Build Your Own Combat Robot

FIGURE 5-4
Internal voltage
drop versus current
draw for 12-volt
batteries/packs.
Chapter 5: It’s All About Power 89
draw 200 amps. The 8 volts lost inside the batteries is turned into heat, and the batteries
get very hot—in this case, 1,600 watts of heat. On the other hand, the motors
will receive only 4 volts and will run much cooler. Of course, the motors will run
much slower and also deliver a great deal less torque. Clever battery/motor designs
might use this fact to raise the normal running voltage to the motors, knowing that
under high load, the voltage will drop and prevent the motors from burning up. If
you do this, remember that many motors and batteries have burned out using this
method during the competitions. On the other side of the spectrum, the SLA battery
will hold the output voltage at 9 volts while delivering 200 amps. This increases the
speed and torque to the motors compared to the other battery types.
When comparing batteries with similar 6-minute capacities, the series resistance
of a particular battery characteristic must be looked at carefully. This data
will be included in the battery data sheets from the manufacturer. For example,
only the high-quality NiCads and NiMHs will have a series resistance as low as
what is shown in Figure 5-4. Another point to consider is that, in general, the
larger the Ahr capacity of the battery, the lower the internal series resistance.
Wrapping Up the Comparison
It is sometimes easier to see the battery comparison in table form. Note the additional
information—the weight of each battery, the rated maximum current, and
the 6-minute power density. Tables 5-1 and 5-2 show the battery performance
characteristics used to create Figures 5-2, 5-3, and 5-4.