Nickel battery charging method and apparatus

Raising the charging acceptability of nickel battery. The setting of the initial SOC (state of charge) of hybrid electric vehicle battery to X% (for example 50%) is performed by two steps. The first one is a constant current charge until a fully charged state (S11, S12). Secondly, a discharging is performed until the target SOC of X% is obtained (S13, S14). This enables the interior part of the nickel positive electrode active particles to be assumed as a charged state so that during normal operation, charging and discharging can be performed relatively in the surface layer. This can increase the efficiency of oxidation-reduction in the nickel positive electrode, thereby raising the charging acceptability.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a charging method and apparatus for nickel 
batteries having a nickel positive electrode with nickel oxide as an 
active material, and more particularly to improvement of the input 
characteristics of nickel batteries. 
2. Description of the Related Art(s) 
Heretofore, various secondary batteries having different characteristics 
are known, among them an appropriate battery type is employed according to 
the purpose of the application. That is to say, the output and input power 
level and capacity of a battery are the most important characteristics for 
secondary batteries, and these characteristics differ greatly according to 
the purpose of the application for the battery. For example, hybrid 
electric vehicles (HEV) require batteries with high power input and output 
but low capacity. 
Hybrid electric vehicles operate small internal combustion engines (ICE) at 
a point of maximum efficiency to yield a constant average power. The 
operation of this sort of engine at optimum efficiency both improves fuel 
efficiency and reduces the amount of exhaust gas emissions. Furthermore, 
sufficient treatment of the exhaust gas emissions can be performed at a 
subsequent stage until the pollutants in the exhaust gas emissions reach a 
small and fixed amount, thereby enabling the emission of pollutants to be 
kept to a minimum. 
The secondary battery to be used in hybrid electric vehicles has the 
following two objectives. 
(1) To furnish the necessary power for the extra demand, such as during 
acceleration or hill climbing, when the required driving power exceeds the 
average output power of the engine. On the other hand, when the required 
driving power is less then the average output of the engine, the battery 
accepts the output power of the engine for charging. Therefore, the 
driving of the engine is maintained at an optimum point. For this reason, 
an output performance of high discharge power is required for the battery. 
(2) To increase the energy efficiency of the vehicle through the use of 
regenerative braking during deceleration by converting the braking force 
into electrical energy for use in charging the battery. For this reason, 
acceptability of high charging power is also required for the battery. 
To achieve these objectives, the battery for the hybrid electric vehicle 
must be capable, at any time during vehicle travel, of outputting and 
accepting high electric power. Thus, the state of charge (SOC) of the 
secondary battery for the hybrid electric vehicle is initially set to 
approximately 50%. As a result, the secondary battery is charged or 
discharged according to the output state of the driving power, such as for 
acceleration or deceleration of the vehicle. Generally, since an 
overcharged or overdischarged state must be avoided for the secondary 
battery, charging and discharging are controlled so that the SOC falls 
approximately within 20% to 80%. 
To increase the output performance and electric power acceptability of the 
battery, the capacity of the battery should be increased. However, hybrid 
electric vehicles carry a dynamo with the initial objective of reducing 
the capacity of the battery, and the basic requirement is to reduce the 
capacity of the battery as much as possible. Therefore, it is desirable 
for the battery in the hybrid electric vehicle to have a small capacity 
and to be capable of high electric power input and output. 
Nickel hydride batteries are a known type of secondary battery. The nickel 
hydride battery uses a positive electrode of nickel oxide and nickel 
hydroxide as the active materials and a negative electrode of hydrogen 
occluding alloy. The nickel hydride battery has high electric power output 
and electric power acceptability relative to its capacity, which is 
considered preferable as a battery for hybrid electric vehicles. 
Furthermore, nickel hydride batteries basically do not emit gases and are 
maintenance free. In this specification, the term nickel battery is 
understood to mean any battery using nickel oxide as the active material, 
such as nickel hydride batteries, nickel cadmium batteries, and so forth. 
The nickel hydride battery is disclosed, for example, in Japanese Patent 
Laid-Open Publication No. Sho 8-124596. 
