Battery with electrochemical tester

The present invention relates to an electrochemical cell and a related state of charge indicator comprising an electrochemically generated display. The state of charge indicator comprises two electrical contacts and an electrochemically generated display connected therebetween. The display comprises an electrochemical cell that may be permanently connected to a main cell in a parallel configuration via the contacts. The condition of the main cell may thus be continuously displayed on the indicator.

This invention relates to an improved combination of an electrochemical 
cell and an integrally related battery condition indicator comprising an 
electrochemically generated display. 
Electrical primary cells which include a means for visually indicating the 
condition or state of charge of the cell are known. The known indication 
means include, but are not limited to, chemical indicators which react 
with materials inside the battery, chemical indicators located externally 
to the battery, elements embedded within an electrode that become visible 
during discharge, and thermochromic materials in thermal contact with a 
resistive element that is adapted to be connected across the battery. A 
problem with many of these indicators is the timing of their indication is 
sensitive to the construction geometry of the indicator on or within the 
battery. Therefore, natural variations which inherently occur during 
manufacture lead to variability, from battery to battery, in the time 
during discharge when the indication occurs. 
A preferred battery tester is one which measures the voltage of a battery 
(main cell), since a voltage measurement, per se, is not sensitive to 
construction geometry. One type of tester which provides an indication 
that is proportional to voltage comprises a thermochromic material in 
thermal contact with a resistive element. Non-limiting examples of such 
testers are disclosed in U.S. Pat. Nos. 4,835,476, 4,726,661, 4,835,475, 
4,702,563, 4,702,564, 4,737,020, 4,006,414, 4,723,656, and U.S. Ser. No. 
652,165 filed Feb. 7, 1991 issued as U.S. Pat. No. 5,128,616. These 
testers work well for intermittent testing of a battery during its useful 
life. They are more difficult to permanently attach to a battery because 
the visual indicator is a thermochromic material. Care must be taken to 
thermally insulate the indicator from the battery casing in order to 
prevent heat transfer that would interfere with proper operation of the 
indicator. Additionally, these testers comprise a resistor that is 
connected in series with the battery during the voltage measurement. 
Therefore, the electrical contacts of the tester can not be permanently 
attached to the battery terminals in the absence of a switch, otherwise, 
the battery would be prematurely discharged through the tester. Several 
thermochromic testers are disclosed which can be manufactured already 
attached to a main cell as disclosed in commonly assigned U.S. patent 
application Ser. No. 07/730,712 filed Jul. 16, 1991 and in U.S. Pat. No. 
5,059,895. 
The present invention overcomes the problems associated with the above 
described testers by employing a battery tester comprising an 
electrochemically generated display that is permanently electrically 
connected in parallel to the battery. Heat transfer is not an issue 
because the principle of operation is electrochemical, not thermochromic. 
Premature discharge is not a problem because the electrochemical tester is 
connected in parallel to the battery and therefore, can not act as a 
series resistor. The voltage of the electrochemical cell which generates 
the display follows the voltage of the battery during discharge and 
thereby provides an accurate determination of the useful life remaining in 
the battery. 
In particular, the present invention relates to an electrochemical cell 
comprising a container and a top and an integrally related state of charge 
indicator positioned externally both to said cell top and said container. 
The state of charge indicator has two electrical contacts and an 
electrochemically generated display connected therebetween. A first 
contact is permanently connected to a first cell terminal and a second 
contact is permanently connected to the other terminal. In a preferred 
embodiment the indicator has an anode active layer electrically connected 
to the negative terminal of the battery and a cathode active layer 
electrically connected to the positive terminal of the battery. The 
indicator is so designed that no part thereof is positioned where it could 
interfere with insertion of the battery in a device such as would be the 
case if wires or tabs were associated therewith for connecting terminals 
at one or both ends of a cell, and the addition of chemicals in order to 
operate is not required. 
In one embodiment the indicator is integrally related to the cell label. In 
a second embodiment the condition indicator is located between the cell 
top and an opposing end cap.

For purposes of the following discussion the electrochemical cell or 
battery that is being measured will be referred to as the "main cell" and 
the electrochemical cell that generates the display will be called the 
"indicator cell". In accordance with the present invention, an integrally 
related battery and condition indicator is constructed by permanently 
connecting a condition indicator comprising an indicator cell in a 
parallel configuration with the main cell. The indicator cell indicates 
the condition of the main cell using an electrochemically generated 
display that is constructed as follows. 
