A method and apparatus are described for reconditioning batteries utilizing a dc-dc converter. During a discharge of the batteries, each cell is monitored by the converter. When the voltage of a cell decreases to a predetermined level, a converter will assume the load of this cell and inhibit the voltage from reaching zero, thereby preventing voltage reversal of that cell

Kapustka, R. E.

English

NASA Technical Reports Server

United States Patent [191 P11 4,084,124 
Kapustka [45] Apr. 11, 1978 \ 
[54] METHOD AND APPARATUS FOR 
CONDITIONING OF NICKELCADMIUM 
BATTERIES 
[75] Inventor: 
[73] Assignee: 
[21] Appl. No.: 
[22] Filed: 
Robert E. Kapustka, Huntsville, Ala. 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
744,573 
Nov. 24,1976 
[51] Int. (3.2 ................................................ HOW 7/00 
[52] U.S. CI .  .......................................... 320/9; 320/13; 
320/15; 320/32; 320/39 
[58] Field of Search ..................... 320/2, 9, 21, 31, 35, 
320/39, 40,48, 14, 15, 13, 32 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,305,754 2/1967 Oaks et al. ......................... 320/40 X 
3,454,859 7/1969 Ford et al. ......................... 320/31 X 
3,930,192 12/1975 Dinkler .............................. 320/21 X 
3,997,830 12/1976 Newel1 et al. ..................... 320/35 X 
Primary Examiner-Robert J. Hickey 
Attorney, Agent, or Firm-George J. Porter; J. R. 
Manning; L. D. Wofford, Jr. 
[571 ABSTRACT 
A method and apparatus for reconditioning batteries 
utilizing a DC-DC converter. During a discharge of the 
batteries, each cell is monitored by the converter. When 
the voltage of a cell decreases to a predetermined level, 
a converter will assume the load of this cell and inhibit 
the voltage from reaching zero, thereby preventing 
voltage reversal of that cell. 
11 Claims, 2 Drawing Figures 
f 32 N LOWVOLTAGE 
OUTPUTS BATTERY + TO CHARGER/REG!J@K)J 
RECON 01 T IO N I N G 
LOA0 RESISTOR I CELL I CELL 2 24/ 
RECONOITIONI NG 3 CONTROL RELAY 
IBATTERY-1 TO CHARGER/  
REG U L A T O  R 
w 
i T 0  CHARGER I' REGULATOR 
https://ntrs.nasa.gov/search.jsp?R=19780017588 2020-03-20T13:29:51+00:00Z
U.S. Patent April 11, 1978 4,084,124 
f 32 FIG. 1 .  N LOWVOLTAGE 
OUTPUTS BATTERY + TO CHARGER/RE@JLJQ Po 2 2 3  4 28,J 
CELL 
24 
RECONDITIONING 
LOAD RES1 STO R 
RECONDITIONING 
CONTROL RELAY 
IBATTERY-1 TO C H A R G E R /  ~ 
REG U LATO R 
T E L E M E T R Y  
TO CH A R GE R / REGULATOR 
/ 20 
FIG. 2 .  
OUTPUT 
VOLTAGE 
. 4  
.3 
. 2  
. I  
(VOLTS) 
. 5  
I I 
0 .5 I ( A M P E R E S )  
OUTPUT CURRENT 
4,084,124 
1 
METHOD AND APPARATUS FOR 
BATTERIES 
CONDITIONING OF NICKEL-CADMIUM 
ORIGIN OF THE INVENTION 
The invention described herein was made by an em- 
ployee of the United States Government and may be 
manufactured and used by or for the Government for 
governmental purposes without the payment of any 
royalties thereon or therefor. 
FIELD OF THE INVENTION 
The present invention relates to a method and appara- 
tus for reconditioning batteries and has particular appli- 
cation in the field of reconditioning nickel-cadmium 
batteries used in outer space applications. 
