JP2007026686A - Battery pack and charge control method for it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and a system for preventing overcharge of each cell in charging a battery pack constructed of cells connected in series and to provide the battery pack realizing the control method and the system. <P>SOLUTION: In this battery pack charge control method, the battery pack 10 including a charge controlling cell 5 with a lower salt level in addition to cells 3, in which a salt concentration in an electrolyte is set to a predetermined level, is used, and a voltage value of the charge controlling cell is monitored. When the voltage value of the charge controlling cell is lower than a predetermined voltage value, charge processing is carried out. When the voltage value of the charge controlling cell reaches the predetermined voltage value, charge processing is stopped. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a charge control method and a charge control system for an assembled battery including a plurality of secondary batteries connected in series. Furthermore, the present invention relates to an assembled battery whose charging can be controlled by the charging control method and the charging control system.

An assembled battery in which a plurality of secondary batteries are connected in series is increasing in importance as a power source for mounting on a vehicle or a power source for a personal computer and a portable terminal as a power source for obtaining a high output. In particular, an assembled battery in which a plurality of lithium secondary batteries that are lightweight and have a high energy density are connected in series is preferably used as a high-output power source.
By the way, an assembled battery mounted on an apparatus as described above is often charged as it is connected in series. However, individual batteries constituting the assembled battery (hereinafter referred to as “single cells”) have some variations in battery performance due to slight differences in conditions during battery manufacture and slight changes that occur during use. For this reason, an individual difference may arise in the charge capacity between single cells.
When a plurality of single cells having different charge capacities are charged in a serial state, a specific battery having a small charge capacity is charged first to be overcharged. And the performance of this specific battery selectively falls by overcharge. When the battery performance of a specific battery is lowered, the performance of the assembled battery itself including the battery is lowered, and the life of the assembled battery tends to be shortened.
When using an assembled battery as a power source for various devices (vehicles such as electric vehicles and hybrid vehicles, or portable terminals such as personal computers), it is important to ensure safety and extend the life for cost reduction. Protecting the battery from overcharging is a major challenge.

  In response to this problem, Patent Document 1 discloses a control method in which a monitor for measuring a terminal voltage is connected to each of the batteries constituting the assembled battery, and the charging is stopped by checking the full charge state for each battery. Is described. In such a control method, each battery needs to be provided with a switch necessary for switching, such as voltage monitoring and charging stop, and there is a problem that a circuit necessary for charge control becomes complicated and cost increases.

JP 2003-157908 A

  Therefore, the present invention has been made in view of the above problems, and its purpose is to prevent a specific battery from being selectively overcharged when charging an assembled battery composed of a plurality of unit cells connected in series. The present invention provides a charge control method for controlling the control method, a system that can effectively realize the control method, and a battery pack that can be applied to the method and system.

The present inventor has found that a battery in which the concentration of the supporting salt contained in the electrolyte is set low has a rapid voltage increase during charging, and has completed the present invention.
The battery pack charge control method provided by the present invention is a battery pack charge control method in which a chargeable / dischargeable secondary battery is a single battery and a plurality of the single batteries are connected in series.
In this method, the assembled battery includes at least one unit cell for charge control set to a salt concentration lower than the predetermined salt concentration in addition to the unit cell having a predetermined salt concentration in the electrolyte. Using the assembled battery, the voltage value of the charging control cell is monitored.
Then, according to the monitoring, a charging process is performed when the voltage value of the charging control cell is lower than a predetermined voltage value, and the charging process is performed when the voltage value of the charging control cell reaches a predetermined voltage value. It is characterized by stopping.

In this specification, “secondary battery” refers to a battery that can be repeatedly charged, and includes so-called storage batteries such as lithium secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, and lead-acid batteries.
In addition, the plurality of single cells included in the assembled battery are typically single cells of the same standard made up of substantially the same constituent elements (for example, positive electrode, negative electrode, electrolyte material, blending ratio, etc.). In addition, the unit cell used as the unit cell for charge control is different from other unit cells in that the salt concentration of the electrolyte is set low, but the other components may be unit cells configured according to the same standard. .
The charge control method having such a configuration is a method using the fact that the voltage increase of the low salt concentration battery (charge control cell according to the present invention) is faster than that of the other cells when charging. An assembled battery in which a low-salt concentration cell for charging control is incorporated in advance, stops charging when the voltage value of the charging control cell having a fast potential rise reaches a predetermined voltage value, so that other cells It is possible to prevent the battery from exceeding a predetermined voltage. Therefore, overcharging of each unit cell can be prevented and deterioration of the unit cell can be prevented without monitoring the voltage values of all the unit cells included in the assembled battery as in the prior art. As a result, the life of each individual cell is increased, and as a result, the life of the entire assembled battery is extended.