The indicator cell of the invention contains a cathode active layer, and an 
anode active layer, and an electrolyte layer therebetween. The cathode 
active layer and an anode active layer are selected such that the 
indicator cell will have a voltage substantially similar to the voltage of 
the main cell, preferably just slightly less than the voltage of the main 
cell. This ensures that the indicator cell will be discharged when the 
main cell is also being discharged. The anode and the cathode of the 
indicator cell can be selected to be the same as the anode and the cathode 
of the main cell, e.g. zinc and manganese dioxide. However, an anode and 
cathode pair different from the main cell can also be used, provided that 
the voltage of the indicator cell is such that it will at least begin to 
discharge before the main cell voltage drops to a value that is no longer 
useful. Otherwise, the indicator cell would not be discharged and a 
display would not be generated before the end of the useful life of the 
main cell. 
As discussed further below, the capacity of the indicator cell is much less 
than the capacity of the main cell. For example, the capacity of the 
indicator cell can be as low as 1/1000 of the capacity of the main cell. 
Therefore, it is preferred that the impedance of the indicator cell is at 
least 10 times, more preferably at least 100 times, and most preferably 
1000 times the impedance of the main cell. A high impedance will cause the 
indicator cell to discharge at a lower rate than the main cell so that the 
discharge of the indicator cell is timed to coincide with the time 
corresponding to the useful discharge of the main cell. In fact, the 
impedance of the indicator cell can be specifically tailored by adding a 
resistor in series so that the combination of the indicator cell and 
resistor causes the indicator cell to discharge at a predetermined rate 
that is proportional to the rate of discharge of the main cell. A resistor 
could also be added in series with the battery to alter the impedance of 
the battery. 
Desirably the indicator's voltage profile during discharge is also similar 
to the voltage profile of the main cell. Thus, the anode, cathode and 
electrolyte layers of the indicator cell are preferably selected to obtain 
such a matched voltage profile. During discharge of the indicator cell the 
anode and cathode are gradually electrochemically depleted. Thus, the 
extent of the discharge of the main cell is determined by observing the 
depletion of the indicator cell anode or cathode, typically by observing 
the disappearance of the indicator cell anode. 
The preferred indicator cell is one in which the anode disappears and its 
disappearance creates an observable display. The display is completed by 
including an indicia bearing layer beneath the anode layer. The indicia 
may be coated as tiny granules of fluorescent material on the surface of 
the cathode layer at the electrolyte/cathode interface. The indicia could 
be a fluorescent color or convey a message to the observer, such as the 
word "Replace" and the like. The layers between the anode layer and the 
indicia should be clear so that the indicia or color is readily observable 
when the anode layer disappears. The amount of anode metal in the 
indicator cell is chosen so that enough metal is removed to reveal the 
indicia at a time when the main cell is approaching the end of its useful 
life. 
The indicator cell is preferably made very thin so that it can be 
permanently attached to an external surface of the main cell without 
noticeably adding to the dimensions of the main cell. If the thickness of 
the indicator cell becomes significantly large, then the diameter of the 
main cell would have to be reduced for the overall diameter to remain 
about the same. This of course would cause a reduction in the capacity of 
the main cell. Therefore, it is desirable for the indicator cell to be 
made very thin. The anode, cathode and electrolyte layers which form the 
indicator cell may be in a stacked arrangement. More preferably, the anode 
and cathode layers may be laterally spaced apart from each other with the 
electrolyte contacting at least a portion of the surface of each. This 
latter embodiment provides a moving boundary during discharge whereby a 
"fuel gauge" effect is created. The stacked or laterally spaced apart 
construction for the indicator cell of the invention desirably may have a 
thickness of less than 100 mil (2.5 mm) but preferably may be made very 
thin to a thickness less than about 15 mil (0.4 mm), preferably a 
thickness less than about 10 mils (0.25 mm). The indicator cell thickness 
is typically between about 4 and 15 mils (0.1 and 0.4 mm). 
Thin metal foil strips, or thin insulated wires and the like, may be used 
to connect the cathode to the positive terminal of the main cell and the 
anode to the negative terminal of the main cell. The anode layer is 
visible from the outside through either a transparent portion of the main 
cell label that is juxtaposed to the indicator cell or through a clear 
substrate that covers the outer surface of the indicator cell. Specific 
embodiments are discussed further below. 
The features and advantages of the present invention will now be discussed 
in connection with a specific embodiment and by making reference to the 
drawings. A condition indicator comprising an electrochemically generated 
display for a "AA" size zinc/manganese dioxide alkaline cell is 
constructed as follows. All parts are parts by weight unless indicated 
otherwise. 
A cathode layer for the indicator cell may be prepared by mixing manganese 
dioxide powder, and about 6 wt % of a conductive agent such as carbon 
black powder (e.g. acetylene black) and/or graphite and 5 wt % 
polytetrafluoroethylene powder. 200 mg of the cathode mixture is added to 
a round mold cavity (diameter of about 0.5 in.) having a flat bottom. A 
closely fitting mold die having a flat surface is inserted into the cavity 
and manually pressed down to compress and flatten the cathode mixture. 
During compression a disc-like cathode pellet having a thickness of about 
20 mils (0.5 mm) is formed. The disc-like cathode pellet is then easily 
removed from the mold. 