In this manner, the nickel hydride battery is the preferred battery for 
hybrid electric vehicles. However, when various types of tests were 
actually conducted using nickel hydride batteries in hybrid electric 
vehicles, it was found that the acceptability of charging current was less 
than the demand. In the application of nickel hydride batteries in various 
types of electrical apparatus, the charging efficiency does not present 
much of a problem. In other words, some electric power loss during 
charging does not present much of a problem since the accurate state of 
charge during charging is not necessary to know and the battery is 
overcharged with electric power from commercial power sources. However, 
any loss during charging in hybrid electric vehicle batteries makes it 
difficult to control the battery SOC to be operated in predetermined SOC 
range. Therefore, there remains a problem where it is desirable to improve 
the acceptability of charging current for the battery. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to improve the acceptability of 
charging current by modifying the method for initial charging. 
The present invention is characterized by a charging method for a nickel 
battery having a nickel positive electrode with nickel oxide as the active 
material, providing the initial charging steps of a full charging step for 
fully charging, and after full charging, an initial discharging step for 
discharging the above-mentioned nickel battery until a predetermined 
amount of initial charge remains whereby execution of the initial charging 
steps causes charging to occur near the center of the material that forms 
the nickel positive electrode. 
This invention is also characterized by the above-mentioned nickel positive 
electrode which is formed from fine particles, where the particles have 
diameters from 1 .mu.m to 100 .mu.m. 
This invention is also characterized by charging and discharging with 
constant current in the above-mentioned full charging step and in the 
above-mentioned initial discharging step. 
This invention is also characterized by the above-mentioned nickel battery 
with nickel positive electrode being carried in a hybrid electric vehicle 
equipped with a dynamo for generating the charging current for the nickel 
battery and a motor that is driven by the discharging current from the 
nickel battery. 
Furthermore, this invention is characterized by a charging apparatus for a 
nickel battery, which has a nickel positive electrode with nickel oxide as 
the active material, comprising charging means for charging by supplying 
charging current to the above-mentioned nickel battery discharging means 
for discharging by causing discharging current to be released from the 
above-mentioned nickel battery full charge detecting means for detecting a 
full charge of the above-mentioned nickel battery, and initial discharge 
detecting means for detecting the completion of discharge until a 
predetermined amount of initial charge whereby in performing initial 
charging of the above-mentioned nickel battery, the above-mentioned nickel 
battery is discharged until a predetermined amount of initial charge after 
being initially charged to a fully charged state. 
In this manner, this invention uses one initial charging step to perform 
full charging, then discharging until a predetermined amount of initial 
charge. As a result, the positive electrode first completely turns into a 
nickel oxide material, the material of the charged state. Then, the 
positive electrode partially turns into a material of the discharged state 
(nickel hydroxide) from the subsequent discharging. At this time, the 
charging and discharging are performed in a direction from the surface to 
the interior. According to this invention, the positive electrode to its 
interior turns into the material of the charged state due to the initial 
full charge, and its outer shell turns into the material of the discharged 
state from the subsequent discharging. 
In the prior art, initial charging was terminated by charging until the 
amount of initial charge as the target. In this case, initial charging 
terminates in the middle of charging from the outer surface to the 
interior. Therefore, the outer shell of the positive electrode active 
particles had turned into the material of the charged state while the 
interior material remained the discharged state. If charging and 
discharging are repeated using this initial state, charging will occur at 
the interface of the material of the charged state located in the out 
shell (charged area) and the material of the discharged state in the inner 
core (discharged area). Therefore, charging does not occur until protons 
migrate from the surface to the interface. For this reason, the 
overpotential is high, making the side reaction more likely to occur, 
which reduces the charge efficiency. 
Furthermore, if the positive electrode is formed from particles, such as of 
nickel oxide, the charging efficiency can be improved by increasing the 
reaction area. In prior art, charging is performed at the interior, the 
area of the interface of the charged area and discharged area decreases. 
However, according to this invention, charging proceeds at the outer 
surface which can further raise the charging efficiency. 
In particular, when a battery is used in a hybrid electric vehicle, it is 
necessary for the battery to accept the high electric power that is 
generated from regenerative braking. Therefore, it is extremely important 
to raise the charging efficiency in the manner of this invention. Raising 
the charging efficiency raises the fuel economy of the hybrid electric 
vehicle and the accuracy of the battery state of charge control. 
Furthermore, this invention is applicable not only to nickel hydride 
batteries but also to batteries that use nickel oxide as the positive 
electrode active material, such as nickel cadmium batteries.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
Preferred embodiments of the present invention will be described with 
reference to the drawings. 
