Chief Delphi - Longer battery wires

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Re: Longer battery wires

Quote:

Originally Posted by philso (Post 1561153)

I'll take your word that the crimps were good and the screws and nuts are tight. What sort of motors did they have in their drivetrain? How much wire was removed from the loop in total?

My gut feeling is that if cutting out a few feet of wire made that much difference (you probably didn't cut out 12 feet or even 6 feet if they started with 6 feet), they are probably still just above the brownout threshold of 6.8 V. It is likely that they still have some other issues such as batteries with high internal resistance and that it is likely that their brownout problems will return.

6 CIM drivetrain and a couple of minicims on an auxiliary mechanism.

The loop was 6 ft for both black and red wires (12 ft in total). When I told them to chop it off to a minimum, I came back and saw that they had just under 1 ft for both black and red wires.

The problem went away.

High current draw across 6AWG *DOES* result in a large voltage across a long span of wire.

Physics doesn't lie unless you do the math wrong.

Re: Longer battery wires

Quote:

Originally Posted by kiettyyyy (Post 1561201)

If they really are pulling that much current and had an extra 6 feet in both the red and black, it makes sense that it would make a difference. I am very familiar with how to do the math but this information was not in your original post so I could only make assumptions about your example. Most of the time, when I see a robot with funky wiring (including that much extra), it was built by someone who also does bad crimps, leaves connections loose and abuses their batteries.

Re: Longer battery wires

Quote:

Originally Posted by Al Skierkiewicz (Post 1560892)

OK,
running worse case calaculations, a four motor CIM drive will draw ~480 amps at stall. 480 amps will drop .48 volts for every 2 feet of #6, please include the red and black wire in your calculations. The 6 foot run would be 12 feet of wire added to the two feet on the battery and about another equivalent 2 feet for properly crimped connectors and terminals. That adds up to almost 4 volts dropped in just the wiring feeding the PDP whenever you start driving or reverse direction. Add that to the internal resistance of the battery and you potentially have only 3 volts available even on a fully charged battery.
Jumping back to my easy calculator the WIRE FOOT, every WF will drop 0.1 volt at 100 amps.
Battery Internal Resistance=11WF
1 ft. of #6=0.5WF
1 ft. of #10=1WF
1 ft. of #12=2WF
When you view your voltage logs, those short dips to the 4 volt level you see are real. Make your #6 run as short as possible. Often you can significantly shorten the run by simply turning the PDP 180 degrees.

But if there were only 3 volts at the motors, can they still pull 480 amps? I know it is a problem, but I don't know if it is that bad.

Re: Longer battery wires

Quote:

Originally Posted by Bennett548 (Post 1561275)

But if there were only 3 volts at the motors, can they still pull 480 amps?

No.

Remember Al said it was "worst case".

In this case, the estimate is quite pessimistic.

You could create a more complete model, including:

battery internal resistance
PDB resistance
wire resistance
crimp resistance
connector resistance
voltage drop across motor controller
motor resistance

... and then do a more accurate calculation to determine the various voltages.

Re: Longer battery wires

Guys,
What I am trying to get you to think about is this...
1. All robot current flows through the #6 wire from battery to PDP.
2. Any resistance in that loop will drop voltage at a predictable value.
3. Worst case voltage drop will affect the voltage available for other systems.
4. The RoboRio is the most important followed by the radio for keeping available voltage as high as possible.
5. Regardless of your design, all motors draw stall current when starting.
6. Brownout voltage doesn't matter to motors if the control system has already died.
7. The voltage dips you see in your logs are real as measured by the RoboRio. This confuses most people but those dips do occur. The log is not lying to you. WildStang did some extensive research into this more than ten years ago. We were able to compare video and voltage data to show that certain behavior leads to voltage drops. We had developed an app for Blackberrry at that time to put it all together and generate warning messages for our drivers at that time.

Yes, motors will draw less current at a lower battery terminal voltage but they will also likely stay at near stall longer as drivers push the throttles to max to get the robot to move as they want. Turns will also push the current to extremes if you have a lot of tires or tank treads on the ground.