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Figure 2 shows a cross section of a single cell of an alkaline battery (the nine
volt battery uses six series connected rectangular cells). The chemical reactions
that produce energy in the alkaline battery (e° is the electrochemical potential;
[s] is a solid material; [aq] is an aqueous solution; [l] is a liquid material) are:
Zn(s) + 2OH−(aq) → ZnO(s) + H2O(l) + 2e− [e° = - 1.28 V] anode reaction
2MnO2(s) + H2O(l) + 2e− → Mn2O3(s) + 2OH−(aq) [e° = +0.15 V] cathode reaction
Zn(s) + 2MnO2(s) ⇌ ZnO(s) + Mn2O3(s) [e° = 1.43 V] net reaction
When a battery is connected to a load, an electrical current is produced by
the voltage potential produced by the chemical reactions in Figure 2, as shown
in Figure 3. When current flows due to the battery’s chemical reactions, the
original chemicals (Zn(s) - solid zinc metal and MnO2(s) solid manganese oxide)
are no longer available for further reactions unless the battery is recharged.
Potassium hydroxide (KOH) is the source of the hydroxide ion (OH--) in the
alkaline battery (the potassium is not involved in the reaction; it
just brings along the hydroxide ion). Potassium hydroxide is related to sodium
hydroxide which we know as lye, which is used in some drain cleaners. Thus,
potassium hydroxide is highly corrosive to all of the metals in the battery (zinc
anode, steel can, and top and bottom caps). If the KOH corrodes through the
steel can, the electrolyte leaks out and ruins the device in which the battery is
In a battery which is sitting on the
shelf during storage, the potassium
hydroxide electrolyte continues
to react with the zinc anode and
manganese oxide cathode, so there
are less of them left to produce more
energy. As the anode and
cathode material are made inert by
these chemical reactions, the voltage
of the battery deceases. At some
point, the voltage of the battery is too
low to power our electronic devices
and needs to be disposed of in
compliance with local ordinances.
A battery not connected in a
circuit does not have a current as we
know it (there is an internal flow of
electrically charged atoms), so you
cannot calculate the current based on
the battery’s amp-hour rating. Self-discharge is really deterioration of the
energy producing internal components
of the battery. Figure 4 shows the
service life (time until the terminal
voltage is five volts) for the Duracell
MN1604 nine volt battery. For the
different load currents, I calculate the
amp-hour ratings as 250 mA - 2. 50,
100 mA - 0.32, and 50 mA - 0.43
which shows that the amp-hour rating
is dependent on the load current.
Battery service life is highly
dependent on the device it is
powering, the load current, ambient
temperature, and load cycling.
Tim Brown N&V Q&A
QUESTIONS and ANSWERS
Post comments on this article at
n FIGURE 4.
n FIGURE 3.
n FIGURE 6.
Re: Furnace Data Acquisition
#1 Hello, Tim! I enjoyed reading your reply to my comments
(Mailbag, February 2016) regarding furnace data acquisition.
Thank you for the nice words. I was impressed that you pointed
out to your readers that thermostats can differ markedly. In
addition to the terminals you mentioned, I have E, L, G1, and G2
terminals, although some are not connected.
With my electric heat pump system, I would definitely want
to also monitor contact closure for stage two, which signals the
addition of resistance heating coils on extremely cold nights. I
would like to contribute two more comments to the discussion
regarding your schematic shown in Figure A. Following the
recommendations given in the datasheet for the 7805 (ST
Microelectronics), I would definitely add .1 µF capacitors to
ground on the microcontroller side of the regulators. You do
not specify the value for C1 and C2, but I would assume you
were intending a substantial value electrolytic capacitor. I
might consider paralleling those with . 33 µF caps based on the
datasheet, but perhaps that would be unnecessary and wasteful.