The present invention also provides an assembled battery charging control system.
The charge control system disclosed here is a battery pack charge control system in which a chargeable / dischargeable secondary battery is a single battery and a plurality of the single batteries are connected in series, and the charge control method described above is suitable. It is a system that can be implemented.
In the assembled battery applied to the present system, in addition to the unit cell having a predetermined salt concentration in the electrolyte, at least one unit cell for charge control set to a salt concentration lower than the predetermined salt concentration is used. One assembled battery.
This system includes a charging device that applies charging power to the assembled battery, and voltage measuring means that can measure the voltage value of the charging control cell. And a control unit connected to the charging device and the voltage measuring unit, wherein the battery pack is charged when the voltage value measured by the voltage measuring unit is lower than a predetermined voltage value, and the voltage value is A control unit that controls the charging process so as to stop the charging when the voltage value reaches the above value. That is, the system disclosed here is configured so that charging of the assembled battery can be controlled by the control unit.
According to the system having such a configuration, it is possible to appropriately charge the entire assembled battery by monitoring the voltage value of the charging control cell.

The control unit may further include a counter that counts an elapsed time after stopping charging. The system including such a control unit is set so that the voltage value of the charging control cell is measured by the voltage measuring means when the counter counts a predetermined elapsed time. The charging process is performed when the voltage value is lower than the predetermined voltage value, and the elapsed time count is reset when the voltage value is equal to or higher than the predetermined voltage value.
According to the system having such a configuration, the state of charge of the assembled battery after a predetermined period can be confirmed regardless of whether the assembled battery is used. Moreover, since the necessity of charge of an assembled battery can be confirmed by measuring the voltage value of the cell for charge control regularly, deterioration of an assembled battery by unnecessary charge processing can be prevented.

The present invention also provides an assembled battery that can be applied to the charge control method and the charge control system according to the present invention.
In the assembled battery disclosed herein, a plurality of single cells, which are chargeable / dischargeable secondary batteries, are connected in series, and the plurality of single cells have a single concentration of salt in the electrolyte. In addition to the above, at least one unit cell set to a salt concentration (low salt concentration) lower than the predetermined salt concentration is included.
According to the assembled battery having such a configuration, the above-described charge control method and charge control system can be suitably implemented.

The invention disclosed herein can be suitably applied particularly to a lithium secondary battery. That is, in a preferred aspect of the charge control method, the charge control system and the assembled battery provided by the present invention, the unit cell includes a positive electrode containing a positive electrode active material capable of occluding and releasing lithium ions, and occluding and absorbing lithium ions. A lithium secondary battery comprising a negative electrode containing a negative electrode active material that can be released and a non-aqueous electrolyte containing a lithium salt. Preferably, the lithium salt is lithium hexafluorophosphate (LiPF ₆ ). Particularly preferably, the salt concentration at the predetermined level is 1.0 to 1.5 mol / L, and the salt concentration at the low level is 50 to 80% of the predetermined level. More preferably, the low level salt concentration is 0.8 mol / L or less.
Lithium secondary batteries have high energy density and high output. For this reason, the assembled battery using a lithium secondary battery as a single battery can constitute a high energy density and high output assembled battery with a small number of single batteries. For this reason, the assembled battery itself can be reduced in size and weight, which is preferable.
Also, the battery pack comprising a LiPF ₆ lithium secondary battery including an electrolyte used as the lithium salt can be particularly suitably carried out in the charge control method and charge control system described above.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than the matters specifically mentioned in the present specification (for example, the salt concentration of the electrolyte of the battery for charge control) and matters necessary for the implementation of the present invention (for example, the type of power source in the charging device) Can be understood as a design matter of those skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

A battery pack according to the present invention is a battery pack in which a chargeable / dischargeable secondary battery is a single battery and a plurality of the single batteries are connected in series, and the electrolyte of another single battery is used as a charge control single battery. A unit cell including an electrolyte set to a salt concentration lower than the salt concentration is provided. Therefore, as long as the assembled battery according to the present invention has the above-described characteristics, there is no particular limitation on the contents of other components and manufacturing processes.
The secondary battery (unit cell) mounted on the assembled battery disclosed herein is not particularly limited as long as it is a chargeable / dischargeable storage battery. For example, a lithium secondary battery, a nickel hydride battery, a nickel cadmium battery, a lead storage battery, and the like can be given. In particular, since the lithium secondary battery has a high energy density and a high output voltage, a high-performance assembled battery can be realized even if the number of unit cells is small.

