The batteries’ primary purpose is to keep your robot powered during the competi-tion. These competitions can last up to 5 minutes, so the battery must supply all the power to the robot during that time. Selecting an appropriately sized battery that will confidently run your robot throughout the entire match can be a significant competitive advantage. The lightest battery will allow the robot to use the weight savings for other things, such as weapons and armor. A properly selected battery will have enough capacity to supply full running current continuously to your ro-bot’s motors; and it will be able to supply the peak currents that will allow your robot’s motors to deliver the maximum torque, when needed.
Measuring Current Draw from the Battery
You can find out from the motor specification sheets exactly what current draw to expect when running the motor. Adding up all the currents from the various mo-tors on your robot will tell you the maximum and typical motor running currents to expect.
Because many of us use motors that come without data sheets, we have to mea-sure the running currents ourselves. To do this, you need to have a good battery
from which to draw the current. You might ask yourself, “Do I really need to buy a battery to test what size battery I need?” Yes, you do, if you want to be able to mea-sure the current draw. The battery voltage of this test battery must not droop while testing for the current draw. In other words, the voltage must remain constant throughout the tests. The advantage of using a large lead acid battery for the current draw tests is that, because it will provide a long run time, you can use this battery during the initial testing phases of the robot. After you have selected the appropriate batteries for your robot, you can use them for all of the final test phases.
In most cases, fighting robots will draw a lot of current—much more than the maximum current rating of most multimeters. The best tool to use to measure the current draw is a high-current ammeter capable of measuring more that 100 amps.
Using Ohm’s Law to Measure Current Draw
You can also measure the resistance of the motor and calculate the current draw from this measurement using Ohm’s Law. The formula to do this iscurrent = volt-age / resistance.This formula doesn’t necessarily provide a reliable measure, however, because, first, the resistances are very low for competition motors and most ohm meters are not accurate at such low resistance levels. Second, if this measurement is made accurately, it must be made considering the resistances of the complete wiring harness, motor drivers, and motor. Last, even if the measurement is done accurately, the calculated current will be much higher than actual due to frictional and heat losses.
In all fairness, if measured accurately, the peak motor currents can be deter-mined using an ohm meter and this formula:
Here, the current,I, is in amps; the voltage,V, is in volts; and the resistance,R, is in ohms. To use this method, place a high-power, small-resistance-value resistor in series with your robot’s battery supply. Then, using a voltmeter, measure the voltage across this resistor.
Suitable Resistor and Measurement Basics
If you have access to a low-value, high-wattage resistor, you should use it to per-form your measurements—but resistance, high-wattage resistors are hard to find.
The resistance should be less than 0.01 ohms. If your motor’s expected peak cur-rent draw is 100 amps, you will need at least a 100-watt resistor. If you don’t have access to such a resistor, a 0.01-ohm resistor can be made with 6.2 feet of readily available #12 copper wire. The wire needs to be slightly longer than 6.2 feet, but you can connect the voltmeter at the place on the wire that is 6.2 feet from the bat-tery. In addition, it is a good idea to keep the insulation on the wire and to coil up
5.1
the wire so that it is easy to handle. Temperature causes the resistance to change, so use the wire at room temperature and don’t use it so long that it heats up.
1. Place the resistor in series with your robot’s battery.
2. Measure the voltage across the resistor (a 6.2-foot-long coil of #12 wire, or the high-wattage resistor) with the robot running in normal battle-like conditions. When measuring this voltage, the value will likely be variable and may appear unstable. Take the maximum reading, and then take a reading that appears to be the nominal or average value. The robot’s motors must be loaded to simulate those of a real battle, or else you will measure a value that is much too low—up to five to ten times too low than battle-use values.
3. When you have gathered these voltage values, calculate the current by placing the voltage readings into the formulacurrent = voltage/ 0.01 ohms.
The 0.01 ohms is the resistance of the 6.2-foot-long wire. If you are using a high-wattage resistor, then substitute the 0.01 ohms for the resistance of your resistor. For example, suppose that when running the experiment, you noted a maximum voltage of 1.2 volts and an average of 0.5 volts.
Plugging these values into the formula yields a maximum current value of 120 amps (120 amps = 1.2 volts / 0.01 ohms) and a typical current of 50 amps (50 amps = 0.5 volts / 0.01 ohms). After you have found the maximum current value and the typical current value, you have the information that you need to choose the correct battery for your robot.
Blowing Fuses on Purpose?
An alternative method for measuring current draw is one of the easiest methods and is fairly accurate. You can use the fuse holder that is in-line with your robot’s battery to measure draw. Fuses are commonly used for testing, but few people use fuses during an actual competition. It is usually better to risk an electrical fire than to blow a fuse and be a sitting duck for your opponent to destroy your bot with impunity. A blown fuse in battle also means an automatic loss!
To use this method, you’ll need a handful of fuses of various amperages. Start with a fast blow fuse, and select values that you think it will survive. Install this fuse and test run your robot in battle-like conditions.
n o t e It is important that you test your robot in battle-like conditions, or else the measurement will yield a current draw that is lower than what will occur in the robot arena.
Keep changing the fuse values until you find the fuse value that will survive and the highest fuse value that fails. Between these two values is your robot’s maximum
current draw. Using this method, you can find the maximum running current of your robot to within 5 amps. Next, switch to slow-blow fuses. You want to find the fuse that lasts for about 1 minute while running your robot in battle-like con-ditions. This fuse value will yield your typical running current.
After you have found the maximum current value and the typical current value, you have the information that you need to size your battery.