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184 ALL ABOUT BATTERIES

Battery Type* Nominal Voltage
Alkaline 1.5 volts
NiCd, NiMH 1.2 volts
Sealed lead- acid 2 volts

* Individual cells.

To achieve higher voltages, you can link cells together, like lights on a Christmas tree. See
“Increasing Battery Ratings,” later in this chapter, for more details. By linking together six
1.5- volt cells, for example, your battery “pack” will provide 9 volts.

When Decreasing Voltage Becomes a Problem
The varying voltage of a battery as it discharges doesn’t usually present a problem. That is,
unless the voltage falls below a certain critical threshold. That depends on the design of your
robot, but it usually affects the electronic subsystems the most.
Batteries are considered dead when their power level reaches about 70 to 80 percent of
their rated voltage. That is, if the cell is rated at 6 volts, it’s considered “dead” when it puts
out only 4.8 volts. Some equipment may still function below this level, but the efficiency of the
battery is greatly diminished.
Most electronics systems in robots use a voltage regulator of some type, and this regulator
requires some overhead . . . usually a volt or two. As the battery voltage drops below that
needed for the regulator, the electronics go into a “brownout” mode, where they still receive
power but not enough for reliable operation.
Brownouts are a common and damnable problem in robotics. You want to avoid this
scourge at all costs. See Chapter 19, “Robot Power Systems,” for some ideas. If your robot
has an onboard computer, you want to avoid running out of juice midway through some task.
At best, this is a nuisance; at worst, damage to the robot or its surroundings could occur. See
Chapter 19 for ways to add a simple battery voltage monitor to your bot.

CAPACITY
A common analogy for explaining electricity is to compare it to water going through a pipe.
If voltage is pressure, then current is the amount of water that flows through every second.
Current in a battery determines the ability of the circuit it’s connected with to do heavy
work. Higher currents can illuminate brighter lamps, move bigger motors, propel larger robots
across the floor, and at higher speeds.
Because batteries cannot hold an infinite amount of energy, the current of a battery is most
often referred to as an energy store, and is also referred to as capacity, abbreviated C.

Capacity Expressed in Amp- Hours
Battery capacity is rated in amp- hours, or roughly the amount of amperage (a measure of
current) that can be delivered by the battery in a hypothetical one- hour period. In actuality, the
amp- hour rating is an idealized specification: it’s really determined by discharging the battery
over a 5- to 20- hour period, as shown in Figure 18- 5.
What exactly does the term amp- hour mean? Basically, the battery will be able to (again,
theoretically) provide the rated current for 1 hour. This current is expressed in amps— short

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