The setting of the salt concentration of the cell for charge control may vary depending on the applied secondary battery, the type of electrolyte salt contained in the electrolyte, and the like. For example, when charge control of an assembled battery using a lithium secondary battery as a single battery is performed (for example, when the electrolyte salt is LiPF ₆ ), the salt concentration of the main single battery of the assembled battery is 1.0 to 1.5 mol. In the range of / L, the charge control cell preferably has a salt concentration of 50 to 80% of the salt concentration of the main cell (preferably further 0.8 mol / L or less).
If the difference in salt concentration is too small, it is difficult to produce a difference in the rate at which the charging voltage rises between the charge control cell and another cell, making it difficult to achieve the effects of the present invention. Moreover, when the salt concentration of the cell for charge control is too low, the electric conductivity of the electrolyte may be excessively lowered, which is not preferable.
The effect of the charge control cell can be sufficiently achieved if one cell is included in the assembled battery, but a plurality of cells may be included. Moreover, the connection site | part of the cell for charge control in an assembled battery is not specifically limited, The aspect pinched | interposed into another cell may be sufficient, and the form arrange | positioned at a connection end part may be sufficient.

Further, in the battery pack charge control method and system disclosed herein, the charging process is performed when the voltage value of the charge control cell in the above-described aspect is lower than the predetermined voltage value, and the predetermined voltage value is obtained. Charging is stopped when it reaches it.
Note that the “predetermined voltage value” may vary as appropriate depending on the type of secondary battery (that is, a single battery) constituting the assembled battery. Preferably, the “predetermined voltage value” can be set to a voltage value within a range of a normal voltage (allowable voltage) assumed for a single battery (secondary battery) constituting the assembled battery. . Normally, charging is performed when the voltage is below the lower limit of the allowable voltage, and charging is stopped when the voltage value reaches the upper limit of the allowable voltage. The voltage value of the charging control cell can be measured using a known measuring device such as a digital multi-tester, for example.

Moreover, in the charge control system of the preferable form disclosed here, the control part is further provided with a counter that measures the elapsed time from when charging is stopped.
In a battery pack composed of secondary batteries, voltage drop due to self-discharge tends to occur regardless of use. In the system of the preferred form disclosed here, the elapsed time from the stop of the charging process is measured by a counter, and the charging process is performed according to the state of charge of the assembled battery when a predetermined period has elapsed. As a result, the output and energy density of the assembled battery are always kept at a high level.
The state of charge when the predetermined period has elapsed is determined based on the voltage value of the charging control cell included in the assembled battery, and the charging process is performed when the voltage is lower than the predetermined voltage value. When the voltage is higher than the predetermined voltage value, the elapsed time count is reset and the elapsed time is measured again. Note that the “predetermined period” may vary depending on the type and application of the secondary battery (that is, the single battery) constituting the assembled battery.

  The charge control method and system for an assembled battery disclosed herein can be implemented by a charge control unit cell having an electrolyte with a low salt concentration having a faster voltage rise during charging than other unit cells. Although the mechanism of the effect due to the low salt concentration of the electrolyte is not necessarily clear, for example, a substance such as a solvent-ion (dissociation ion of a supporting salt) or an ion-ion in the electrolyte, and / or collision or association of ions. It is considered that a rapid increase in voltage is realized because there is little interaction in the electrolyte with a low salt concentration.

The use of the assembled battery is not particularly limited, and examples thereof include a vehicle-mounted battery mounted in an electric vehicle or a hybrid vehicle, a portable terminal power supply such as a mobile phone or a notebook personal computer, or an emergency power supply battery.
The assembled battery has a configuration in which a plurality of single cells are connected in series, and it is sufficient that at least one single cell (charge control single cell) used for the charge control of the above-described aspect is included.
The number and shape of the single cells constituting the assembled battery may be determined in accordance with the use of the assembled battery (for example, power required for the device to be mounted and required capacity), and is not particularly limited. Examples of the shape of the unit cell include a square shape, a coin shape, a cylindrical shape, and a film exterior shape.

