How do the battery testers on battery packages work? The little disposable battery testers that you see on batteries or battery packages are a grea

How do the battery testers on battery packages work?

The little disposable battery testers that you see on
batteries or battery packages are a great example of combined
technologies -- several existing technologies have been
combined in a completely new way! Battery testers depend on
two special types of ink: thermochromic and conductive inks.
Thermochromic ink changes color depending on its temperature.
Conductive ink can conduct electricity. By applying layers of
these special inks along with a layer of normal ink using
a fairly normal printing press, it is possible to create an
extremely inexpensive printed design that changes depending
on the amount of electricity it receives.

There are two types of thermochromic ink: liquid crystal and
leucodye. Liquid crystal based thermochromic ink is sensitive
to very small changes in temperature, but it is fairly
difficult to manufacture. This makes it perfect for use in
items like thermometers where you need the sensitivity, but
troublesome in an item that needs to be inexpensive and in
which a large, abrupt change in temperature will occur.
Leucodyes are specially formulated substances that change
from a specific color, like blue, to a clear state when
subjected to a temperature change of about 5 degrees F or
more. Thermochromic inks can be formulated to change color at
specific temperatures. For battery testers, the desired
temperature is usually around 100-120 degrees F.

To create a battery tester, you start with a layer of
conductive ink that gets progressively narrower as you move
across the tester from "good" to "bad." In the picture above
the tester has 3 bars. In other testers the ink is
wedge-shaped. The narrowest point indicates the weakest
charge; the widest area indicates a full charge. When current
passes through the thin layer of conductive ink, resistance
in the ink creates heat. A small amount of current can
generate enough heat to affect the smallest area of
thermochromic ink; but, as the area widens, more current is
needed to change colors.

On top of the conductive ink is a layer of normal ink that
conveys the design. In most battery testers, this is some
type of "fuel gauge" graphic or text that indicates that
a battery is good. The design can be anything, since the
normal ink layer does not affect the way the conductive and
thermochromic layers interact.

Finally, there is the thermochromic layer. In the photo of
the battery tester above, the thermochromic layer is black
when cool. By touching a battery to the conductive ink on the
back of the paper, a connection between the positive and
negative terminals is created. As a current is generated, the
thermochromic ink will turn clear. This reveals the design
that is printed in normal ink. If there is enough current,
most or all of the thermochromic ink will heat to the
temperature needed to become translucent.

One question you might have right now is, "Doesn't the
battery tester drain some of the battery's energy?" The
answer is, "yes, but not enough to matter." If you tested the
battery every 5 minutes it might be a problem, but most
people don't do that.

One type of battery tester available now has the tester right
on the battery. You press two small dots indicated on the
battery to test it. These points complete a circuit between
the battery and the tester, and electricity flows through the
conductive ink in the same way as in the tester discussed
above.