twice the conductive copper. At 600 amps (the absolute highest current available from a fully charged new battery) would drop .234 volts per foot for the #6 wire and .144 volts per foot for the #4. A typical FRC robot will have about 2 feet of wire length total from battery to PDP, so 4 feet (total for both red and black) would be 0.936 volt for #6 and .576 volts for #4 about 0.4 volts difference when measured at the input of the PDP at max current. Your mileage will vary.
You cannot judge the conductive area by eye. It will vary with strand size. 7 strand wire will have a different diameter than 31 strand wire. Wire purchased on Amazon may not be as advertised as the seller simply measures the cross section and looks up the size on a wire chart. I inspected wire purchased on Amazon as #10 and I measured it at #13. A good hint that the wire is different size is that the wire size is not printed on the wire.
The greatest cause of brownout in my opinion, is slipping battery and wire terminals. I have suggested for many years a method for removing this problem with the addition of a simple #10 external tooth lockwasher. If you place the lockwasher between the battery terminal and the wire terminal, the lockwasher will prevent any movement between the two terminals. No movement results in no loosening of the connection. Please use the locking hardware that comes with most batteries. This would be a split washer and flat washer under the nut. I would estimate over 90% of all brownouts are caused by loose battery terminals. A loose battery connection raises the series resistance of the primary wiring and therefore increases the voltage drop of the primary wiring.
Nord-lock washers are the absolute bomb to replace split washers on the battery terminals. I highly recommend them.