OK, spoiler alert, Al is going to talk about electrical issues again....
In most cases brownouts are preventable with a good electrical system design. To begin, let's talk about real world issues that every team experiences.
1. Wires have resistance! Resistance drops voltage when current flows. V=I*R.
2. The battery has internal resistance. Typical is 0.011 ohms. At 400 amps that resistance drops
4.4 volts. Yes that is real and it occurs with a freshly charged battery. Stall or start four CIM motors and that is what you will actually measure
at the battery terminals.
3. Batteries do age, typical for our batteries is about 400 charge discharge cycles maximum. The Battery Beak will give you some indications but a true test is one that duplicates the curves published by the manufacturer. The West Mountain Radio CBA-IV is capable of performing this test and keeping the data from year to year so that you can overlay the tests and see graphically the deterioration of the battery capacity.
4. Terminals, connectors, breakers, and hardware all add resistance to the robot electrical system.
5. Smart chargers can be confused by certain battery conditions and operation. Read the charger manual and follow the instructions.
Many of us (Inspectors and CSAs) saw these issues throughout the season. They are preventable and we showed teams how to overcome these issues.
1. Make the terminations on the battery
tight and non-moving. Add a #10 external tooth star washer between the battery terminal and the wire terminal to cut through all the surface crud and to prevent the terminals from moving. Use the supplied locking hardware to keep the screw and nut tight as well. These two steps will keep the termination as low resistance as possible over the possible three or fours years of the battery life. If the terminals move, any locking hardware will loosen. Loose connections are high resistance.
2. Terminate the #6 wiring so it won't pull out and it won't loosen. If you use the screw type terminals, strip the wire sufficiently to push bare copper all the way through the connector. This one step will prevent wire pullout. I don't like the type where the screw actually compresses the wire (as shown below). I prefer a clamping type where the wire is held between two moving parts. With the type shown, wire strands will be pushed to the side, reducing the wire gauge and increasing resistance.
If you use this type, attach it to the battery terminal so that the screw faces the center of the battery. This will give you less of a chance of having the terminal catch on mechanical parts in your robot when you install the battery. Follow up with good insulation.
Duct tape is not insulation. Use electrical tape or heat shrink tubing.
3. I prefer the Burndy crimp type terminals. You can obtain these at AndyMark (am-0805) or many other locations. You can use a vise to crimp (see my other posts) these but the tool sold also by AndyMark (am-3326) is the perfect device. We follow up with solder to improve reliability and reduce resistance.
4. Tighten all hardware,
Al tight, not freshmen tight. If you use any terminal that is 1/2" wide, be sure that your tighten the hardware on the PDP sufficiently that the terminal is in firm contact with the PDP terminals. It is easy to think you have done a nice job, but the plastic cover may be preventing you from having maximum contact with the PDP. This is a source of increased resistance.
5. Center the battery, the main breaker and the PDP in the robot to keep all wiring as short as possible. You may have to rotate the PDP to get this right. Every foot of #6 wire will drop 0.2 volts at 400 amps.
All of your robot current flows through this wire. Make sure it is as short as possible.
6. Make sure the fuses at the very end of the PDP are fully inserted. Take the time to inspect this when the PDP is not in the robot. The shape of the fuse has a handle molded into the fuse to let you grab it. That handle should be almost touching the surface of the PDP when correctly inserted. The fuse feeds power to the RoboRio. If it is loose, the Rio gets upset or completely reboots.
7. The push type terminals require that wire insulation be stripped to 3/8" (0.375" or just under 1cm) . If you do not meet that minimum specification, the wire will not be retained in the terminal and the resistance will be high. Be sure to check that there are no whiskers touching the other wire.
8. Inspect your main breaker often for correct operation. Lightly tapping the red button while your robot is on, should not cause any blinking lights. If you see that, replace the main breaker. If there is any damage to the outside of the breaker (cracked or chipped case, broken handle or cover) assume the breaker has been damaged and replace. When you push the flag in, there should be one, smooth, movement. If you feel a slight resistance (as if the handle is moving over a speed bump) and then it pushes the rest of the way, the breaker is bad. This is an indication that the handle is damaged and will fail at some point.
9. It is possible that your drive system is drawing excessive current. This could be do to the type of drive you are using, the friction of the tires with the carpet or how aggressively your driver turns. Tank treads are the highest current demand in turns. Eight (or more) wheel drives with no drop center wheels are next. Knobby tires are also notorious for high current in turns. If you are experiencing problems in turns, it could be any one of these designs. Practice with your driver to not turn in place but do arcing turns to reduce the current draw. High friction drives in turns will put the drive motors into near stall conditions. A CIM motor stalls at 131 amps but typical robots will see current in excess of 100 amps per motor.
10. Finally, too many teams do not follow the instructions that came with their battery chargers. Let your battery rest for at least 30 minutes prior to charging. This will bring the internal temperature down to near ambient and allow your charger to optimize charge current. Charging a 'hot' battery may not get you full charge.