An anode layer for the indicator cell is preferably prepared by vapor 
depositing or electrochemically plating zinc metal onto a clear substrate 
such as polyester film. If the anode layer is electrochemically plated, 
the substrate is a clear conductive substrate. Such a conductive substrate 
may be a polyester film having a coating of indium tin oxide coated 
thereon, such as that designated as "Altair" M-5 film (manufactured by 
Southwall Technology Inc., Palo Alto, Calif.). A rectangular piece of this 
film is plated with zinc using a current density of 10 milliamp/cm.sup.2 
for about 2 to 4 minutes in a plating bath. The plating bath is formed by 
employing a 1 molar ZnSO.sub.4 solution in H.sub.2 O with the pH adjusted 
to 1.5 to 2 using sulfuric acid. The clear conductive layer may typically 
have a thickness of about 1 mil (0.025 mm) and the zinc deposited layer 
typically between about 0.03 and 0.04 micron. Other methods such as 
sputtering techniques may be employed to deposit the zinc anode layer onto 
a film substrate. 
Referring now to FIGS. 1A and 1B, indicator cell 10 is a thin laminate 
containing an anode layer 20 on a film backing 18, an electrolyte layer 12 
and cathode layer 14 with indicia 40 at the cathode/electrolyte interface. 
(The term laminate as used herein shall be defined to include layered 
structures which may contain film, metallic or coated layers or any 
combination thereof.) Indicator 10 may be assembled on battery 50 as 
follows: The above described cathode layer 14 may be first applied with 
one side facing casing 56 of battery 50. The cathode layer 14 may be 
electrically connected to the positive terminal 57 directly or by 
contacting cathode layer 14 against casing 56 which in turn is in 
electrical contact with positive terminal 57. If cathode layer 14 
comprises a pellet as described above, it may have a thickness between 
about 0.3 and 1.0 mm, typically about 0.5 mm. Cathode layer 14 thickness 
may be reduced by employing a coating containing a cathode active material 
in a solvent mixture. (Preferred formulations for such coating is herein 
later described.) After the coating is applied, for example, directly onto 
casing 56, or onto a thin film such as of MYLAR polyester, the solvent may 
then be evaporated. The resulting thickness of the dry cathode coating 14 
may be as low as 1 mil (0.025 mm) and such dry cathode coatings may 
conveniently be made to have thicknesses between about 1 mil (0.025 mm) 
and 5 mil (0.13 mm). The electrolyte layer 12, preferably the electrolytic 
film (herein later described), is applied onto the exposed surface of 
cathode layer 14. Electrolyte layer 12 may typically have a thickness 
between about 0.05 and 0.25 mm. Thereupon a section of polyester film 18 
having zinc layer 20 plated thereon is applied with zinc layer 20 held 
against electrolyte layer 12. The polyester film 18 may typically have a 
thickness of about 0.025 mm and the zinc layer 20 thereon may typically 
have a thickness between about 0.03 and 0.04 microns. The zinc may cover 
the entire surface area of electrolyte layer 12. The zinc layer can extend 
beyond the surface area of the electrolyte layer, which extending portion 
can function as at least part of the electrical pathway for connecting the 
indicator cell anode to the negative terminal of the battery. The 
completed cell has an impedance of between about 500 and 1000 ohms. The 
zinc anode of the indicator cell is electrically connected to the negative 
terminal of a "AA" size zinc/manganese dioxide alkaline cell and the 
cathode is electrically connected to the positive terminal of said "AA" 
size cell. A resistive load is connected across the terminals of the 
battery. As the battery approaches the end of its useful life the zinc 
indicator cell anode disappears alerting the user that the battery needs 
to be replaced. 
In contrast to the single event indicator described above the indicator 
cell can be designed in a manner so that it functions as a "fuel gauge". 
FIGS. 2 and 3 show cross section of indicator cell anode 26 and 36 having 
an increasing thickness from one end to the other. Such an anode would 
first disappear at its thin end and the thickest end would disappear last. 
An indicia layer 40 may be employed, for example, between electrolyte 12 
and cathode 14. As the zinc anode layer 20 disappears on discharge of 
indicator cell 10, the indicia layer 40 becomes visible. When such an 
indicator cell is permanently connected to a battery the user is provided 
with a continuous indication of the state of charge of the battery in the 
same manner as the fuel gauge on a car. An alternative, but less preferred 
embodiment for achieving the "fuel gauge" effect is to vary the thickness 
of the cathode layer (similar to the anodes shown in FIGS. 2 and 3) while 
keeping the anode layer thickness substantially uniform. 
