Page 212 - Robot Builder's Bonanza

BATTERIES FOR YOUR ROBOTS 181

RECHARGEABLE ALKALINE
Rechargeable alkaline batteries are the mass- merchandizing answer to the high cost of regular
alkaline batteries used in high- demand applications— robotics is certainly one such applica-
tion, though battery makers had things like portable CD players in mind when they designed
these puppies. Rechargeable alkalines require a recharger designed for them and can be
revived dozens or hundreds of times before discarding.
Rechargeable alkalines are probably the best choice as direct replacements for regular
alkaline cells. The reason: Most rechargeable batteries put out about 1.2 volts per cell; both
rechargeable and nonrechargeable alkalines are rated at 1.5 volts per cell. See “Understand-
ing Battery Ratings,” later in this chapter, for more details about cell voltage.

NICKEL- CADMIUM
Nickel- cadmium rechargeable batteries are an old technology and, unfortunately, one that’s
unfriendly to the environment— cadmium is extremely toxic. Lately, battery companies have
been favoring the “greener” nickel- metal hydride formulation that follows. Nickel- cadmium
batteries are still plentiful, and you’ll likely use them for at least some of your projects.
Nickel- cadmium (NiCd for short) cells are available in all standard sizes, plus special- purpose
“sub” sizes for use in sealed battery packs for consumer products— things like rechargeable
handheld vacuum cleaners, cordless phones, and so forth. Most battery manufacturers claim
their NiCd cells last for a thousand or more recharges.
A new, higher- capacity NiCd battery is available that offers two to three times the service
life of regular nickel- cadmium cells. More important, these high- capacity cells provide consid-
erably more power and are ideally suited for robotics work. Of course, they cost more.
Earlier NiCd batteries suffered from “memory effect,” whereby the useful capacity of the
battery was reduced the more times it was recharged. The newer NiCd batteries— those made
within the past 10 years or so— are said not to exhibit this memory effect, or at least not as
much as the older variety.

NICKEL METAL HYDRIDE
Nickel metal hydride rechargeable batteries not only offer better performance than NiCds,
they don’t make fish, animals, and people (as) sick when they are discarded in landfills. They
are the premier choice in rechargeable batteries today, including robotics, but they’re not
cheap. They require a recharger made for them (Figure 18- 2). Many of the latest rechargers
will work with rechargeable alkalines, NiCds, and NiMHs; just don’t use a NiCd- only recharger
with NiMH.
NiMH batteries can be recharged 400 to 600 times and have what’s known as a low inter-
nal resistance. That means they can deliver high amounts of juice in a short period of time.
Unlike NiCds, NiMH batteries of any type don’t exhibit a memory effect. NiMH cells can’t be
recharged as many times as NiCd batteries: about 400 full charge cycles for NiMH, as opposed
to 2000 cycles for NiCd.

LITHIUM- ION

Lithium- ion cells are frequently used in the rechargeable battery packs for laptop computers
and high- end camcorders. They are the Mercedes- Benz of batteries and are surprisingly light-

18-chapter-18.indd 181 4/21/11 11:49 AM