Alkaline-type coulometer cell for primary charge control in secondary battery recharge circuit

Stroup, E. R.

English

NASA Technical Reports Server

8,196 
~~~~~ Sopt. 12, 1967 
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ondary alkaline type formed by flooding the cell with elec- 15 
trolyte and passi~g 8 forming current in two directions 
through the cell with the attainment of oxygenol volt- 
age and hydrogennl voltage in each direction. 
20 
s made by an em- 
ployee, of the United States Government and may be 
manufaqtured and used by or for the government for gov- 
ernmental purposes without the payment of any royalties 
thereon or therefor. 25 
This inventian relates to a secondary alkaline electro- 
chemical cell and a method of making same. More par- 
ticularly, this investion relates to a novel type of coulom- 
eter which exhibits a high degree of reliability and sta- 
bility. 30 
m e  coulameter, as its name implies, is a device that 
(ampere-seconds) and which may 
a signal after a specific quantity of 
electrical charge has passed through the device. The sig- 
nal produced may in turn be used to control an electric 35 
circuit, e.g. a recharge circuit for a secondary battery. 
The use of coulometers in recharging circuits for sec- 
both the coulometer and the space cell yield comparable 
lifetime performance characteristics. The coulometer of 
this invention comprises an electrochemical cell having 
a pair of reversible electrodes immersed in an electrolyte. 
The electrodes should have substantially the same e!ec- 
trode potential so that the voltage across the cell is 
small. The cell is adapted to develop a substantial and 
abrupt change of voltage between its electrodes when more 
than a definite quantity of electricity has passed through 
the cell. This abrupt voltage change corresponds to tho 
onset of a charge in the electrochemical process occurring 
at one or both electrodes, such as the onset of gas evolu- 
tion. This increased voltage is then used as a controlling 
signal in an electric circuit. For example, in a recharge 
circuit for a secondary battery the coulometer is placed 
in series with the battery so that the amount of charge is 
measured as a function of the amount of discharge and 
a voltage signal is emitted when the charge equals the 
amount discharged and the charge rate can be reduced to 
a safe level. 
Preferably, the electrodes are chosen to have the same 
electrode potential, but they may differ as long as the 
difference between the electrode potentials is less than the 
voltage which develops across the cell when mQre than 
the definite quantity of electricity has passed through 
the cell so that the change in voltage can control an 
electric circuit. Usually, the difference between the elec- 
trode potentials will be less than .050 volt. 
In the preferred embodiment, cadmium is employed 
in the electrodes. Thus, the electrodes may consist of a 
suitably conductive support material with the active ma- 
terial thereon consisting either of Cd metal or an insol- 
uble salt of that metal, e.g. Cd(OH)2. In operation, the 
active material goes into its higher valence state at the 
oxidation electrode and its lower state at its reduction 
electrode. Using cadmium, therefore, the oxidation re- 
action at the positive electrode would be 
ondary batteries has been of recent impwtance due to Cd+20H-+Cd (OH) 2+2e- 
the need for effectively controlling the recharge of sec- 
oodav  cells when attendant monitoring is not possible, 40 and the reduction reaction at the negative electrode would 
e.g. in satellite aper@igns. Generally, a storage or sec- be C ~ ( O H ) Z + ~ ~ - + ~ ~ + ~ ~ H - *  As is apparent to one 
ondary battery is recharged by the passage of electric skilled in the art, other types of electrode systems are 
current therethrough until the quantity of charge passed Possible as long as an voltage change through the 
(the ampere-hours) is eqyal to or greater than the charge cell is realized upon the commencement of a change in 
removed during discbarge of the battery. When a bat- 43 the electrochemical process occurring at one or both of 
tery attains its fuf ly  charged state, further passage of the electrodes. Examples of such systems include those 
current in the charging direction results in wasteful disclosed in Patent 3,302,091, Table I. 
electrolysis of the electrolyte with attendant gas evolu- The electrolyte used may be any enabling ion transfer 
tion. In order to maintain stability and reliability of op- within the cell. A typical example is an alkaline solution 
eratiqn of a storage battery in a circuit, it is necessary to 60 of Water and 30% KOH. 
