Page 129 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 5 Mechanical and Electrical Power, Work, and Energy 109

Nickel-Cadmium
Nickel-cadmium (NiCad) batteries are an older rechargeable battery technology. Some
sources say they show a memory effect, which leads to diminished capacity if you fail
to drain a battery completely before charging it to full. While the memory effect is
debatable, these batteries more likely suffer from voltage drop. This means that if a
battery is repeatedly used only partway before recharging, it will start delivering lower
and lower voltages. This is true, but it’s also true of all rechargeable batteries.

Once charged, NiCads maintain their voltages reliably until they are almost completely
empty (unlike alkalines). They won’t self-discharge as quickly as NiMH batteries do
during periods of disuse. Like NiMH batteries, NiCad batteries are only 1.2V.

A downside is that cadmium is highly toxic and does not belong in a landfill. Disposing
of NiCad batteries in the trash is illegal in many countries and states, so special recycling
is necessary. For these reasons, properly charged NiMHs are usually a better choice.

Lead-Acid
Lead-acid batteries are found in your car and also come in smaller sizes like 6V, 12V,
or 24V versions that power motorcycles, computers, and boats. They are useful to us
because of their size. However, they are notoriously heavy. The trade-off for their
weight is that these batteries are cheap and last a long time. Most lead-acid batteries
can be recharged with a special high-current battery charger. Look for one that is
sealed (denoted SLA for sealed lead acid or VRLA for valve-regulated lead acid) to
avoid accidents with leaking battery acid.

Gel cells are just lead-acid batteries with a jello-like filling instead of a liquid acid.
These are generally safer and cleaner. Look for the SLA batteries used in SADbot in
Project 10-3 for reference.

Lithium, Lithium-Ion, and Polymer Lithium-Ion
Lithium-type batteries are the rechargeable batteries most commonly used in laptop
computers and portable electronics. They’re relatively expensive, but pack a lot of
power for their size, and they will retain a charge for many months. The little coin cell
batteries in watches and calculators are also sometimes lithium cells, but they aren’t
rechargeable.