Page 118 - Build Your Own Combat Robot

It’s All About Power
Chapter 5:
The alkaline battery works best when powering low-current devices. When used 99
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 alka-
line 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:
In the long run, they are the most expensive battery type.

They have the poorest 6-minute energy density of all the batteries.

Lithium Ion
Lithium ion batteries are common rechargeable batteries used in computing appli-
cations. 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 addi-
tion, 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 neg-
ative 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.