Page 102 - Build Your Own Combat Robot

Chapter 5:
Potting the Battery…NOT! It’s All About Power 83
Potting is encasing the battery in epoxy or some other compound. At first, this
might seem like a good idea because it will protect the battery. Don’t do it! All
batteries have internal gas vents. If you were to pot the batteries and then overcharge
one or more of them, the buildup of internal pressure inside the battery would cause
the battery to explode! If you want to encase the battery, put it in a well-vented but
protected place in your robot.

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.

B attery Capacity Basics

Batteries come in several varieties:

Sealed Lead Acid (SLA)
Nickel Cadmium (NiCad)
Nickel Metal Hydride (NiMH)
Alkaline

Lithium Ion

Each of these will be discussed later in the chapter in the section “Battery Types.”
The amp-hour (Ahr) rating of a battery specifies its capacity to hold energy. In
simple terms, it can be viewed as the number of amps that the battery will supply
during a 1-hour period. Even so, all batteries’ amp-hour ratings are specified at the
place where that particular battery technology will be the most efficient, any-
where from dozens of hours for alkaline batteries to 1 hour for NiCads and
NiMH. In addition, some battery types are specified at various run-time capaci-
ties. Because competition matches only last for 2 to 5 minutes (at BattleBots, the
preliminary elimination rounds are 2 minutes, finals are 3 minutes, and rumbles
are 5 minutes), the results for how the various battery types compare may surprise
you. One surprise is alkaline batteries. Although they are considered to have the
highest energy density of almost any common battery type, they end up dead last
when evaluated for high-current, short-run applications.
When purchasing batteries, always check their Ahr ratings because many
name-brand battery manufacturers are selling subcapacity cells. For example, a