FIG. 1 is a block diagram showing a general configuration of a hybrid 
electric vehicle using a charging system concerning an embodiment of this 
invention. An engine 10 outputs the rotary power that is generated by an 
internal combustion engine, such as gasoline engine. A dynamo 12 converts 
the rotary power to electric power, where the output electric power is 
rectified and smoothed into DC electric power. The rotary power of engine 
10 is also transmitted to wheels 14, thereby causing the hybrid electric 
vehicle to move. 
A battery 16 accepts electric power from dynamo 12 for charging and 
supplies electric power to motor 18. Once the main switch is turned on to 
place the hybrid electric vehicle in a state ready for traveling, engine 
10 is always driven under constant conditions so that constant electric 
power is generated from dynamo 12. If the electric power from dynamo 12 is 
less than the power consumption of motor 18, battery 16 supplies the 
needed amount to the motor by discharging current. Conversely, if the 
electric power from dynamo 12 is larger than the power consumption of 
motor 18, battery 16 accepts the surplus electric power for charging. 
Furthermore, regenerative braking is used for braking in the hybrid 
electric vehicle and the regenerative electric power is output from motor 
18. The regenerative electric power charges battery 16. An induction motor 
is used, for example, for motor 18, and the DC electric power from battery 
16 is converted into a predetermined AC current by an inverter (not shown) 
to drive the induction motor. Therefore, the regenerative electric power 
is also output from the inverter. 
A nickel hydride battery is used for battery 16. The nickel hydride battery 
uses for the positive electrode a sintered positive electrode plate with 
nickel oxide (or nickel hydroxide) as the main active material. For the 
negative electrode, a hydrogen occluding alloy is used. Since the capacity 
of the negative electrode is made larger than that of the positive 
electrode, the battery capacity is determined by the capacity of the 
positive electrode. In this embodiment in particular, particulate nickel 
oxide (nickel hydroxide) is used for the positive electrode, and each 
particle undergoes oxidation-reduction respectively from charging and 
discharging. With smaller particle diameters, the surface area increases, 
thereby raising the charge-discharge performance. However, to maintain the 
shape of the positive electrode, a particle diameter of a certain size is 
necessary. In this embodiment, particle diameters of approximately 1 .mu.m 
to 100 .mu.m are employed. 
A battery control unit 20, connected to battery 16, detects and controls 
the state of charge (SOC) of the battery by detecting the discharging 
current from battery 16 and the charging current to battery 16. 
Charger/discharger 22 is provided separately from the hybrid electric 
vehicle and is connectable to a commercial power source. 
Charger/discharger 22 both rectifies AC electric power from the commercial 
power source into DC electric power at a predetermined voltage for output, 
and dissipates the electric power from battery 16, such as through a 
discharge resistor. Charger/discharger 22 may be carried in the hybrid 
electric vehicle, or dynamo 12 may be made to perform the function of the 
charger and motor 18 may be made to perform the function of the 
discharger. 
In this sort of configuration, charger/discharger 22 is not connected to 
battery 18 during normal traveling. Engine 10 is driven and the power 
consumption of motor 18 varies with the traveling conditions of the hybrid 
electric vehicle. In other words, the power consumption of motor 18 
increases during acceleration and decreases when the vehicle is stopped or 
traveling at a constant speed. Furthermore, during braking, regenerative 
electric power is generated by motor 18. Thus, the hybrid electric vehicle 
travels while battery 16 charges and discharges. 
Next, the initial charging of battery 16 in this embodiment is described 
with reference to FIG. 2. Charging and discharging in initial charging is 
performed using charger/discharger 22, however, dynamo 12 and motor 18 may 
also be used as described above. 
First, with charger/discharger 22 connected, battery 16 is charged (S11) by 
supplying a constant charging current to battery 16. Next, the SOC of 
battery 16 is detected by the battery control unit 20, and a judgment 
(S12) is made as to whether or not battery 14 is fully charged (SOC=100%). 
If battery 16 is not fully charged in S12, the operation returns to S11 
and charging is continued. The detection of the SOC of battery 16 may 
employ any suitable method, such as using a current integrating meter 
provided in battery charger/discharger 22 or detecting the fully charged 
state from the voltage of battery 14. 