An example of a system that can effectively implement the battery pack charge control method of the present invention will be described with reference to FIG.
As shown in FIG. 1, an assembled battery charge control system 1 roughly includes an assembled battery 10 in which a plurality of single cells 3 and a single charge control single cell 5 are connected in series, and a single charge control unit. From the voltage measuring means 12 for measuring the voltage value of the battery 5, the charging power source 18 for performing the charging process on the assembled battery 10, and the electronic control unit (ECU) 14 as the control means for controlling the charging process. It is configured.
In addition, as shown with the broken line in FIG. 1, the electrical component 20 and the assembled battery 10 are not removed at the time of charging (for example, the assembled component 10 is a battery mounted on an electric vehicle and the electrical component 20 is a motor. Alternatively, in the case where the assembled battery 10 is a mobile phone battery pack and the electrical component 20 is a cellular phone body), the electrical component 20 remains connected even during charging.
The assembled battery charging control system according to the present invention can also be applied to a configuration in which the electrical component 20 and the charging power source 18 are shared. In the case where the electrical component 20 and the charging power source 18 are shared, it is not necessary to separately provide a charging power source. As a form in which the electrical component 20 and the charging power source 18 are made common, for example, a hybrid vehicle system can be cited. In the hybrid vehicle system, the motor has a function of an electrical component 20 to which power is supplied from a battery (assembled battery 10) and a function of a power supply 18 that supplies power to the battery (assembled battery 10). When the hybrid vehicle operates using the motor drive as a power source, the motor functions as an electrical component 20 that receives power from the battery (the assembled battery 10). When the hybrid vehicle decelerates or brakes, the motor operates as a generator that converts kinetic energy of the vehicle into electric energy, and functions as a power source 18 that supplies power to the battery (the assembled battery 10). When the charge control system according to the present invention is applied to a hybrid vehicle system, the motor serves as both the electrical component 20 and the power source 18, so that it is not necessary to separately provide a power source for charging.

The ECU 14 is electrically connected to each of the voltage measuring means 12 of the assembled battery 10, the charging power source 20, and the charging control cell 50. The ECU 14 selects an appropriate charging process according to the voltage value obtained by the voltage measuring means 12, and configures the system 1 so that the process is performed on the assembled battery 10 under the control of the ECU 14. Can do.
Further, the ECU 14 can include a counter 16 that measures a period after the previous charging process of the assembled battery 10 is stopped.

The operation of the system having such a configuration will be described with reference to the flowchart shown in FIG.
First, the voltage value of the charging control cell is measured by the voltage measuring means (S1). Then, the voltage value is determined (S2), and when the voltage value (V) is lower than the predetermined voltage value (V ₀ ) (Yes determination), the charging process is performed by applying power from the charging power source (S3). Then, the voltage value of the voltage control cell is monitored (S4), and when the voltage value reaches a predetermined voltage value (S5, Yes determination), charging is stopped (S6).
Next, using the counter provided in the control unit, the elapsed time from when the charging process was stopped is counted, and the voltage value of the charging control unit cell is again reached when it has reached a predetermined elapsed time (for example, one day). Is measured (S7). When the voltage measurement value is equal to or lower than the predetermined voltage value (S8, Yes determination), the charging process is performed in the same procedure as described above. When the voltage measurement value has reached the predetermined voltage value (S8, No determination), the count of the elapsed time after the charging is stopped is reset (S9), and the elapsed time is counted again until the predetermined time (S7). .
By repeating the above process, the state of charge of the assembled battery is suitably controlled.

  Next, several lithium secondary batteries (unit cells) equipped with electrolytes having different salt concentrations were produced, and the difference in charging characteristics due to constant power charging was evaluated.

<Production of lithium secondary battery>
The lithium secondary battery (18650 type) used in the test examples was produced as follows. That is, a carbon material as an anode active material (here, artificial graphite powder having an average particle size of about 10 μm was used) was mixed with carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) with ion-exchanged water, A paste-like negative electrode composition was prepared. The approximate mass ratio of each material (other than water) contained in this composition is 98% by mass for the carbon material, 1% by mass for CMC, and 1% by mass for SBR. This negative electrode composition was applied to both sides of a long copper foil as a negative electrode current collector to prepare a sheet-like negative electrode (negative electrode sheet) having negative electrode active material-containing layers on both sides of the negative electrode current collector. .