BACKGROUND O F  THE INVENTION 
Due to the unique problems of space exploration, 
different power sources had to be developed for power- 
ing the many electrical components which are utilized 
in space rockets, capsules or other devices. Many of 
these power sources contained a battery having a num- 
ber of nickel-cadmium sintered plate cells, since these 
cells have a high energy density and can be recharged, 
A primary factor in the power system that used a nickel- 
cadmium battery is the capacity degradation of the 
batteries. Since obviously, these batteries while used in 
space cannot be physically replaced, an approach must 
be developed which would extend the capacity life by 
periodically reconditioning the batteries. A prior 
method of reconditioning these batteries has been ac- 
complished by using relays to individually control the 
discharge of each cell. However, it has been determined 
that this method is not practical for inflight applications 
because of the bulk and complexity of using a relay, cell 
sensing and relay control circuitry for each individual 
cell. 
In developing a method which can be utilized in 
inflight applications, it should be remembered that the 
voltage of a nickel-cadmium cell should not be reversed 
during a discharge since there is a chemical reaction 
which produces hydrogen gas. This gas can build up 
considerable pressure in a sealed cell or cause a vented 
cell to emit the hydrogen gas. Either situation can be 
detrimental to the battery plus hazardous to any person- 
nel which are in the area. Since these cells are never 
identical, there will always be some cells which would 
go through zero and reverse before other cells are dis- 
charged. Therefore, a method must be developed which 
prevents the cell reversal of the nickel-cadmium bat- 
tery. 
The need for reconditioning a nickel-cadmium bat- 
tery is important since it has been demonstrated that as 
the result of “memory” or “fading” the effective energy 
capacity of the nickel-cadmium battery can be de- 
creased by as much as 60-80% under operating condi- 
tions. ’ 
A review of the prior art indicates that while it is old 
to increase the life of a nickel-cadmium battery by re- 
conditioning, no prior art reference was discovered 
which efficiently monitors each cell of the battery for 
preventing cell reversal. A typical prior art reference is 
British patent 1,256,980 issued to Brinsley Hodge. This 
patent is directed to a method for restoring the electri- 
cal capacity of nickel-cadmiuy cells by subjecting each 
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cell to a complete discharge by connecting a resistance 
means across its electrodes. Additional prior art devices 
include British patent 864,844 issued to John Jephcott 
and French patents 1,203,578 and 1,217,578. However, 
none of these references disclose a device which e%- 
ciently monitors each cell of a nickel-cadmium battery 
to prevent the cells of the battery from reversing polar- 
ity. 
SUMMARY OF THE INVENTION 
The present invention, as described herein, is directed 
to a method and apparatus for reconditioning a nickel- 
cadmium, or for that matter, any battery, by using a 
DC-DC converter to monitor the voltage of each cell in 
the battery. When the voltage of the cell is reduced to 
a predetermined small voltage, the converter assumes a 
portion of the load on that cell and thus prevents the 
voltage of that cell from ever becoming zero. Since the 
cell voltage is never zero, it would thus be impossible 
for the cell to reverse its polarity. 
BRIEF DESCRIPTION OF THE DRAWING 
These and other features of the present invention may 
be more fully appreciated when considered in light of 
the following specification and drawings, in which: 
FIG. 1 is a oircuit diagram of the apparatus of the 
present invention; 
FIG. 2 is a graph of typical low voltage outputs of the 
DC-DC converter having input voltages of 24 and 32 
volts. 
DETAILED DESCRIPTION OF THE 
PREFERRED EMBODIMENT 
As shown in FIG. 1, a DC-DC converter 10 is used to 
recondition a battery 32 containing a plurality of cells 1, 
2 . . . N-1, N. The converter 10 is managed by a 
DC-DC converter control box 12 containing the stan- 
dard and necessary control electronics to establish the 
converter frequency of operation, the necessary drive 
and the required commands. Transistors 14 and 16 are 
utilized as power switches and are alternatively turned 
on and off as is common in any typical converter. A 
transformer 22 containing primary and secondary wind- 
ings is connected to the power switches 14, 16 and 
provides the proper turns ratio to convert the input 
voltage (V,) according to the level and number of low 
voltage outputs required for the battery cells. These 
outputs contain two rectifying diodes 28 and 30 con- 
nected to a single cell of the battery. Each cell of the 
battery is connected to only one low voltage output. 
The turns ratio of the transformer is configured so 
that the open circuit output voltage of each of the N 
low voltage outputs is lower than the battery cell volt- 
age for normal operations (for example, less than 0.7V 
dc). Therefore, since the open circuit output voltage is 
less than the battery cell voltage, the cell voltage will 
reverse bias (turn off) the two diodes in each output. 