When battery 16 is charged to the fully charged state and the judgment in 
S12 becomes YES, discharging is initiated (S13). Then, the SOC of battery 
16 is checked by battery control unit 22 and a judgment is made as to 
whether the SOC has reached a predetermined initial set value X% (S14). If 
SOC has not reached X% in S13, the operation returns to S13 and 
discharging is continued. In S14, SOC reaches X% and the judgment becomes 
YES so the operation for initial charging is terminated. 
The initial set value is assumed, for example, to be 50%. The reason is a 
value of this extent is preferable in order for battery 16 to have 
suitable discharging performance and current acceptability. The hybrid 
electric vehicle travels using battery 16 for which this sort of initial 
charging has been completed. During traveling, the SOC of battery 16 
varies with the traveling conditions. Generally, the output of motor 18 is 
regulated when the SOC approaches 20% so that the SOC during traveling is 
kept within a range of 20% to 80%. On the other hand, when the SOC 
approaches 80%, the generation of regenerative electric power is regulated 
through the use of a mechanical brake and the regenerative electric power 
is dissipated, such as through a discharge resistor. 
Therefore, in this embodiment, after being fully charged, battery 16 is 
discharged to set its SOC to the predetermined initial set value (X%). 
This is described next. 
FIG. 3(A) shows a conceptual view of the charged state of one particle of 
the positive electrode in initial charging. First, in state I of SOC=0%, 
the positive electrode particles are all in the discharged area 
(non-charged area) with the nickel being Ni(OH).sub.2. When charging is 
performed in this state, an oxidation reaction progresses from the surface 
of the positive electrode particles, protons (H.sup.+) are released from 
the surface, and the charged area (area of NiOOH) increases. In other 
words, the protons diffuse from the inner surface of the charged area 
(interface of charged area and discharged area) to the surface. In state 
II where charging has reached SOC=X%, the charged area assumes a donut 
shape from the surface. When charging is further continued and a full 
charge is attained, the entire particle turns into a charged area as shown 
in state III. 
Next, discharging is described with reference to FIG. 3(B). When 
discharging is performed from a fully charged state, protons are taken in 
from the surface of the particle and NiOOH in the charged area is reduced. 
As a result, the discharged area expands from the surface, and a 
donut-shaped discharged area forms around the perimeter with the center 
still charged (state I to state II). At this time, protons diffuse from 
the particle surface to the interface of the charged and discharged area, 
and NiOOH of the charged area is reduced. Then, when state III is reached 
for a completely discharged state, the entire particle becomes a 
discharged area. 
In the prior art, initial charging is terminated by charging until the 
amount of initial charge as the target, for example, by charging until 
SOC=50%. Therefore, the state at this time is given by state II in FIG. 
3(A). On the other hand, in the present embodiment, discharging is 
performed until a target amount of charge (such as 50%) after initially 
charging until a full charge. The state at this time is given by state II 
of FIG. 3(B). 
In this manner, the charged area gradually decreases in the prior art due 
to the discharging that occurs from near the surface. On the other hand, 
charging basically occurs at the inner surface of the charged area. 
Accordingly, the proton diffusion length increases during charging. 
On the other hand, when discharging is performed in this embodiment, the 
charged area decreases in the way shown in FIG. 3(C). The state of Soc 
decreasing from X% to Y% is shown in states I and II. Then, when charging 
is performed in this state, a charged area is formed on the surface of the 
particle separately from the charged area in the center. In other words, 
as shown in state III, charging occurs from the surface and the charged 
area is formed near the surface. In this state, discharging occurs from 
the outer surface of the charged area that was formed on the surface, and 
charging occurs from the inner surface. Therefore, during normal use, most 
of the reaction occurs at a thin area at the surface. Then, when 
discharging continues and the charged area at the surface disappears, 
discharging occurs from the surface of the charged area at the center and 
the charged area decreases. 
Basically, in this manner, the probability increases in this embodiment for 
the occurrence of charging and discharging near the surface of the nickel 
oxide particles which form the positive electrode. Therefore, the proton 
diffusion length during charging shortens and the surface area to be used 
by the reaction (area of interface of charged area and discharged area) 
can be increased. For this reason, the polarization of nickel electrode is 
lowered, thereby the acceptability of charging current is raised. 