On the other hand, lithium nickel oxide (LiNiO ₂ ) as a positive electrode active material and acetylene black as a conductive material are mixed with polytetrafluoroethylene (PTFE) and CMC with ion-exchanged water to obtain a paste-like positive electrode composition. Prepared. The approximate mass ratio of each material (other than water) contained in this composition is 95% by mass of the positive electrode active material (LiNiO ₂ ), 3% by mass of acetylene black, 1% by mass of PTFE, and 1% by mass of CMC. is there. This positive electrode composition was applied to both sides of a long aluminum foil serving as a positive electrode current collector to prepare a sheet-like positive electrode (positive electrode sheet) having a positive electrode active material-containing layer on both sides of the positive electrode current collector. .

  As the separator, a long porous polyethylene sheet having a thickness of about 25 μm was used. The positive electrode sheet and the negative electrode sheet were stacked so as to face each other through this separator sheet, and this was wound in the longitudinal direction to produce a wound electrode body. After the obtained electrode body was housed in a bottomed cylindrical aluminum container, an aluminum battery lid was welded to the open end of the container. Here, the positive electrode sheet which comprises an electrode body is electrically connected with the positive electrode terminal which protrudes from a battery cover through the positive electrode current collection tab made from aluminum. Moreover, the negative electrode sheet which comprises an electrode body is electrically connected with the negative electrode terminal which protrudes from a battery cover through the copper current collection tabs.

As the electrolytic solution, a solution obtained by dissolving LiPF ₆ (supporting salt) in a 3: 7 (volume ratio) mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was used.
In addition, the density | concentration of the support salt of the battery which concerns on a present Example is 0.75 mol / L (sample No. 1), 1.0 mol / L (sample No. 2), and 1.25 mol / L (sample No. 1), respectively. It adjusted so that it might become 3).
The electrolyte was poured into a battery container from a through hole (a liquid injection hole) provided in the battery lid, and then the container was sealed. The lithium secondary battery thus assembled was aged for several days and then charged and discharged for several cycles. The battery after performing such aging and charging / discharging was used as a lithium secondary battery for testing (sample Nos. 1 to 3).
Using the test lithium secondary battery thus produced, a charge characteristic evaluation test shown in the following test example was performed.

<Test Example 1: Charging characteristic evaluation in 25 ° C., 600 W constant power charging>
Each of the test lithium secondary batteries (samples Nos. 1 to 3) having different salt concentrations was charged to 60% charge (SOC) under the condition of 25 ° C., and then subjected to 600 W constant power charging. The time to reach a voltage value of 2 V was measured. The results are shown in FIG.
As shown in FIG. 3, the sample No. 1 having the lowest salt concentration of 0.75 mol / L. In the battery No. 1, the voltage arrival time up to 4.2 V is 20 seconds. It reached about 5 seconds earlier than the batteries of 2 and 3.

<Test Example 2: Charging characteristic evaluation in 25 ° C., 200 W constant power charging>
The test lithium secondary batteries (Sample Nos. 1 to 3) were each charged to 60% charge (SOC) under the condition of 25 ° C. and then charged with 200 W constant power, and a voltage value of 4.2 V The time to reach was measured. The results are shown in FIG.
As shown in FIG. In each of the batteries 1 to 3, the voltage arrival time up to 4.2 V was about 200 seconds, and no significant change was observed in the charge characteristics of the individual batteries.

<Test Example 3: Charging characteristic evaluation in 0 ° C., 200 W constant power charging>
Each of the test lithium secondary batteries (Sample Nos. 1 to 3) was charged to 60% (SOC) under the condition of 0 ° C., and then charged with 200 W constant power, and a voltage value of 4.2V. The time to reach was measured. The results are shown in FIG.
As shown in FIG. The batteries Nos. 1 to 3 have the lowest sample number of 0.75 mol / L. In the battery No. 1, the voltage arrival time up to 4.2 V is 27 seconds. It reached about 10 to 1.5 seconds earlier than the batteries of 2 and 3.

<Test Example 4: Charging characteristic evaluation in 0 ° C., 150 W constant power charging>
Each of the test lithium secondary batteries (Sample Nos. 1 to 3) was charged to 60% charge (SOC) under the condition of 0 ° C., and then was charged with 150 W constant power and a voltage value of 4.2 V The time to reach was measured. The results are shown in FIG.
As shown in FIG. In each of the batteries 1 to 3, the voltage arrival time up to 4.2 V was about 200 seconds, and no significant change was observed in the charge characteristics of the individual batteries.