FIG. 5 shows an embodiment of an indicator cell 60 (essentially indicator 
cell 10) permanently connected to battery 50. Indicator cell 60 is a 
laminate comprising an anode 20, electrolyte layer 12, and cathode layer 
14 such as shown in FIG. 1B and may or may not include polyester film 
layer 18 for anode layer 20. Indicator 60 is applied to battery 50 
preferably with cathode layer 14 closer to cell wall 56 than anode layer 
20, e.g. as shown best in FIG. 5A. The anode layer 20 may be printed, 
electrodeposited, or otherwise affixed to the inside surface of cell label 
52. Typically, however, anode 20, may be a thin layer of zinc deposited 
onto a polymeric substrate such as a polyester film 18. 
Reference is now made to FIGS. 5A-8 which show several embodiments for 
achieving the preferred "fuel gauge" effect. As illustrated in a preferred 
embodiment shown in FIG. 5A indicator 60 is a thin laminate formed of 
anode layer 20, cathode layer 14 and electrolyte layer 12 in stacked 
arrangement with the electrolyte layer 12 physically contacting both anode 
layer 20 and cathode layer 14. The anode layer 20 of indicator cell 60 may 
be permanently connected to the negative terminal 54 of cell 50 by a 
conductive element 62 as shown in FIG. 5A. Conductive element 62 can be an 
extension of the deposited anode layer 20 as described above or it can be 
a different conductive material that is fixed to the inside surface of the 
label or it may be an insulated wire. If conductive element 62 is itself 
not insulated, then an electrically insulating layer (not shown) must also 
be interposed between conductive element 62 and casing wall 56, otherwise 
the indicator cell and battery would be short circuited. The cathode layer 
14 is electrically connected to the positive terminal 57 by a wire 63 or 
the like (FIG. 5A) or or by directly contacting casing 56 which in turn 
may be in electrical contact with positive terminal 57. If cathode layer 
14 contacts cell casing 56, then the anode layer 20 will disappear 
uniformly over its entire length during discharge of cell 60. However, 
cathode layer 14 may alternatively be connected at one end (A) directly to 
positive terminal 57 by an insulated wire 63 or the like (FIG. 5A) and may 
be insulated from contact with cell casing 56 by an insulating substrate 
73, e.g. a polymeric film of MYLAR polyester or the like (FIG. 5A). In 
this latter embodiment as indicator cell 60 discharges, the anode layer 20 
will begin to disappear first at point A (FIG. 5A) and then gradually from 
point A to point B gradually shrinking anode 20. Thus during the discharge 
process as more of anode 20 disappears, more of underlying indicia layer 
40 becomes exposed. This conveys a "fuel gauge" effect permitting the user 
to determine at any time the remaining capacity of main cell 50 by simply 
viewing the portion of anode layer 20 remaining or by reading the message 
on exposed indicia layer 40. The overall thickness of indicator cell 60 
(FIG. 5A) is less than 100 mil (2.5 mm), preferably less than 15 mm (0.4 
mm), more preferably less than 10 mm (0.25 mm), typically between about 4 
and 15 mils (0.1 and 0.4 mm). 
The indicator cell 60 in the preferred embodiment of FIG. 5A may preferably 
have the electrolyte layer 12 formed of an electrolytic film comprising a 
porous polymeric film containing the liquid electrolyte solution entrapped 
within the porous film. The electrolyte film is herein later described in 
detail. Anode layer 20 (FIG. 5A) is preferably a zinc layer of thickness 
of between about 0.03 to 0.04 microns on backing 18 which may typically be 
a 1 mil (0.025 mm) thick clear MYLAR polyester film. Cathode layer 14 
(FIG. 5A) may be a coating having an active cathode material and 
conductive agent such as a mixture of carbon black and graphite. The 
conductive agent preferably comprises at least 4 per cent by weight of the 
mixture of active cathode material and conductive agent. Preparation of 
the coating is discussed in detail later in the description. It is 
preferably applied as a solvent based coating onto polymeric substrate 73 
(FIG. 5A). The coating is then dried. The dried cathode layer 14 (FIG. 5A) 
typically has a thickness between about 0.3 to 3 mil (0.008 to 0.08 mm), 
preferably between 0.5 and 1 mil (0.013 and 0.025 mm). The indicia layer 
40 (FIG. 5A) may typically have a thickness together with any imprinted or 
coated ink layer thereon of about 1 and 2 mil (0.025 and 0.05 mm). A 
moisture barrier film, preferably of mica as discussed in commonly 
assigned U.S. patent application Ser. No. 07/914,943 (Treger) filed of 
even date with the present patent application, may be inserted between 
label 52 and indicator 60 to protect indicator 60 from exposure to 
deleterious amounts of ambient moisture. This patent application is herein 
incorporated by reference. The moisture barrier film may be adhesively 
secured along its border to casing 56 as described in the above referenced 
commonly assigned patent application. A label 56, typically of 
polyvinylchloride, may then be tightly applied around casing 56 and over 
indicator 60 to tightly encase the indicator 60 and moisture barrier 
against casing 56. 
