SD540 Motor Controller

[notmattlythgoe]

Originally Posted by pnitin

If you are letting down your battery 9.5 V your are basically killing your battery( actually 10.8V that is 1.8V per cell).
Lead-Acid battery will quickly built lead sulfide layers on plates ( no matter what is battery technology and what manufacturer claims)
you are basically screwing up battery and now it will have much high internal resistance so the you will start seeing voltage drop when you try to take out good amount of current.

Don't think I've seen a robot get through a match without dipping that low in their battery voltage.

[Andrew Schreiber]

Originally Posted by notmattlythgoe

Don't think I've seen a robot get through a match without dipping that low in their battery voltage.

Pretty sure I've seen that on a non aggressively geared kitbot that only weighed 80 pounds... Unless the 9.5V limit only kicks in after a period of time in which case I'd like to see that in the documentation (and still complain because my use case for low cost controllers involves lower voltages anyway)

[notmattlythgoe]

Originally Posted by Andrew Schreiber

Pretty sure I've seen that on a non aggressively geared kitbot that only weighed 80 pounds... Unless the 9.5V limit only kicks in after a period of time in which case I'd like to see that in the documentation (and still complain because my use case for low cost controllers involves lower voltages anyway)

I guess a robot that doesn't move would probably keep a pretty high voltage too.

[geezloueez]

Originally Posted by notmattlythgoe

Don't think I've seen a robot get through a match without dipping that low in their battery voltage.

I am a first year mentor for a rookie team. May seem like a dumb question, but I have not been involved first hand in a match yet. What is causing the batteries to drop so low? Are you using old batteries?

[notmattlythgoe]

Originally Posted by geezloueez

I am a first year mentor for a rookie team. May seem like a dumb question, but I have not been involved first hand in a match yet. What is causing the batteries to drop so low? Are you using old batteries?

Running any number of motors at the same time. Compressors running a lot. Pushing matches with other robots. FRC robots can use a lot of power and usually do.

[geezloueez]

Originally Posted by notmattlythgoe

Running any number of motors at the same time. Compressors running a lot. Pushing matches with other robots. FRC robots can use a lot of power and usually do.

What is the lowest you have witnessed in a match?

[marshall]

Originally Posted by geezloueez

What is the lowest you have witnessed in a match?

I've seen my share of robots in brown out conditions. It's less severe these days than it used to be but it can still cause you some heartache at competitions. This is from one of the presentations on the control system last year:



And this is current (no pun intended) info about what happens and when:

http://wpilib.screenstepslive.com/s/...g-current-draw

EDIT: You can help prevent this by keeping happy batteries around and always keeping good ones in the robot.

[notmattlythgoe]

Originally Posted by geezloueez

What is the lowest you have witnessed in a match?

A temporary drop to 8V is a pretty common occurrence. I don't know what the absolute lowest is though. Current draw is going to be a very important thing to watch in the coming years.

The compressor turning on can drop the voltage a good 1-1.5V itself.

Edit: See Marshall's post.

[Andrew Schreiber]

Originally Posted by geezloueez

What is the lowest you have witnessed in a match?

<6V followed very closely by the robot stopping moving as the CRIO reset. Though occasionally we did see it spike that low and come back up.

[Jared Russell]

Originally Posted by geezloueez

I am a first year mentor for a rookie team. May seem like a dumb question, but I have not been involved first hand in a match yet. What is causing the batteries to drop so low? Are you using old batteries?

Internal resistance is usually on the order of ~0.01 ohms for the sealed lead-acid batteries that we use.

If your battery is charged to, say, 12.7 volts and you are running 4 CIM motors at stall (= 4 * 131A = 524A current), you can expect a voltage drop of ~5.2V just due to battery resistance (in reality, there are other losses in wiring, connectors, and speed controllers as well, so treat this as an approximate). 12.7 - 5.2 = 7.5V.

This situation happens (instantaneously) any time you rapidly change direction assuming your wheels don't slip on the ground first. Once the drive is moving, your motors draw less and less current, and battery voltage quickly recovers.

You can imagine that a 6 CIM drive, or simultaneously driving while powering mechanisms or the compressor, will only make things worse. I have seen robots drop below 6V relatively frequently in other seasons. Design with care!

[Mark McLeod]

Originally Posted by geezloueez

I am a first year mentor for a rookie team. May seem like a dumb question, but I have not been involved first hand in a match yet. What is causing the batteries to drop so low? Are you using old batteries?

Here are a couple of real life examples to give you an idea of what you have to be prepared to manage in your power design.
Both Driver Station logs are from different teams that borrowed one of my laptops during competition.
Learn how to examine your own DS logs after a match. They are automatic and just brimming with useful data about how your robot performed.
Remember, too, that the power drawn during practice at home is tame compared to power drawn during a real match with competitors.

The yellow line shows the battery voltage for the duration of the match.
No roboRIO brownouts were experienced by either team during these logged events.

Both of these robots had good batteries.
These voltages are what the roboRIO and speed controllers directly experienced.
The first example is from a robot during an off-season event this past October with a large number of motors - drive, lift, tote grabbers all running.

The second example is also from an off-season event, but one held in November. It was a robot with four drive motors and one lift motor, and shows a lot less stress.

The biggest dips are when the motors are starting up from a complete stop, lifting a heavy load, or suddenly reversing.
These logs are taken from a game without active opposition. Expect much worse this coming season.