Inside the Dry Cell
cell chemistry has come a long way since this article appeared in
the April 1959 edition of Popular Electronics. Yes, you can still
buy a basic carbon battery, but much superior cells are available
now that perform over much wider temperature ranges, have nearly
flat discharge curves throughout their rated voltage range, and
offer standard chemistries with voltages other than 1.5 V per cell
April 1959 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Inside the Dry Cell
By Saunder Harris WINXL
dry cell is a "package of electricity" which produces electrical
energy by chemical means. A quick look inside a portable radio,
a flashlight or a hearing aid will reveal one or more of these compact
power sources ready to deliver the juice at the flip of a switch.
How does the dry cell produce electricity? Without going into chemical
reaction formulas, let's take a look at what goes on inside a dry
What Goes On. In Fig. 1, a typical
dry cell is shown in cross section. Its zinc outer case serves as
the negative electrode of the cell. The positive electrode is formed
by a cylindrical carbon rod in the center of the cell. Separating
the two electrodes is a pasty substance composed of an electrolyte
and a depolarizing mix.
Due to chemical action between the
electrolyte (ammonium chloride) and the zinc case, electrons pile
up on the sides of the zinc container and bubbles of hydrogen gas
travel through the electrolyte and cling to the sides of the carbon
rod. As the bubbles pile up, they tend to choke off the action of
Here is where the depolarizing mix goes to work.
Since it is composed of manganese dioxide, which has a high content
of oxygen, it mixes its oxygen with the hydrogen bubbles and water
is formed. This gets rid of the unwanted hydrogen and also keeps
the electrolyte from drying up.
When someone connects the battery into a circuit, the electrons
leave the zinc and travel into the circuit in the form of an electrical
current. Then they return through the carbon rod to complete the
circuit. But what's happening to the zinc while all this is going
As the battery action takes place, the zinc is gradually eaten away.
While the battery is in use, hydrogen bubbles are formed even faster
than they can be removed by the depolarizing mix. This is why your
battery must be given a rest every so often. The depolarizer must
be allowed to catch up on its work.
Fig. 1. Cross-section view of a typical dry cell.
Fig. 2. Internal condition of dry cell at exhaustion.
Fig. 3. Gradual battery volt-age drop with time and use.
Now let's see what happens
when the battery is exhausted. The zinc walls of the cell get thinner
and thinner, and the electrolyte - instead of being in paste form
- dries out into powder. The depolarizing mix stops doing its job
and the hydrogen bubbles around the positive carbon element just
about stop all flow of electrons. Figure 2 will give you some idea
of the condition of the cell at this point. This dry cell has had
Life of a Battery. Knowing what goes
on inside the cell, it is now time to answer the question most important
to you as a battery user. How long will your batteries last?
According to the National Bureau of Standards, in 1910, under
standard testing procedures, a size "D" cell would give 260 minutes
of service under intermittent use. In 1951, the testing of 12 to
15 brands of the same size "D" cell showed an average service life
of over 800 minutes, with some cells giving 1000 to 1100 minutes
of service. Today's batteries will do even better than that with
proper care. The five factors that determine the life of a dry cell
(1) Initial current drain
(2) Hours of
use per day
(3) End point voltage
Storage period prior to use
It is impossible to say that
any battery has an exact number of hours of service life. If a battery
is operated under conditions which draw a large current from it
in a short period of time, the depolarizing mix cannot do its job
properly and the voltage will drop off very rapidly. This is the
situation we have already discussed. On the other hand, if the battery
is used too slowly, its normal aging will cause the output to be
reduced. The shelf life of a battery can range from a few months
to as long as two years, depending upon the type of battery and
the conditions of storage.
While in use, your battery should
periodically be given "time off" to allow the depolarizing mix to
work and remove the hydrogen and other waste products developed
in the cell. This point is emphasized because it is so important
in proper battery care. It will pay you to have two sets of batteries
for frequently operated devices. By switching from set to set, you
will increase the operating life of both sets.
The end point
voltage is of interest mainly to the designer of the battery-operated
device rather than the user, and so we will only touch on it here.
The end point voltage is the voltage below which the battery can
no longer operate the device in question. If it takes one volt per
cell to operate a radio receiver, the unit will not operate when
the battery voltage goes to 0.9 volt per cell. The best designs
make the end point voltage as low as possible to allow the maximum
to be gotten from a battery as its voltage drops off with time and
use. Figure 3 will give you some idea of the manner in which battery
voltage drops off with time.
Temperature Story. Dry cell batteries designed for normal
use operate best at room temperature, about 70° F. When batteries
are exposed to continual high temperatures, they will break down
in a much shorter time due to increased chemical action and a drying
up of the electrolyte. Low temperatures, however, are a different
From the standpoint of storage, a battery loves cold
weather. For example, as you may know, batteries which were frozen
in the Arctic ice on polar expeditions were thawed out years later
by other explorers and found quite usable. Cold will slow up the
chemical action within a dry cell battery and in some cases make
it inoperative; but when it is brought back to room temperature,
the battery will return to normal operation none the worse for the
If you are going to store a dry cell battery for
any length of time, store it in a cool spot. A temperature of about
45° F is ideal, and a shelf in the refrigerator is an excellent
storage spot (if you can get away with it). According to tests made
by the National Carbon Company's Battery Engineering Department,
a battery which has been stored in this manner for nine months will
give as much useful service as will a battery stored at room temperature
for three months. Just remember to take the battery out of the cold
about six hours prior to using it, and allow it to get back to room
temperature for normal operation.
"Cells." Before we complete this discussion of basic dry
cells, there are a few things which deserve clarification. You will
note that we have used the term "cell" and "battery." A cell is
one unit consisting of positive and negative electrodes separated
by an electrolyte. A battery is two or more cells, connected in
series to add their voltage, and packaged in one container.
Actually, the "battery" you put in your flashlight is a cell,
while the B+ unit you use in your portable radio is a true battery
because it consists of more than one cell.
You should also
be aware that every cell, no matter what its electrode materials,
develops about 1 1/2 volts. The physical size of the cell determines
the amount of current that can be drawn from it.
of course, much about dry cell batteries that we haven't discussed.
There are also many new types of batteries such as the mercury battery,
the solar battery and the new rechargeable dry cells, all of which
are now important means of develop-ing portable power. These we
will cover in a future issue. However, the dry cells that we have
considered are still the workhorses of the battery world and, with
a little care, will pay you many dividend hours of extra, dependable