FIG. 6 shows an alternative embodiment of the indicator cell, namely 
indicator cell 92, which is also a thin laminate. In the embodiment shown 
in FIG. 6 the cathode and anode layers, 74 and 76 respectively, are 
laterally separated from each other instead of being in stacked 
arrangement as shown in FIG. 1B. Thus, in indicator cell 92, no portion of 
the anode active layer overlaps any portion of the cathode active layer. 
The electrolyte layer 77 is placed over and contacts the same side of both 
cathode layer 74 and anode layer 76 as illustrated best in FIG. 6. Cathode 
layer 74, is laterally separated from anode layer 76 by gap 85. In the 
embodiment shown in FIG. 6 electroyte layer 77 is placed on the side of 
cathode layer 74 and anode layer 76 facing away from battery 50. The 
battery 50 shown in FIG. 6 is representative of a conventional main cell, 
typically an alkaline cell, having a negative terminal 54, a positive 
terminal 57 and a casing 56. Casing 56 is typically in electrical contact 
with positive terminal 57. Indicator cell 92 may also contain a color or 
indicia layer 83 which may be advantageously located against casing 56 of 
battery 50. Color or indicia layer 83 may be a layer of colored polymeric 
film, for example colored polyester (MYLAR) film. Alternatively, layer 83 
may be a clear polymeric film, preferably a MYLAR film which is printed on 
one side with a message. Preferably the imprinted side of layer 83, faces 
casing 56. Layer 83 also functions as an electrolyte barrier layer, that 
is, it prevents electrolyte from layer 77 from contacting and corroding 
casing 56. Layer 83 thus should be impervious to electrolyte from layer 77 
and also should be sufficiently heat resistant that it does not distort 
when exposed to heat during conventional labeling of cell 50. Typically 
layer 83 together with any print layer thereon may have a thickness of 
between about 0.5 and 1 mil (0.013 and 0.025 mm). 
As illustrated in FIG. 6 anode layer 76 is electrically connected to the 
negative terminal 54, for example by an insulated electrical wire 81. 
Cathode layer 74 is electrically connected to the positive terminal 57, 
preferably by an insulated electrical wire connecting cathode layer 74 
directly to positive terminal 57 or alternatively to casing 56 which in 
turn is in electrical contact with positive terminal 57. Typically anode 
layer 76 is a thin metallic layer, for example vapor or electrodeposited 
zinc. In such case it is desirable to provide a backing layer, e.g. layer 
75, onto which the metal may be deposited. Backing 75 is preferably a 
clear polyester film, e.g. a MYLAR film. As shown in FIG. 6 the backing 
layer 75 may contact indicia layer 83. It may be advantageous with cathode 
materials that are not highly conductive to employ a metallic current 
collector 73 in contact with cathode layer 74. Preferably current 
collector 73, if employed, is a thin sheet of stainless steel, aluminum or 
a conductive plastic which may contact the inside surface of cathode layer 
74 as shown in FIG. 6. If such a current collector is employed, cathode 
layer 74 may be electrically connected by means of insulated wire 82 
connecting current collector 73 to positive terminal 57 or casing 56. It 
may be desirable to employ a moisture barrier layer 98 (FIG. S) over 
indicator cell 92, namely over electrolyte layer 77 to protect indicator 
cell 92 from exposure to deleterious amounts of ambient moisture. Moisture 
barrier layer 98 is preferably a thin sheet of adhesively secured mica as 
discussed in the above cited commonly assigned U.S. patent application. 
Indicator 92 may be held in place against cell 50 by label 99 (FIG. 8). 
Label 99 may typically be a heat shrinkable protective film of 
polyvinylchloride applied around cell 50 and indicator cell 92. As heat is 
applied label 99 shrinks tightly encasing indicator 92 (and moisture 
barrier layer 98) against casing 56. 
In operation as main cell 50 discharges, indicator cell 92 discharges in 
proportional amount. During discharge of indicator cell 92 the anode layer 
76, typically of zinc, begins to electrochemically erode and disappear 
beginning at point A at the end of anode layer 76 nearest gap 85 (FIG. 6). 
As discharge continues gap 85 becomes greater as anode layer 76 gradually 
disappears from point A towards point B exposing more and more of 
underlying indicia layer 83. This creates a "fuel gauge" visual effect. 
Indicia layer 83 can be imprinted with words reflecting the extent to 
which main cell 50 has been depleted at any point in the discharge cycle. 
Electrolyte layer 77 and anode backing layer 75 are preferably clear 
making it easy to view indicia layer 83 as gap 85 becomes greater. The 
overall thickness of indicator cell 92 is less than 15 mils (0.4 mm), 
typically between about 4 and 15 mils (0.1 and 0.4 mm). 