EXAMPLE(S) 
Charge acceptability at different current magnitude is tested by repeating 
a predetermined charging-discharging pattern for a battery that has 
undergone initial charging according to the present embodiment and for a 
battery that has undergone initial charging according to the prior art. 
FIGS. 4(A) and (B) show the repetitive patterns for charging and 
discharging in the test that was conducted. FIG. 4(A) shows the pattern of 
current for charging and discharging, wherein one cycle comprises 
processes of charging at Ia for period ta, stopping the current for rest 
period ts, then discharging at Ic for period tc, and stopping the current 
for period ts. Thus, by sequentially repeating this cycle, charging and 
discharging with rest time ts were repeated. 
Accordingly, the electric charge Qai (AH) that is charged in one pulse 
becomes 
EQU Qai=Ia*ta (1) 
On the other hand, the electric charge Qci (A.multidot.H) that is 
discharged in one pulse becomes 
EQU Qci=Ic*tc (2) 
Therefore, the total amount of charge Qa and amount of discharge Qc in the 
entire sequence become 
EQU Qa=.SIGMA.Qai (3) 
EQU Qc=.SIGMA.Qci (4) 
If Qa=Qc is set, there should be no change in the SOC of the battery 
resulting from the execution of the above-mentioned sequence. The SOC of 
the battery is detected before and after the above-mentioned charging and 
discharging to determine the amount of change .DELTA.Q. 
The amount of charge to the battery can be checked by first charging the 
battery under constant conditions, then discharging the battery at a 
relatively small constant current. After the sequential test above is 
performed, the battery is discharged at the same relatively small constant 
current mentioned above. Checking the remaining capacity after the test 
enables .DELTA.Q to be detected. 
Once .DELTA.Q is determined, the charging efficiency (.eta.) can be 
determined from 
EQU .eta.=(Qa-.DELTA.Q)/Qa 
Rest time ts is provided to release the heat generated from the previous 
pulse so that the temperature is kept constant during charging and 
discharging with all pulses, and does not influence the calculation of 
charging efficiency. 
In the measurement of charging efficiency, the magnitude of current and the 
length of time are automatically controlled by the charger and the 
corresponding voltage is measured with the elapse of time. The behavior of 
the measured voltage is shown in FIG. 4(B), where Va is the pulse voltage 
while charging with pulse current and Vc is the pulse voltage while 
discharging pulse current. 
During the operation of charging and discharging through pulse current, V0 
exhibits two types of behavior according to the charged state of the 
surface. When the magnitude of the pulse current that was set can be 
sufficiently accepted, the relationship of Qai=Qci is maintained in the 
surface layer for all cycles, and there is no substantial change in SOC in 
the surface layer for all cycles. Since there is no change in the total 
SOC, V0 remains constant as shown by line P in FIG. 4(B). However, if the 
discharging from the surface layer of the particle advances while electric 
charge can be accepted in the interior of the active particle and the 
charging acceptability is lower than the magnitude of the charging current 
that was set, the electric charge that was actually accepted in the 
surface layer of the particle is less than that actually discharged so 
that SOC essentially decreases even though the condition of Qai=Qci is 
maintained. 
Under these sort of conditions, V0 gradually drops down (line S in FIG. 
4(B)). 
Examples of various tests are described in the following. In comparative 
examples, conditions different from those in the embodiment of this 
invention (amount of pulse current) are employed. The reason is if the 
amount of pulse current identical to that of this invention is used, the 
amount of heat generated would be high and would prevent stable tests from 
being conducted. Therefore, for the comparative examples, conditions are 
used where the amount of pulse current is lower and the charging 
efficiency rises. When the amount of pulse current in the comparative 
examples is made identical to that of the invention, the charging 
efficiency drops further. 
Example 1 
A sealed, maintenance-free nickel oxide/metal hydride battery (nickel 
hydride battery) having a rated capacity of 6.5 (25.degree. C.) AH and a 
rated voltage of 7.2 V was employed as a typical example of the present 
invention. The battery cell comprises a Ni oxide positive electrode with 
nickel oxide (and nickel hydroxide) as the active material, a negative 
electrode formed from hydrogen occluding alloy that can perform hydrogen 
absorption-desorption reactions, an alkaline electrolyte, and a separator. 