With the diode off, no power is drawn in the secondary 
and only a very low “stand by” power is drawn in the 
primary of the transformer. When the voltage in a sin- 
gle cell becomes slightly less than the voltage of its 
Corresponding low voltage output, the voltage from the 
transformer winding would forward bias the rectifier 
diodes. When this occurs, the converter assumes the 
load of the low capacity cell and the input current of the 
converter is increased as a result of the added load. 
Therefore, it can be seen that the converter senses the 
output of each of the cells of the battery during its 
4,084,124 
3 
normal operation and if a single cell falls below a prede- 
termined voltage set according to the voltage of the low 
voltage outputs, this cell is effectively taken out of the 
system and the converter assumes the load for that cell. 
Since the voltage of each cell cannot become zero, it is 
impossible for a polarity reversal to occur. 
This application of a DC-DC converter has several 
inherent advantages. For example, if one cell in the 
battery has degraded more rapidly than the others, it 
can be protected from voltage reversal by the converter 
and thus allow operational uti!ization of more of the 
capacity in the other cells. Furthermore, even if the 
converter were not designed to carry the full opera- 
tional load of a cell, it could be used to detect the pres- 
ence of a low voltage cell, thereby allowing automatic 
removal or reduction of the operational load on the 
battery. 
As one or more cells lose capacity and their voltages 
drop below the converter output voltage, the converter 
will assume a large portion of the load. Depending upon 
the system requirements, the converter could continue 
to assist the “weak” cells until a certain fraction (for 
example, 1/5 to f )  of the cells needed help. A current 
sense resistor 18 and input current sense control 20 can 
be used to supply analog or digital signals to a remote 
point for information and decision making. If the bat- 
tery is utilized in an inflight application, the current 
information would be sent through standard telemetry 
to the ground control. According to the information 
receivzd, a decision can now be made whether to termi- 
nate the use of the battery, reduce the battery discharge 
rate, increase the battery charge current or initiate the 
battery reconditioning. Since the input current (the 
current in the primary winding) above the ambient 
current level is indicative of the current drawn in the 
secondary caused by the addition of a load due to a low 
cell voltage, this current would also indicate the num- 
ber of weak cells. Decisions may also be made automati- 
cally, based upon previously established criteria, and 
discrete signals transmitted to a charger and/or regula- 
tor for system operation. 
When the decision has been reached to recondition 
the battery, a signal is sent to remove the battery from 
its energy source and load (typically a charger and 
regulator in space flight). A reconditioning control 
relay 26 is then closed placing a reconditioning load 
resistor 24 across the battery. Each battery cell then 
continues to discharge as long as the load is present and 
the cell’s voltage was above the converter low voltage 
outputs voltage. No cell could be reversed, Le., driven 
to the opposite polarity since, as each cell voltage fell 
below the level of the converter low voltage output 
voltage for the cell, the converter assumes the load. 
This limits the minimum voltage to which the cell 
would fall. This mode of operation would continue until 
all cells had reached some minimum voltage and/or 
some definite time limit had elapsed. After completion 
of the reconditioning operation, the reconditioning con- 
trol relay 26 is then opened and the battery is recon- 
nected to the bus or the primary energy source through 
a current limit and recharged at a trickle to some pre- 
scribed level (such as 30-80% of rated capacity). By use 
of this method, the cell voltage is reduced to just a few 
tenths of a volt and cell reversal is prevented. The type 
of charging used is immaterial and methods such as 
constant current, constant potential and modified con- 
stant potential can be utilized. Additionally, the type of 
charging equipment is also immaterial. After the battery 
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has been fully charged, it is then returned to normal 
system operation. 
FIG. 2 shows the electrical characteristics of a typi- 
cal low voltage output of the DC-DC converter. The 
input current to the DC-DC converter is the current in 
the primary winding of the converter transformer. Due 
to the specific turns ratio, this primary current is pro- 
portional to the secondary c s in the low voltage 
outputs. The current in each 
until the voltage of the cell 
enough to allow the voltage from the transformer wind- 
ing to forward bias the rectifier diodes. This graph 
shows the current conducted through the rectifier di- 
odes (the “output current”) versus the voltage of the 
cell it is connected to (the “output voltage”) for con- 
verter input voltages of 24 and 32Vdc. The main point 
to be emphasized is that before a cell reaches 0 volts, it 
will cause current to be conducted in a transformer 
output winding associated with that cell. This current is 
such that it relieves the cell from carrying all the battery 
discharge current and also reflects itself as increase in 
the current in the primary winding. 