effectively and precisely control the recharge of the In order to electrically separate the plates of one 
batteries. pole from those of the other, non-conductive sheets are 
It is therefore the object of the instant invention to placed therebetween. These separators should be porous 
provide coulometers which will exhibit the necessary Sp as to permit the gases formed by the natural func- 
stability and reliability of operation as primary charge 83 tioning of the cells to pass easily therethrough. A non- 
control dewicps in secondary battery recharge circuits. woven nylon fabric has proven to be particularly suit- 
It is still another object of the present invention to able. The plate separation in a cadmium-cadmium cou- 
provide a methd  for forming coulometers which may lometer should be at least 10 mils and may be as large 
be used in satellite applications. as 20 mils for most applications. 
In the embodiment of the invention illustrated by FIG- 
come apparent from a copsideration of the drawing and URE-1, the coulometer consists of a number of plates 
the following detailed description and claims. X and V alternately arranged and a thin non-woven fabric 
In the drawing which illustrates an embodiment of 2 separating the plates. The plates can be obtained from 
the invention and its formation: a discharged, dismantled alkaline storage battery of the 
FIG. 1 is a Rerspective view partly in section of one 05 sintered plate nickel-cadmium type, consisting of a porous 
embodiment; and sintered nickel support 3 in the pores of which a thin layer 
FIG. 2 graphically represents the forming cyclc for a has been deposited. Metal tabs 
cadmium-ciidmiuni cciulometer. nated X are connected together 
The cells of this invention are constructed in the Sitme and to an external connection 8 labeled positvie. The 
general fashion as the nickel-cadmium cells used in space- 7 0  plates designated Y arc similarly connected together and 
crYft and commercial batteries that powcr buch items as tq the external electrical connection 9 labeled negative. 
flrgshliphts, knivcg, drills, etc. Due to this relationship, The platc assembly is placed in an outer container 1 of 
These and other objects of the present invention will be- GO 
f cadmium hydroxide 
welded to the plates d 
3,463,678 
3 
alkali inert matsrial smh as plastic, nylon, or metal such 
as nickel or stainless stgel, @ped as a rectangular solid. 
A top 6 is provided for the, wntainer through which the 
electrical eonncctiaas 5 and 8 to the plates are passed, and 
throu& which a vent 7 is provided for filling the con- 
lyQ e.g. 2 4 0 %  KUH solution, i s  added which will com- 
pletely saturate the separatw material 2, fill the pores in 
the plates and fully coyer the plates with liquid. This so 
called "flded" condition is necessary to insure proper 
formation of the coulonoceter. 
FIGURE 2 is a graphical representation of the forming 
operation using as an example, a cadmium-cadmium cell. 
After the cell is "flooded" with electrolyte a current of 
6/10. where C=rated capacity of the electrode, is passed 
through the vented cell. Just before the current is passed 
through the cell substantially all the active material on 
the plates is in the form of cadmium oxide or hydroxide. 
Starting at point A in FIGURE 2, at the instant of com- 
mencement of the forming cycle, one finds that the volt- 
age rises directly to an oxygen generating plateau. This 
oxygenol voltage is described electrically as approximately 
1.45 volts. During this time the material on plate Y is 
reduced, evolving gaseous oxygen. In five to seven hours, 
the voltage rises to the hydrogen generation plateau, D. 
This hydrogenol voltage is  described electrically as ap- 
proximately 1.55 volts. At $ome time after the hydrogen 
plateau is reached, the forming current is reversed through 
the cell and the voltage falls to point B where it continues 
to point A, requiring the same amount of time as the time 
spent on the oxygen plateau. It than falls from point A to 
point E, the opposite oxygen plateau at approximately 
The time it remains on this plateau as shown by the 
dotted line is a function of the amount of electrolyte in 
the cell. At 8 given point in time, the opposite hydrogen 
plateau F will be reached at approximately -1.55 volts. 