Battery 1 has its initial SOC set by the new method of this invention at 
40.degree. C. This new method includes two basic steps. The first step is 
a 7 AH (ampere-hour) charge at a rate of 1 C, with the actual accepted 
charge being 6.31 AH (detected by discharging to 6.0 V at a rate of C/3 
40.degree. C.). The second step is 3 AH discharge at a 1 C rate at 
40.degree. C. The result of these two steps was a final SOC of 3.31 AH. 
These two steps are shown in FIG. 5(B) as curves 1 and 2, respectively. 
For the nickel oxide positive electrode, the entire charge is actually 
stored in the inner core of the active particle. This is clearly 
illustrated in state II of FIG. 3(B) with X%=50%. 
For comparison, the initial SOC of battery 2 was set by the conventional 
method with a 3.35 AH (ampere-hour) charge at a 1 C rate. The actual 
accepted charge was measured as 3.29 AH by discharging to 6.0 V at a C/3 
rate. This is shown as curve 1 in FIG. 6(B). For the nickel oxide positive 
electrode, the entire electric charge is actually stored in the outer 
shell of the active particles. This is clearly illustrated in state II of 
FIG. 3(A) with X%=50%. 
After the initial SOC was set for battery 1 by the method of the present 
invention, a charging and discharging program was performed with a 
constant 100 A pulse current at 40.degree. C. One charging and discharging 
cycle comprises a 10 second charging period with the 100 A pulse current 
followed by a 10 minute rest, then a 10 second discharging period with the 
100 A pulse current followed by a 10 minute rest. Under these conditions, 
Qai is equal to Qci. The entire pulse current charging and discharging 
sequence causes a total amount of charge of 4.15 AH and a total amount of 
discharge of 4.16 AH to occur. The voltage corresponding to the 100 A 
pulse current charging and discharging program is shown in FIG. 5(A). V0 
is basically the same for all cycles. The value of V0 after pulse current 
discharging is slightly lower than after pulse current charging. The 
reason is that after pulse current discharging, V0 lowers because the 
electric charge in the surface layer decreases slightly. 
After this 100 A pulse current charging and discharging, the change in the 
SOC of battery 1 was checked by discharging it to 6 Vat a C/3 rate 
(discharging until the depth of discharge DOD=100%). The measured SOC is 
3.37 AH as shown by curve 3 in FIG. 5(B). Taking into account the maximum 
error of the charger (charger/discharger 20) of 0.3% at 400 A, the error 
of the charger becomes .+-.0.1 AH. Therefore, the difference between the 
final SOC and the initial SOC lies completely within the error range of 
the charger. This result clearly proves that a 100% charging efficiency 
can be obtained through 100 A pulse current charging at 40.degree. C. when 
the initial SOC is set using the method of the present invention. 
For battery 2, the initial SOC was set by the conventional method with 
initial charging until SOC=50% through 3.35 AH charging at a 1 C rate. In 
this case, the entire electric charge is stored in the outer shell, having 
a predetermined thickness, of the active particle. A 25 A constant pulse 
current charging and discharging program was performed at 40.degree. C. 
Except for the magnitude of the pulse current, the pulse duration times 
and rest times are the same as those for the above-mentioned battery 1. 
The total amount of charge is 2.01 AH and the total amount of discharge is 
2.01 AH. The voltage corresponding to the 25 A pulse current charging and 
discharging is shown in FIG. 6(A). Here, V0 decreases slightly indicating 
that the SOC of the surface has changed during pulse charging and 
discharging. 
A quantitative test of the change in SOC was conducted by discharging at a 
C/3 rate until 100% DOD, resulting in a discharge of 3.11 AH. This is 
shown as curve 2 in FIG. 6(B). Therefore, the change in SOC .DELTA.Q is 
-0.18 AH, which exceeds the error range of the charger. This decrease of 
SOC is caused by the low charge acceptability of battery 2 for which the 
initial SOC was set by the conventional method. The calculated charging 
efficiency is 91%. 
The result of this example shows that the method of the present invention 
for setting the initial SOC of the battery is extremely effective in 
raising the charge acceptability of the battery at 40.degree. C. 