Although this invention has been described with 
reference to a nickel-cadmium battery, it should not be 
construed to be so limited. The circuit of the present 
invention would be applicable to any battery in which it 
is desirable to prevent cell voltage reversal during nor- 
of the present invention does 
type of battery charger or PO 
tery is disconnected from the 
ditioning period. The recond 
battery be discharged to a very low voltage, therefore, 
the charger must be shut off or removed to allow thisto 
be accomplished. In the same sense, it does not matter 
what the battery power is normally used for. It could 
feed its DC voltage directly to a power bus or to a 
regulator which would then supply whatever kind of 
voltage, AC or DC, which is required by the power 
system. Furthermore, this particular type of recondi- 
tioning is not limited to just inflight applications, but 
may be utilized even in a ground,application if it were 
desirable to have a compact, reliable circuit to accom- 
plish reconditioning and/or low cell voltage detection. 
While this invention has been described in great de- 
tail, it would be obvious to those skilled in the art that 
various changes may be made without departing from 
the scope of the invention and the invention is not to be 
considered limited to what is shown in the drawing and 
described in the specification. For example, the DC-DC 
converter need not be constantly connected across the 
battery, but can be activated after the battery has been 
in operation for a certain amount of time. 
What is claimed is: 
1. A method for reconditioning a battery, having a 
plurality of cells, connected to an energy source com- 
prising the steps of: 
connecting across each cell a means for assuming the 
load of each cell when the voltage of each cell is 
reduced to a predetermined value; 
removing the battery from the energy source; 
placing a load across the battery; 
allowing the cells of the battery to discharge until all 
of the cells have reached said predetermined value; 
removing said load from across the battery; and 
recharging the battery. 
4,084,124 
5 6 
2. A method for reconditioning a battery according 
to claim 1 wherein a DC-DC converter is utilized in 
said connecting step. 
3. A method for reconditioning the battery according 
input current of the DC-DC converter to properly 
determine the correct time to remove the battery from 
its energy source for initiating the reconditioning pro- 
cess. 
4. A method for reconditioning a battery, having a 
plurality of cells, connected to an energy source com- 
prising the steps of: 
connecting across each cell a means for assuming the 
its energy source for initiating the reconditioning pro- 
cess. 
7. A device for reconditioning a battery having a 
plurality of cells comprising: 
to claim 2 further including the step of monitoring the monitoring means containing a plurality of outputs, 
one output placed in parallel with a single cell of 
the battery, for sensing the voltage of each of the 
cells; 
a control relay placed all of the cells of the 
a load resistor placed in series with said relay 
whereby each of the outputs of said monitoring 
means senses the voltage of each cell and if the 
voltage of each cell reaches a predetermined value, 
said monitoring means assumes the load thus pre- 
venting the voltage in each cell from becoming 
zero. 
8. A device for reconditioning a battery according to 
claim 7 wherein said monitoring means is a DC-DC 
9. A device for reconditioning a battery according to 
claim 7 further including a means for sensing the input 
current of said monitoring means. 
to claim 4 wherein a DC-DC converter is utilized in 25 to claim 7 further including a transformer for convert- 
said connecting step. ing the input voltage of said monitoring means to the 
6. A method for reconditioning a battery according level and number of outputs required for the battery. 
to claim 5 further including the Steps Of monitoring the 11. A device according to claim 7 wherein the battery 
input current of the DC-DC converter to properly is of the nickel-cadmium type. 
10 battery; 
load of each cell when the voltage of each cell is 15 
reduced to a predetermined value; 
removing the battery from the energy source; 
placing a load across the battery; 
allowing the cells of the battery to discharge for a 
predetermined period of time; 
removing said load from across the battery; and 
recharging the battery. 
20 converter. 
5. A method for reconditioning a battery according 10. A device for reconditioning a battery according 
determine the correct time to remove the battery from 30 * * * * *  
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Method and apparatus for conditioning of nickel-cadmium batteries

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