Another embodiment of an indicator cell of the invention having laterally 
spaced-apart anode and cathode active layers is represented as indicator 
cell 93 in FIG. 7. Indicator 93 is a laminate essentially the same as 
indicator 92 except that the portion of electrolyte layer 77 nearest anode 
layer 76 is interposed between anode layer 76 and casing 56. To 
accommodate this change the film backing 75 for anode layer 76 appears on 
the outside surface of anode layer, that is, away from casing 56, as 
illustrated in FIG. 7. In indicator 93 no portion of anode active layer 76 
overlaps any portion of cathode active layer 74. Indicator 93 is connected 
in parallel to main cell 50. That is, anode layer 76 is electrically 
connected to negative terminal 54 preferably by insulated wire 81, and 
cathode layer 74 is preferably electrically connected to positive terminal 
57 directly by insulated wire 82 or through a metallic current collector 
73 which in turn is connected to positive terminal 57. As above described 
with reference to indicator 92 it is desirable to secure a moisture 
barrier film, preferably of mica over indicator cell 93. The moisture 
barrier film may be placed over indicator 93 and adhesively secured along 
its border to casing 56 as described in the above referenced commonly 
assigned patent application. Indicator 93 and any moisture barrier film 
thereover may be tightly held in place against casing 56 by a heat 
shrinkable label 99 which is fitted around main cell 50 (FIG. 8). 
In operation as main cell 50 discharges, indicator cell 93 discharges in 
proportional amount. As indicator cell 93 discharges anode layer 76 begins 
to disappear gradually from the end of anode layer 76 (point A) nearest to 
gap 85. As in indicator 92 a "fuel gauge" visual effect is created in 
indicator 93 as gap 85 lengthens during the disappearance of anode layer 
76, gradually, from point A towards point B. The overall thickness of 
indicator cell 93 is less than 100 mil (2.5 mil), preferably less than 15 
mil (0.4 mm ), typically between about 4 an 15 mils (0.1 and 0.4 mm). It 
is surprising that so thin an indicator cell 60, 92 or 93 can be made to 
continually reflect the state of condiction of main cell 50. 
Indicator cells 92 and 93 have anode and cathode layers which do not 
overlap as shown in FIGS. 6 and 7, respectively. These are preferred 
embodiments. However, embodiments of indicator cells 92 and 93 are 
possible wherein cathode and anode layers overlap and yet the indicator 
still obtains the "fuel gauge" effect, above described. If such an overlap 
configuration is employed, it is desirable to increase the resistance of 
the cathode layer and to take precaution that the electrolyte layer is 
sufficiently thick between the anode and cathode overlapped portion so 
that the indicator cell does not short circuit. 
The anode active layer 76 for indicator cell 92 and 93 is preferably of 
zinc which may be vapor deposited or electrochemically plated onto a clear 
substrate, that is, backing 75. If plated, backing 75 is a clear 
conductive substrate, preferably a polyester film having a coating of 
indium tin oxide. The composition of the plating bath which may be 
employed and method of plating has already been described in the foregoing 
and applies in its entirety to the preparation of anode active layer 76. 
The thickness of anode active layer 76 may typically be between about 0.03 
and 0.04 micron and the thickness of backing layer 75 may be about 1 mil 
(0.025 mm). 
The cathode layer 74 for indicators 92 and 93 or cathode layer 14 for 
indicators 10 and 60 may contain any type of known cathode active 
material. Preferably cathode layers 74 and 14 contain a cathode active 
material which produces an open circuit voltage (OCV) for the indicators, 
which is substantially similar to the open circuit voltage of main cell 50 
throughout the life of the main cell. Desirably, the cathode layers 74 for 
indicator 92 and 93 or cathode layer 14 for indicator 10 and 60 contain a 
cathode active material which produces an open circuit voltage for these 
indicators, which is between about 80 and 120 percent of the open circuit 
voltage of main cell 50 throughout the life of the main cell. If the 
voltage of the indicator is very low in relation to the battery voltage, 
the indicator will not begin to discharge early enough during the life of 
the battery. If the voltage of the indicator is very high in relation to 
the battery voltage, then corrosion problems in the indicator can develop, 
since the battery will tend to charge the indicator as the battery 
discharges. Cathode layer 74 may be applied as a solvent based coating 
onto a thin sheet of metallic current collector 73. The coating is 
thereupon dried to evaporate solvent leaving behind a thin dry coating 
containing the manganese dioxide. The preferred cathode layer 74 may be 
prepared as a coating mixture containing a) active cathode material b) a 
conducting agent c) a binder and d) solvent. The cathode active material 
preferably may include CoO.sub.2, NiO.sub.2, lambda MnO.sub.2 or mixtures 
thereof. (These compounds may be produced from the chemical or 
