Is it legal to use a 5v 5a BEC on the robot?
*(http://www.hobbyking.com/hobbyking/store/_18789__5V_5A_UBEC_2_5S_Lipoly_7_2_21v.html) for my quadrotor. It only weighs 11g compared to the 5v DC/DC converter on the robot that is 136g.
It would be used to power an arduino micro with LEDs, and possibly a RPi or a Beaglebone.
The main reason why I want to use this is because of the weight.*
There’s nothing that prohibits using it as a custom circuit. Make sure you read and follow all custom circuit rules, though! You could not, for example, have used this to replace the converter for the robot radio last year, per R43:
The Wireless Bridge power must be supplied by the 12VDC-to-5VDC converter (P/N: CLL25-24S05) connected to the marked 12VDC supply terminals at the end of the PD Board, and not the main WAGO connectors along the sides of the PD Board shown in Figure 4-12. No other electrical load may be connected to these terminals.
Cor custom circuits, see R53
CUSTOM CIRCUITS shall not directly alter the power pathways between the ROBOT battery, PD Board, motor controllers, relays, motors, or other elements of the ROBOT control system (items explicitly mentioned in R64). Custom high impedance voltage monitoring or low impedance current monitoring circuitry connected to the ROBOT’S electrical system is acceptable, if the effect on the ROBOT outputs is inconsequential.
and R71
All outputs from CUSTOM CIRCUITS shall connect to only the following:
other CUSTOM CIRCUITS, input ports on the Digital Sidecar, input ports on the Analog Breakout Board, the RS-232 port on the cRIO, the Ethernet network connected to either Port 1 or Port 2 of the cRIO, the CAN-bus if and only if all Jaguar motor controllers on the CAN-bus are wired in full compliance with R67and R68, or the sensor inputs on the Jaguar motor controller.
Of course, these rules may change for next year!
First, no one here can post anything accurately with regards to what may or may not be legal next year.
The big thing to remember is that the new control system has a voltage regulator module. This provides 12 volts and 5 volts outputs, both at 500ma and 2 amps. It is 1.8 ounces or 50 grams.
They say the power is ±50mv @ 2a, which is good enough for what I need. I won’t realistically be pulling 5a, more like 1-2. I am powering LEDs through an arduino. I have powered about a meter with like 1 amp and thats more than enough.
I am pretty sure the ardunio 5v output isn’t designed to power anything, and they recommend against using it power anything more then an LED or two.
I’ve had one of those Hobbyking BEC, it had a habit of running hot and wasn’t the cleanest output causing lines on my VtX. And running about 3-4amp though it blew it up. I would avoid HK electronics in general on anything but the cheapest toys, there QC for electronics is non-existent and they over-rated everything horrifically. But it may work. 
Yes, but I’m most likely going to only use 1-2 amps. And the LEDs have a 5v in on them so they can be powered directly by the 5v BEC.
We used something very similar to power our LEDs and Arduino on our robot last year, but it was a 3A one from Adafruit and it worked perfectly.
Chad,
As Jon pointed out, you may not use this in place of the 5 volt convertor for feeding the radio. You might also look at the input specs on the board. It is only good down to 7.2 volts and the robot regularly can draw battery terminal voltage below 5 volts for short durations. Depending on how you wire the LEDs, you may exceed the output capability of the Arduino.
You really might want to invest in either getting a buck-boost voltage regulator or some sort of an (possible illegal. Al is the guru here) isolated capacitor bank to feed the reg when the voltage drops under 7.2 momentarily.
I, myself, would use a secondary “bridge converter”. They is rated for 5v@1A, so it will directly work with your design. You also get the bonus that they have gone through quite rigorous testing in FRC, because every team uses one to power their communications. This would probably be your best bet if your team has any spare converters laying around! 
I power our team ODROID using a bridge converter. The voltage is quite stable and barely dropped bellow 4.98v and went over 5.03v when I tested it with a battery connected directly!
Keep in mind that the 12v/5v bridge converter is a buck topology. It’s connected to a 12V regulator on the PDU, so it won’t drop out. You can’t connect a custom circuit to that regulator, and I doubt that will change next year.
Personally I haven’t seen our battery voltage drop below 8, but we also haven’t been as aggressive with motors as other teams. If a system is consistently dropping that low, I would be worried about a bad cell in the battery.
The regulators our team will use next year are these: http://www.pololu.com/product/2866 and they maintain a sub 1V drop out up to 5A.