Build Your Own Combat Robot

The alkaline battery works best when powering low-current devices. When used
to power high-current devices, the performance is dismal. Even so, many robot
kits use AA alkaline batteries to drive servos and the onboard electronics. When
stalled, these servos can try to draw 1 ampere, bringing short order to the AA alkaline
batteries. Usually, these robots will see a performance increase if the alkaline
batteries are changed over to standard NiMH or NiCad cells.
Following are some advantages of alkaline:
■ They are readily available.
■ They have the least expensive startup cost.
■ It is easy to replace the battery with a known fresh battery.
■ They are low maintenance—you can throw away the old ones.
Here are the disadvantages of alkaline:
Lithium Ion
■ In the long run, they are the most expensive battery type.
Chapter 5: It’s All About Power 99
■ They have the poorest 6-minute energy density of all the batteries.
Lithium ion batteries are common rechargeable batteries used in computing applications.
They have high-energy density when current is pulled out at a moderate
rate. However, the voltage drops when pulling current out at a high rate. In addition,
the battery can fail when pulling out current at a higher than moderate rate.
Therefore, lithium ion batteries do not work well for combat robots. Another negative
factor is that the typical shelf life of the lithium ion rechargeable battery is
only two years if stored at 25° C.
Combining Drill Motors, Batteries,
and Battery Chargers
Many cordless power drills come with rechargeable batteries and fast chargers.
Many competition winners have used these drill/battery/charger combinations to have
a complete solution to the problems of supplying motors for the robot, getting good
batteries, and getting fast chargers. In addition, spare batteries for these motors are
readily available. Four-wheel-drive robots using four power drill motors have had
good success in the combat arena. If you go this route, use good-quality cordless
power drills with NiCad or NiMH battery types.