pic: Battery connector.....

CD-Media: Photos
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#1

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There was recent discussion regarding to the safety of the FRC battery connectors. This particular connector was used on 33’s 2012 practice robot. That practice robot melted more than one battery connector, this is the only one I saved and now took pictures of.

When the cable and connector get extremely hot, the contact begins to sink into the red plastic housing, causing an increase in resistance which rapidly heats that terminal causing the wire to begin to melt through the housing. Eventually the connection will break and the robot will not work, or somebody will notice and replace it.

I have seen several failures like this on our 2011 and 2012 practice robots (and once on our 2011 competition robot). In 2013 we switched to a 4AWG main battery cable and had no issues with the battery cable (the main breaker tripped twice).

#2

Yowza. I haven’t been in FRC too long, but I’ve never seen a battery cable melt like that.

#3

I actually see this on the larger versions of these connectors on the fork trucks we drive at work. Not too often, but try getting a battery off a charger when the two connectors melt together.

#4

Someone once told me that the 6AWG connectors were only rated for 80Amps…

#5

I once met a rep from Anderson who was amazed and seemed concerned at how many amps we pull through them. So take that for what you will.

#6

This connector is the Anderson SB-50. The 50 is for 50 amps.

Anderson makes larger connectors, the SB-120, SB-175, and SB-300. Those are larger connectors, commonly used on electric forklifts and other really big things.

Using #4 wire certainly helped with this, but the SB-50 does not go up to #4 wire so we had to play with it a lot to make it work. It would be nice to use the SB-120 which comes with #4 size crimps.

#7

Around 10 years ago, they did an analysis that showed that for the short time that FIRST robots are run, there was enough margin. However, since that time, average current draws have increased significantly, and many more teams are running practice robots for much longer then 2 minutes. Because of that, I hope that in the near future, FIRST will allow a better connector.

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#8

Would you mind if I used the image at the top of this thread in a blog post for Mooshim? We’re posting a video on how to measure contact resistance to identify this type of situation before it manifests as shown.

#9

Was that pre-assembled or did your team fasten the wires?

I’ve seen students (and mentors) get the connector in the shell upside down/backwards.

#10

Eric: Sure, go ahead, I have 2 pics of this one if you want them.

This particular connector came from the KOP, where it was pre-terminated on the Anderson end and we crimped/dip soldered the other end with a large ring terminal.

We now buy fully terminated cables from AM or CTRE for the batteries, and make our own from #4 wire for the robot. We crimp both ends on a large press crimp, and dip solder the ring ends (not the connector ends) before assembly.

This cable came from the 2012 practice robot which melted 3 connectors. we also melted two on our 2011 comp bot during the off-season. This is the least bad failure of all of those, in two cases the black wire came through the red connector so the copper was showing all the way down the connector.

FIRST should really switch to the next size up connector.

#11

Especially since the curent rating on the next size up connector matches the size of the robot main breaker.

#12

Andrew,
I would like to examine this connector up close. I may need to disassemble the connector. Any way you can ship this to me? Jim or Ike has my contact info.
What hit me first off in the picture is why the red wire is not melted as the black wire is. They both pass the same amount of current.

#13

Both wires were hot when the connector failure happened.

When this happens, the terminal begins to sink into the plastic connector. As soon as this happens, it rapidly increases in resistance and melts away (at this point, the current drops, so the other wire would not fail).

I have seen both wires fail, it seems random.

While getting battery cable today (several feet of #4 red and black), I saw some #1 high flex battery cable and smiled. Maybe when we go to the SB-120 I’ll use it.

#14

I’m suprised that nobody has said anything about the fact that a robot that can do that repeatedly is designed wrong. If you stay within the limits of your supplied hardware you wouldn’t have this issue. Nowhere in the rules does it say you have to draw the amperage that you are. Would it make for a slightly les competitive machine? Maybe. Would it reduce the risk of melting connectors and destroying components? Definitely.

It just seems to me the a lower amperage rated connector is something that would result in an engineering challenge. I’m pretty sure that is what most of us are here for.