Example 2 
For the same battery used in the above-mentioned example 1, initial 
charging was performed by the method of the present invention and by the 
conventional method. Initial charging was performed for battery 1 by using 
the two-step method of the present invention. In step 1, a 7 AH charge was 
performed at a 1 C rate at 40.degree. C. The actual accepted charge was 
6.23 AH as measured from discharging to 6.0 V at a C/3 rate (40.degree. 
C.). Step 2 is a 3 AH discharge at a LC rate at 40.degree. C. These two 
steps resulted in an SOC of 3.23 AH. The two steps are shown as curves 1 
and 2 in FIG. 7(B). For the nickel oxide positive electrode, the entire 
electric charge is actually stored in the center of the active particle. 
For comparison, the initial SOC of battery 2 was set by the conventional 
method. A 3.35 AH charge was performed at a 1 C rate at 40.degree. C. The 
actual accepted charge was measured at 3.26 AH from discharging to 6.0 V 
at a C/3 rate. This is shown as curve 1 in FIG. 8(B). 
After the initial SOC was set for battery 1 by the method of the present 
invention, a charging and discharging program was performed with a 
constant 100 A pulse current at 50.degree. C. One charging and discharging 
cycle comprises a 10 second charging period with the 100 A pulse current 
followed by a 10 minute rest, then a 10 second discharging period with the 
100 A pulse current followed by a 10 minute rest. Here, Qai is equal to 
Qci. The voltage corresponding to the 100 A pulse current charging and 
discharging is shown in FIG. 7(A). V0 is basically the same for all 
cycles. The total amount of charge is 3.59 AH and the total amount of 
discharge is 3.61 AH. 
After the 100 A pulse current charging and discharging program, battery 1 
was cooled to 40.degree. C. Then, battery 1 was discharged to 6 V at a 1 C 
rate. The measured amount of discharge is 3.28 AH. This is shown as curve 
3 in FIG. 7(B). In comparison with the initial SOC, the change in SOC 
after 100 A pulse charging and discharging is +0.05 AH. This result shows 
that for the battery, for which the initial SOC was set by the method of 
the present invention, the charge acceptability is 100% for a 10 second 
charge of 100 A pulse current at 50.degree. C. and 50% SOC. 
A comparative test for charge acceptability was conducted for battery 2 for 
which the initial SOC was set by the conventional method. After the 
setting of the initial SOC was completed by the conventional method at 
40.degree. C., the temperature of battery 2 was raised to 50.degree. C., 
and a constant 10 A pulse current charging and discharging program was 
performed for battery 2. This program comprises a number of charging and 
discharging cycles as in the above-mentioned case. Each cycle comprises a 
10 second charging period with the 10 A pulse current followed by a 5 
minute rest, then a 10 second discharging period with the 10 A pulse 
current followed by a 5 minute rest. FIG. 8(A) shows the voltage 
corresponding to the charging and discharging with the 10 A pulse current. 
The total amount of charge by the 10 A pulse current is 1.00 AH and the 
total amount of discharge by the 10 A pulse current is 1.00 AH (Qai=Qci). 
The final SOC of battery 2 after 10 A pulse current charging and 
discharging was quantitatively measured by discharging to 6 V at a C/3 
rate at 40.degree. C. The resulting amount of discharge is 2.82 AH. This 
is shown as curve 2 in FIG. 8(B). This calculation shows that the charging 
efficiency is only 55.1% in spite of the fact that the charging current of 
10 A is considerably lower than the requirement of the charge 
acceptability for hybrid electric vehicle's battery. 
Actually, all the conditions of example 2 are identical to those of example 
1, except that the temperature is 40.degree. C. in example 1 and 
50.degree. C. in example 2. From a comparison between examples 1 and 2, it 
is evident that the charge acceptability of battery 2, for which the 
initial SOC was set by the conventional method, is extremely sensitive to 
battery temperature. At elevated temperature, the charge acceptability of 
battery 2 greatly decreases. However, for battery 1, for which the initial 
SOC was set by the new method of the present invention, the influence of 
battery temperature is extremely small, and the high charge acceptability 
can be maintained even at elevated temperature. 
Example 3 
For the same battery used in the above-mentioned example 1, initial 
charging was performed by the method of the present invention and by the 
conventional method. Initial charging was performed for battery 1 using 
the two-step method of the present invention. In step 1, a 7 AH charge was 
performed at a 1 C rate at 40.degree. C. The actual accepted charge was 
6.18 AH as measured from discharging to 6.0 V at a C/3 rate. This is shown 
as curve 1 in FIG. 9(B). Step 2 is a 1.1 AH discharge at a 1 C rate at 
40.degree. C. This is shown as curve 2 in FIG. 9(B). Therefore, the actual 
SOC that was set by these two steps is 5.08 AH (approximately 82%). With 
the outer shell of the active particle in the discharged state, the entire 
electric charge was stored in the center. 