electrochemical deintercalation of LiCoO2, LiNiO2, or LiMn204, 
respectively.) Advantageously the cathode active material contains 
CoO.sub.2, NiO.sub.2 or lambda MnO.sub.2 (or mixtures thereof), alone or 
in mixture with a second active material selected from LiNiO.sub.2, 
LiCoO.sub.2 or LiMn.sub.2 O.sub.4 (or mixtures thereof). They are desired 
over cathode materials containing montmorillonite (disclosed in the parent 
application) because they are more compatible with the preferred 
electrolyte layers described herein and because they can lead to thinner 
indicators. Specific examples of suitable cathode active material 
containing the above components are (parts are by weight): i) CoO.sub.2, 
NiO.sub.2 or lamda MnO.sub.2, or mixtures thereof (100 wt %), ii) 
CoO.sub.2, NiO.sub.2 or lambda MnO.sub.2 or mixtures thereof (100 parts) 
and LiMn.sub.2 O.sub.4 (10 to 50 parts), iii) CoO.sub.2, NiO.sub.2 or 
lambda MnO.sub.2, or mixtures thereof (100 parts) and LiCoO.sub.2 (10 to 
50 parts) and iv) CoO.sub.2, NiO.sub.2 or lambda MnO.sub.2, or mixtures 
thereof (100 parts) and LiNiO.sub.2 (10 to 50 parts). The conducting agent 
is preferably a mixture of carbon black powder (e.g., acetylene black) and 
graphite. The binder may be selected from polymeric binders such as 
polyacrylonitrile, polyethylene terephthalate, polybutylene terephthalate, 
polyvinylidene fluoride (homopolymer and copolymer) and polyvinyl 
fluoride. The solvent may desirably be selected from N-methyl 
pyrrolidinone, pyrrolidone, dimethyl formamide (DMF), acetone, 
acetonitrile, tetrahydrofuran, methylethylketone (MEK), tetramethyl urea, 
dimethyl sulfoxide, and trimethyl phosphate. A preferred cathode active 
layer 74 or 14 may be prepared by mixing any one of the above active 
cathode formulations together with the conducting agent (carbon black and 
graphite) into a mixed composite powder. The conducting agent may 
desirably comprise 2 to 50 wt % of the mixed composite powder. The binder 
(above indicated) is then dissolved in the solvent, typically in a weight 
ratio of 1 part binder to 10 parts solvent to form a binder/solvent 
solution. The composite powder is then mixed with the dissolved binder, 
typically at ambient temperature employing an electric mixer until a 
homogeneous (ink) mixture is formed. In application the ink mixture may be 
coated directly onto subtrate 73, preferably a cathode collector formed of 
a sheet of stainless steel or aluminum sheet of thickness between about 
0.3 and 1 mil (0.008 and 0.025 mm). The stainless steel or aluminum sheet 
can be replaced with a non conducting polymeric film for substrate 73 of 
indicator 92 or 93 when the amount of conducting agent in the mixture of 
conducting agent and active cathode material exceeds about 10 wt %. Such a 
nonconducting polymeric film may for example be selected from polyester 
(MYLAR), polyethylene, polypropylene and fluoropolymers of thickness 
typically of about 1 mil (0.025 mm). The ink may be coated onto substrate 
73 typically at ambient temperatures using conventional coating techniques 
such as by brush or spray. The coated subtrate 73 is then dried by 
convective air at a temperature of between about 25.degree. and 
300.degree. C. until the solvent has evaporated. The resulting dried 
coating preferably has a thickness between about 0.3 and 3 mil (0.008 and 
0.08 mm) and may form the cathode active layer 74 for indicator 92 and 93 
or the cathode active layer 14 for indicator 60. The above described 
cathode coating wherein the amount of conducting agent in the mixture of 
conducting agent and active cathode material exceeds 10 wt % is also 
preferably employed for the cathode layer 14 of FIG. 5A. (Any of the 
cathode coatings as above described may also be employed for cathode layer 
14 in indicator cell 10 of FIG. 1B.) 
The electrolyte layer 12 for the indicator cell 10 or 60 or electrolyte 
layer 77 for indicator 92 or 93 desirably has a conductivity of at least 
1.times.10.sup.-7 ohm.sup.-1 cm.sup.-1 preferably between about 
1.times.10.sup.-4 and 1.times.10.sup.-3 ohm.sup.-1 cm.sup.-1 and even 
higher and a thickness of between about 0.05 and 0.25 mm. A preferred 
electrolyte layer for layers 12 or 77 is an electrolytic film composed of 
a porous polymeric film-like matrix containing an electrolye solution 
composed of ionic salts dissolved in organic solvants. The ionic salts 
desirably have high solubility in the organic solvents and the electrolyte 
solution has a high boiling point so that it does not volatilize during 
assembly or operation of the indicator cell. Any salt that has been found 
useful in electrochemical cells would also be useful in the indicator 
cell, non-limiting examples of which include LiCF.sub.3 SO.sub.3, 
LiClO.sub.4, Zn(CF.sub.3 SO.sub.3).sub.2, Zn(ClO.sub.4), LiN(CF.sub.3 
SO.sub.2).sub.2 and combinations thereof. The organic solvents desirably 
improve the electrical conductivity of the electrolyte, but primarily 
function as a solvent for the ionic salts and allow the electrolyte as a 
whole to remain liquid at ambient temperature and temperatures as low as 
about -20.degree. C. 
