Page 223 - Robot Builder's Bonanza

192 ROBOT POWER SYSTEMS

You should always pick the capacity based on the estimated needs of your robot, rather than
G just selecting the biggest brute of a battery that you can find. Bigger batteries weigh more and
they cost more. As you work more with robotics you’ll get a sixth sense for the sizes of the
batteries you need to power your creations.

PACKS WITH NICD OR NIMH BATTERIES
As you read in Chapter 18, “All About Batteries,” the two types of rechargeable batteries that
are both easy to find and affordable are nickel- cadmium (NiCd) and nickel metal hydride
(NiMH). Both are frequently used in R/C applications, and both can be recharged many
times. Of the two, NiCd batteries are the least expensive because they’ve been around the
longest. NiMH batteries provide for high capacities, with ratings of 900 to 3000 mAh, and
over.
Both NiCd and NiMH battery packs require rechargers designed for them. The better bat-
tery rechargers work with a variety of pack voltages. I recommend these so you don’t have to
keep multiple chargers around.

Making Your Own Rechargeable Battery Pack

The shape and layout of your robot may make it difficult for you to use a standard- size battery
pack or one of the battery holders detailed in the next section. You can always make your own
battery pack, using rechargeable cells designed to be soldered together. These are special bat-
tery cells with solder terminals on them. They also come in so- called fractional sizes— a 2/3
AA cell is the same diameter as a standard AA cell, but roughly (about, kinda sorta) 2/3 the
length.
You can arrange the cells in whatever layout best suits the space constraints of your robot.
Remember that when batteries are connected one after the other in a string (series), the volts
of each cell are added together. When the batteries are connected side by side (parallel; see
Figure 19- 2), the current- carrying capacity of the cells are added together.
Most battery packs use cells in series. Start with one cell, and wire its positive terminal to
another cell’s negative terminal, like that in Figure 19- 3. Continue until all the cells are con-
nected.

If you need to attach the cells using wire, match the gauge of the wire with the current demand
G from the battery. Thicker wire can handle more current. A good starting size is 14- or 16- gauge
stranded (not solid) wire. See Appendix D, “Electronic Reference,” for a comparison of wire
gauges. Just remember: the lower the number, the larger the wire.

After you’ve soldered the batteries together, use “battery shrink- wrap,” a PVC plastic tube
to enclose everything into one nice package. Use a hair dryer on high to shrink up the plastic
so it makes a snug fit around the batteries. But be sure the PVC doesn’t get so hot it burns.
Battery shrink- wrap is available at many hobby stores catering to builders of R/C airplanes
and cars, as well as online battery outlets.
To complement the pack, you’ll need a compatible battery charger that is designed not
only for the total voltage of all the cells, but for the type of cells and their amp- hour capacity.
Rechargers are available for either NiCd or NiMH (some are switchable), with common volt-
age ratings as follows:

19-chapter-19.indd 192 4/21/11 11:49 AM