SD540 Motor Controller

[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!