The organic solvents should remain liquid during the range of operating 
conditions to which the indicator cell will be exposed, typically between 
-20.degree. C. and 54.degree. C. A preferred organic solvent is composed 
of ethylene carbonate or propylene carbonate, preferably together in 
mixture. The ethylene carbonate has been determined to markedly improve 
the electrical conductivity of the electrolyte, while the addition of 
propylene carbonate assures that the electrolyte remains liquid at ambient 
temperature and temperatures as low as -20.degree. C. 
The porous polymeric matrix is preferably a material which absorbs the 
electrolyte whereby a polymeric-type electrolyte is provided. Such a 
matrix is preferred in order to minimize or even prevent leakage of the 
electrolyte during storage or the usable life of the main cell. The porous 
polymeric matrix disirably has a high void volume to total volume ratio. 
The void volume is desirably at least about 50%. The polymeric matrix has 
a network of microscopic pores which retain the liquid electrolyte 
entrapped therein. The preferred polymeric matrix is a microporous film 
formed of polyvinylidene fluoride (PVDF). Another preferred microporous 
film is polyacrylonitrile. Other suitable microporous films are 
polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, 
polyvinylidene chloride (SARAN), and polyester (MYLAR). These latter films 
may be selected depending on the geometry of the indicator cell and level 
of electrolyte conductivity desired. The electrolytic film, above 
described, has improved conductivity over the electrolytes (disclosed in 
the parent application) containing montmorillonite. 
In preparing the electrolyte layer a mixture of about 2 parts by weight 
propylene carbonate to about 1 part by weight ethylene carbonate is first 
prepared to form the ion-solvating plastiicizer. The ionic salt selected 
preferably from one or more of the components aforementioned, preferably 
LiCF.sub.3 SO.sub.3 (trifluoromethanesulfonate), is then dissolved in the 
organic solvents. This may typically be accomplished by stirring the salt 
and organic solvents together at ambient temperature using a mechanical or 
electric mixer, until a homogeneous electrolyte solution is obtained. The 
concentration of the ionic salt dissolved in the organic solvents may 
desirably be between about 0.5 and 1.5 moles per liter. Next 
polyvinylidene fluoride (PVDF) powder is added to the electrolyte solution 
to yield a concentration of about 27 percent by weight PVDF. The 
components are then mixed, typically at ambient temperature using a 
mechanical or electric mixer, until a homogeneous mixture is obtained. The 
mixture is then be heated at a temperature of about 150.degree. C. for 
about 10 minutes, whereupon the mixture becomes a transparent solution 
typically of glue-like consistency. The solution is then extruded while 
hot by pressing it between two preheated (150.degree. C.) stainless steel 
plates using a pair of rollers. The resulting hot extrudate is allowed to 
cool to ambient temperature whereupon the polyvinylidenefluoride 
percipitates out of solution to form a microporous polymeric film or 
matrix containing the electrolyte solution entrapped therein in liquid 
form. The micropores typically take the form of an interconnected open 
cell structure. The electrolytic film containing the entrapped liquid 
electrolyte may be placed directly between the indicator cathode and anode 
layers so that it contacts all or at least a portion of each of these 
layers. 
Indicator cell 92 or 93 should be assembled so that gap 85 between the 
anode and cathode layers is as short as possible without short circuiting 
the indicator cell. Typically the gap 85 length will be between about 0.5 
mm and 13 mm, more typically about 1 mm. The assembled indicator 92 or 93 
formed of the preferred anode, cathode and electrolyte layers above 
described may have a voltage substantially similar to the voltage of 
conventional alkaline cells, i.e. near 1.5 volts, making it an ideal 
indicator for conventional alkaline cells. Although indicator cell 92 and 
93 have been described within the context of a specific embodiment, that 
is, attached and connected in parallel to a main cell 50, it is not 
intended that these indicator cells be limited to such embodiment. For 
example, indicator cells 92 and 93 may be assembled and offered for sale 
as a separate unit and connected in parallel to the terminals of a main 
cell 50 at a later time. 
The embodiments described above are for illustrative purposes only. The 
specific design of the condition indicator cell will depend, of course, on 
the size and voltage of the associated main cell. Other embodiments for 
fixing the indicator cell to the main cell are, of course, possible and 
are intended to be within the scope of the present invention. Anodes, 
cathodes, and electrolytes other than those specifically described can 
also be used for the indicator cell and are intended to be within the 
scope of the invention as claimed.