The initial SOC of battery 2 was set by the conventional method. A 5.45 AH 
charge was performed at a 1 C rate at 40.degree. C. The actual accepted 
charge was measured as 5.22 AH (approximately 84% SOC) by discharging to 
6.0 V at a 1 C rate. 
After the initial SOC of 82% was set by the method of the present 
invention, a charging and discharging program was performed with a 
constant 80 A pulse current at 40.degree. C. The pulse sequence includes 
the same ta, tc, and ts as in the above-mentioned examples 1 and 2. The 
voltage corresponding to 80 A pulse current charging and discharging 
program is shown in FIG. 9(A). The total amount of charge by the 80 A 
pulse current is 3.09 AH and the total amount of discharge by the 80 A 
pulse current is 3.09 AH. The final SOC after 80 A pulse current charging 
and discharging was measured as 5.00 AH by discharging to 6.0 V at a 1 C 
rate at 40.degree. C. This result is shown as curve 3 in FIG. 9(B). 
Therefore, the change in SOC is -0.08 AH, which is still smaller than the 
error range of the charger. It is evident that the charging efficiency of 
battery 1, for which the initial SOC was set at 80% at 40.degree. C. by 
the method of the present invention, is 100%. 
However, after the initial SOC of 80% was set by the conventional method 
for battery 2, a constant 40 A pulse current charging and discharging 
program was performed at 40.degree. C. The values of ta, tc, and ts are 
the same as those for the 80 A pulse current charging and discharging 
program for battery 1 in this example. The voltage of battery 2 
corresponding to the charging and discharging with the 40 A pulse current 
is shown in FIG. 10(A). The final SOC of battery 2 was measured at 4.69 AH 
by discharging to 6 V at 1 C rate at 40.degree. C. Therefore, the total 
change in SOC is -0.53 AH. This change in SOC is equivalent to a charging 
efficiency of 73%. 
This example shows that the new method of setting the initial SOC in the 
present invention is effective in raising the charge acceptability under 
high SOC conditions. 
Example 4 
In this example, the initial SOC of battery 1 was set by the method of the 
present invention with a charge distribution different from that in 
examples 1 and 2. This example sets the initial SOC in three steps. The 
first step performs a 7 AH charge at a 1 C rate at 25.degree. C. The 
actual accepted charge that was measured by discharging to 6.0 V at 1 C 
rate at 25.degree. C. is 6.62 AH. The second step is a 5.2 AH discharge at 
1 C rate at 25.degree. C. The third step is a 2 AH charge for the battery 
at 1 C rate at 25.degree. C. This results in an actual charge of 1.93 AH. 
Therefore, the final SOC that was set through these three steps is 3.34 
AH, of which 1.42 AH (21% of battery capacity) was stored in the center of 
the active particle, and the other 1.92 AH (29% of battery capacity) was 
stored in the outer layer of the active particle. This state is shown in 
state III of FIG. 3 (C), where Y%=21% and X%=50%. 
When the initial SOC was set, the temperature of battery 1 was raised to 
50.degree. C. and a 35 A constant pulse current charging and discharging 
program was performed. This pulse current charging and discharging program 
comprises a number of charging and discharging cycles, where each cycle 
comprises a 10 second charging period with the 35 A pulse current followed 
by a 5 minute rest, then a 10 second discharging period with the 35 A 
pulse current followed by a 5 minute rest. Therefore, the condition of 
Qai=Qci is satisfied. The voltage response for the 35 A pulse current 
charging and discharging cycle is shown in FIG. 11(A). The total amount of 
charge by the 35 A pulse current is 2.04 AH and the total amount of 
discharge by the 35 A pulse current is 2.04 AH. 
The final SOC after pulse current charging and discharging measured by 
discharging to 6 V at a 1 C rate is 3.34 AH, the value of which is 
identical to the initial SOC. This result shows that the charging 
efficiency is 100% at 50.degree. C. with an initial SOC of 50% (having a 
charge distribution of 21% in the center and 29% in the outer shell of the 
active particle).