The current is then interrupted and the graph returns to 
point C at which time the formation cycle is complete. 
At this point in time, the traces of residual oxygen are 
pumped out and the cell is sealed and ready for used. As 
shown by the graph, the hydragen plateaus in each di- 
rection must be reached before the current is reversed and 
then terminated. It is preferred (not shown by the graph) 
that the time spent on each hydrogen plateau be equal to 
the time on the respective oxygen plateaus. This preferred 
procedure has proven to give superior results as regards 
the reliability and stability of the formed cell. As is 
evident to one skill@ in the art, the capacity stability of 
the coulometer depends upon the removal of as much of 
the "free and mobile oxygen" as i s  possible. 
The magnitude of the forming current and the amount 
of time on each plateau in the foregoing example should 
not be construed 9s limiting in the formation cycle. The 
magnitude of the forming current and the corresponding 
variables, includiqg the amount of time on each plateau, 
may be varied so long as the formation of the cell is com- 
pleted in two directions including the attainment of the 
hydrogenol voltage im each direction. 
As many widely different embodiments of this inven- 
tion may be made without departing from the spirit and 
scope thereof, it is understood thqt the invention is not 
limited to the specific embodirnents thereof except as de- 
fined by the appended claims. 
-1.45 volts. 
P claim: 
11. An electrochemical coulometer cell comprising: 
(a) a sealed vessel containing a body of eiectrolyte; 
(b) a pair of electrodes mounted in contact with said 
electrolyte; 
(c) each electrode consisting essentially of an inert 
suppgrt and solid active material, said active ma- 
terial being present in both reduced and oxidized 
state ;; 
(d) said cell having been formed prior to sealing by 
(i) charging the vented vessel containing the elec- 
trodes therein with electrolyte to substantially 
flood the electrodes; 
(ii) passing forming current through the cell, 
whereupon an oxigenof voltage is realized, fur- 
ther passing the forming current through the cell 
at least until a hydrogenol voltage is attained; 
and 
(iii) thereafter reversing the forming current and 
repeating step ii and interrupting the current 
flow upon completion thereof and sealing the 
cell, 
2. The coulometer of claim 1, wherein the forming cup- 
rent is passed through the cell after the hydrogenol voltage 
25 is reached for a time equal to tho time said current is 
passed through the cell at the oxygenol voltage. 
3. The coulometer of claim I, wherein the active ma- 
terial is cadmium or a salt thereof. 
4. A mcthod of forming a coulometer cell comprised 
(a)  a vented vessel containing a body of electrolyte 
(b) a pair of electrodes mounted in contact with said 
electrolyte; 
(c) each electrode consisting essentially of an inert 
support and solid active material, said active ma- 
terial being present in both reduced and oxidized 
states; 
1" 
l .i 
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30 of: 
33 
(d) which method comprises 
(i) charging the vented vessel containing the elec- 
trodes therein with electrolyte to' substantially 
flood the electrodes; 
(ii) passing forming current through the cell, 
whereupon an oxygenol voltage is realized, fur- 
ther passing the forming current through the cell 
at least until a hydrogenol voltage is attained; 
and 
(iii) thereafter reversing the forming current and 
repeating step ii and interrupting the current 
flow upon comnletion thereof and sealing the 
5. The method of claim wherein the forming current 
is passed through the cell Iter, the hydrogenol voltage 
is reached for a time equal to the time said current is 
d through the cell at the oxygenol voltage. 
The method of claim 5 wherein the active material 
is cadmium or a salt thereof. 
40 
45 
60 4 1 .  
UNITED STATES PATENTS 
2,988,590 611961 Andre _ _ _ _ _ _ _ _ _ _ _ _ _  136-182 
3,302,091 1/1967 Henderson _ _ _ _ _ _  136-182 XW 
(io 
OUCLAS, Primary Examiner 
85 D. &. WAETON, Assistant Examiner 
U.S. 911. X.R. 
136-6; 32Q-48; 324-94 
70 


Electrochemical coulometer and method of forming same Patent

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