pic: Battery connector.....

[apalrd]

loading photo ...

[SenorZ]

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

[m1506m]

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.

[kevin.li.rit]

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

[Michael Hill]

Originally Posted by kevin.li.rit

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

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.

[Joe Ross]

Originally Posted by Michael Hill

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.

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.

[EricVanWyk]

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.

[Dad1279]

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.

[apalrd]

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.

[cgmv123]

Originally Posted by apalrd

FIRST should really switch to the next size up connector.

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

[Al Skierkiewicz]

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.

[apalrd]

Originally Posted by Al Skierkiewicz

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.

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.

[apalrd]

Originally Posted by kevin.li.rit

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

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.

[Nirvash]

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

[Dale(294engr]]

This is a contact resistance problem, (one black, one red)

caused by improper connector seating (i.e. not fully inserted / mated)

exacerbated by crud, dirt, scratches on the contact surfaces +alignment

worsened with use (insertion cycles), years of use / misuse

(so these units more likely to be relagated for use only with practice robots)

analysis:

Two considerations:

P=I^R 1st order heating /melting localized at conductors contact point

2nd order Vdrop=IR = degraded robot performance: less I due to less V

A good estimate for average competition Robot current draw is ~50A

50^2= 2500 * RohmsPerContact =.001 ohm (1 milliohm arbirtary example)

P=2.5W =no problem with heating, no time limit

now include intermittent multi motor high current start / reverse periods .1-.3 sec of currents up to ~400A ^2 = 160,000 *.001 = 160W which is only a problem if it exists too long i.e. allowing build up heat at connector contact(s) junction.

at ~1sec expect some melting!!

Realistically.. Anderson contact resistance greater than .001 ohm happens
then at .002 ohm double the above.. .003 triple, .004 quad etc.

2nd order performance problem is connector contact IR drop,
i.e. Voltage drop (V-IR)
or sag as it may be referred to

400A *.001 = .4v loss * 2 for both sides) = .8v droop or 11.8v available to all robot systems

(brief battery Vsag is ~= .1v/50A so 400A is 4v additional loss
.. 12.6 -4.8 = 7.8v available to motors (and all electronics)
(simplified, disregards that motors will draw less current as Vbatt sags)


(12.6-.8v = 11.8v for new battery, fully charged, discharging at 50A so must subtract an additional .1v for internal battery Resistance sag at 50A delivery, Batt Voltage now is 11.7v Best Case!)

.. critically, motors provide less torque as less peak current is available as more voltage is dropped across contact resistance(s)
(plus all other connections & wire, to & from all motors)

.. before the melt and fall apart.. melt progresses quickly past a certain point, the contact resistance quickly rises, hastening the melting process due to I^2R heating leading to a forced disconnect due to melting apart.

The single surface contact area used by Anderson has always concerned me.
Seems a bifruc pin into a barrel would guarantee more surface area at the cost of a slightly larger physical connector (or perhaps two, one single for each side - just need to assure logistics of improper polarity mating.. opposite sex for + & -and a way to prevent plugging Battery into itself unfused..

Perhaps ALL batteries should be fused at the battery side.

I have witnessed upset robots battery terminals short against aluminum frame, overheat and smoke. Batteries have lead internal "fuse" safety link connections between the 6 ea 2v cells.. so depends on the condition in competition of battery charge state /condition plus resistance of terminal to frame "short" to determine when & if link melts (fuses), opening the circuit, or just smokes until the stored charge is consumed... in heat!

Good luck in competition

.. and may all your connector contact reisistances << .001 ohm until after the championships...