This seems comparable to someone complaining that their drill press doesn’t work very well as a mill…

#15

It’s a safety issue. In ANY electrical system, you have safety measures designed to prevent this from happening. In our system, that is supposed to be the 120amp main breaker. FIRST has determined that drawing excess of 120amps continuous causes an unsafe condition and requires the robot shut down. What Andrew has shown, and has been repeated on several other robots, is that under certain circumstances the main battery connector fails before the main breaker, thus circumventing the entire safety system as it is designed.

#16

I would actually have to disagree with your first statement. It has been repeatedly said that the breakers are in the system to protect the wiring and connections. If the breakers are not doing their job, and wiring is melting before the breakers do break, something in the rules have to change, because thats when things start getting dangerous. Either a new connector that can actually handle how much current the breaker will allow, or a smaller breaker, which is absolutely not the answer. So its a safety issue more then anything.

#17

I had a feeling I would be disagreed with on that. I don’t have a problem with that a all. I feel that the connector is definitely under rated for its purpose. But what I am saying is that we all know that rating going in. It wouldn’t hurt to design for your components.

#18

Forklift at work…

I tend to agree that the source of the problem is not the connectors. It sounds like you have an issue in your robot.

The only way I can think of their being an uneven current flow is if you have a short through the frame to ground. Also, if one connection starts to fail before the other, then the resistance on one side will increase at a higher rate than the other. This could cause uneven heat production. Seems unlikely though.

#19

Just from my experience with electronics and people using undersized connectors, it isn’t odd to see one connector fail first or more dramatically.

#20

Way way way back when the Anderson SB-50 connector was selected and apparently Anderson approved the higher current rating, the kit (this is back when you could only use parts from the kit or Small Parts) contained 2 Bosch drill motors and 2 Fisher-Price motors, and an assortment of small motors. The batteries were the same as they are now, but they used a 60a main fuse instead of the 120a main breaker we now use. Shortly after, the Victor was used, then the IFI control system. Now the rules allow 18 motors of similar power to those 4 drill and FP motors, robots have nearly tripled in speed, and the games are significantly more complex.

Now, we design to the 120 amp breaker, and put a lot of effort into keeping that breaker from tripping. Sometimes we design a bit too close, and it safely trips, and we take steps to prevent it from tripping again. We know what it will take, thanks to data sheets and trip curves. We already do a lot of design based on the main breaker and 40a branch breaker trip curves. We know when they will trip, we can run simulations of accel events, travel times, and distances to optimize our gear ratio to our strategy. We know how far we want to push the main breaker and we certainly know that when we exceed it, we have to scale back a bit but the failure is safe (even if it could cost us a match). In addition, in our pursuit of perfection, we design to push the robot as hard as possible during a competition. Maybe we push too hard. Then we go and drive the competition-designed robot for hours to practice, or at fast pace off-season events, and the issues show up.

We don’t have a number or ‘melt curve’ for the 50 amp plastic connector (and I can not design my robot to 50a instead of 120a and stay competitive) so we can’t design around it. Sure, we could empirically melt a whole bunch of connectors with a several hundred amp current limited lab power supply and huge load (the only reasonably safe way to do it) to get data on connector melting vs time vs load, or we could spend $5 more and buy a properly sized connector so the engineered electrical limiting device (the breaker) trips first and safely instead of the undersized connector.

The whole point of the main breaker is to safely cut power when we exceed the electrical limits imposed by the FIRST electrical system. We shouldn’t rely on a plastic connector to fail (possibly shorting and causing several hundred amps from the battery to start a fire) as an energy limit. The connector is far too small for our application (even if it was approved for the current draw way back when) and the company even makes a connector the right size for us.

It’s not the robot, it’s several robots over several years of practice and off-season events, designed at relatively common FRC speeds with quite efficient drivetrains. We run them (on the practice bot) for hours at a time, running roughly 50% duty cycle, but we can’t upgrade the connector on the practice bot without upgrading all of the batteries (which we also need to take to competition, and plug into the comp bot).