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Two rechargeable AAA Ni-MH
batteries.
5. How Your 3pi Works
5.a. Batteries
Introduction to Batteries
The power system on the 3pi begins with the batteries, so it is
important to understand how your batteries work. A battery
contains a carefully controlled chemical reaction that pulls
electrons in from the positive (+) terminal and pushes them out of
the negative (-) terminal. The most common type is the
alkaline
battery , which is based on a reaction between zinc and
manganese through a potassium hydroxide solution. Once
alkaline batteries are completely discharged, they cannot be
reused. For the 3pi, we recommend rechargeable nickel-metal-
hydride (NiMH) batteries, which can be recharged over and over.
NiMH batteries are based on a different chemical reaction from
alkaline batteries, but you don’t need to know anything about the
chemical details to use a battery: everything you need to know about it is measured with a few simple
numbers. The first is the strength with which the electrons are pushed, which we measure in volts (V),
the units of electric potential. An NiMH battery has a voltage of about 1.2 V. To understand how much
power you can get out of a battery, you also need to know how many electrons the battery can push
per second – this is the electric current, measured in amps (A). A current of 1 A corresponds to about
6×10
18
electrons flowing out one side and in to the other each second, which is such a huge number
that it’s easier to talk about it just in terms of amps. 1 A is also a typical current that a medium-sized
motor might use, and it’s a current that will put a significant strain on small (AAA) batteries.
For any battery, if you attempt to draw more and more current, the voltage produced by the battery will
drop, eventually dropping all the way to zero at the short circuit current: the current that flows if you
connect one side directly to the other with a thick wire. (Don’t try this! The wire might overheat and
melt, and the battery could explode.) The following graph shows a good model of how the voltage on
a typical battery drops as the current goes up:
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