Page 214 - Robot Builder's Bonanza

UNDERSTANDING BATTERY RATINGS 183

• Because of the power demands, larger robots, such as those for machine combat, use
sealed lead- acid batteries whenever possible. The biggest and most brutish robots can use
liquid electrolyte batteries originally intended for use in motorcycles, small boats, or cars.
These batteries are the heaviest of the bunch, but they pack a lot of wallop.

Which is better for the environment: nickel- cadmium or nickel- metal hydride? Actually, both
G contain poisonous materials, and both pose a threat. Never throw away your batteries, no
matter what they’re made of. Recycle instead. If you have a choice, NiMH batteries are
probably the best bet, but remember to recycle those, too.

Understanding Battery Ratings

Batteries carry all sorts of ratings and specifications. The two most critical are voltage and
capacity.

VOLTAGE
The importance of voltage (or V for short) is obvious: the battery must deliver enough volts to
operate whatever circuit it’s connected to. A 12- volt system is best powered by a 12- volt bat-
tery. Lower voltages won’t adequately power the circuit, and higher voltages may require
voltage reduction or regulation, both of which entail some loss of efficiency.

Nominal (“Normal”) Voltage Level
Battery voltage is not absolute. The voltage of a battery may— and usually does— diminish as
it is used.
Take a battery that’s rated at 1.5 volts. It puts out 1.5 volts, give or take. That “give or
take” is important; the rated voltage of a battery may vary as much as 10 to 30 percent. When
fully charged, the typical 1.5- volt cell may deliver 1.65 volts. When fully discharged, the volt-
age may drop to 1.2 volts.
Batteries are rated at a nominal voltage (see Figure 18- 4). Nominal simply means “nor-
mal.” Only for a certain period during the battery’s discharge does it actually deliver this spe-
cific voltage.

2.0
1.5
Volts
per cell 1.2
Lead
NiCd NiMH Alkaline
acid
Figure 18- 4 Simplified but
representative discharge curves for
1.1
several popular types of batteries
used in robotics: NiCd, NiMH,
0 1 2 3 4 5
Discharge (hours) alkaline, and lead- acid.

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