Pololu 3pi Robot - Manual (Page 10)

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:
Battery voltage vs. current.
The power put out by a battery is measured by multiplying the volts by the amps, giving a measurement in watts
(W). For example, at the point marked in the graph, we have a voltage of 0.9 V and a current of 0.6 A, this means
that the power output is 0.54 W. If you want more power, you need to add more batteries, and there are two ways to
do it: parallel and series configurations. When batteries are connected in parallel, with all of their positive terminals
tied together and all of their negative terminals tied together, the voltage stays the same, but the maximum current
Pololu 3pi Robot User's Guide © 2001–2014 Pololu Corporation
5. How Your 3pi Works Page 10 of 63