From the results of the above Test Examples 1 to 4, sample Nos. Provided with electrolytes having LiPF ₆ concentrations of 1.0 mol / L and 1.25 mol / L. 2 and 3 did not differ in voltage arrival time up to 4.2V.
On the other hand, under any temperature condition, when the charging power value is large (600 W at 25 ° C., 200 W at 0 ° C.), the sample No. provided with the electrolyte having a concentration of 0.75 mol / L. It was found that the voltage arrival time up to 4.2 V of 1 was faster than the other samples and could be used as a charge control cell.
Therefore, in order to control charging of a battery pack in which a plurality of battery cells are connected in series, a battery cell containing an electrolyte having a lower level of salt concentration than other battery cells is incorporated in the battery pack in advance. It has been confirmed that it is effective to use as a cell for charge control.

It is the schematic which shows schematic structure of an assembled battery and its charge control system. It is the flowchart which illustrated one operation | movement form of the charge control system of an assembled battery. 6 is a graph showing the results of Test Example 1 and Test Example 2. 6 is a graph showing the results of Test Example 3 and Test Example 4.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Charge control system of assembled battery 3 Single battery 5 Single battery for charge control 10 Assembled battery 12 Voltage measuring means

Claims (10)

A method for controlling charging of an assembled battery formed by connecting a plurality of the single cells in series, wherein the rechargeable secondary battery is a single cell,
The assembled battery includes at least one charge control unit cell set to a salt concentration lower than the predetermined salt concentration in addition to the unit cell having a predetermined salt concentration in the electrolyte. Prepare
Monitoring the voltage value of the charge control cell,
Here, according to the monitoring, when the voltage value of the charging control cell is lower than a predetermined voltage value, a charging process is performed. When the voltage value of the charging control cell reaches a predetermined voltage value, the charging process is performed. A charge control method for an assembled battery, characterized by stopping.   The unit cell includes a positive electrode including a positive electrode active material capable of inserting and extracting lithium ions, a negative electrode including a negative electrode active material capable of inserting and extracting lithium ions, and a non-aqueous electrolyte including a lithium salt. The method of claim 1, wherein the method is a battery. The predetermined level of salt concentration is 1.0 to 1.5 mol / L,
The method according to claim 2, wherein the low level salt concentration is a concentration of 50 to 80% of the predetermined level of salt concentration and a salt concentration of 0.8 mol / L or less. A system for controlling charging of an assembled battery formed by connecting a plurality of unit cells in series, wherein the rechargeable secondary battery is a unit cell,
The assembled battery includes at least one charge control unit cell set to a salt concentration lower than the predetermined salt concentration in addition to the unit cell having a predetermined salt concentration in the electrolyte. Is prepared,
A charging device for applying charging power to the assembled battery;
Voltage measuring means capable of measuring the voltage value of the single cell for charge control alone;
A control unit that controls the charging process so as to charge the assembled battery when the voltage value measured by the voltage measuring means is lower than the predetermined voltage value, and to stop charging when the voltage value reaches the predetermined voltage value. When,
With charge control system. The control unit is equipped with a counter that counts the elapsed time after charging is stopped,
When the counter counts a predetermined elapsed time, the voltage measuring unit measures the voltage value of the charging control cell, and when the voltage value is lower than the predetermined voltage value, a charging process is performed. The system of claim 4, wherein the elapsed time count is reset when the value is greater than or equal to a predetermined voltage value.   The unit cell includes a positive electrode including a positive electrode active material capable of inserting and extracting lithium ions, a negative electrode including a negative electrode active material capable of inserting and extracting lithium ions, and a non-aqueous electrolyte including a lithium salt. The system according to claim 4 or 5, which is a battery. The predetermined level of salt concentration is 1.0-1.5 mol / L,
The system according to claim 6, wherein the low level salt concentration is a concentration of 50 to 80% of the predetermined level of salt concentration and a salt concentration of 0.8 mol / L or less. A rechargeable secondary battery is a single battery, and a plurality of the single batteries are connected in series.
In addition to the unit cell whose salt concentration in the electrolyte is a predetermined level, the plurality of unit cells includes at least one unit cell set to a salt concentration at a level lower than the predetermined salt concentration. Assembled battery.   The unit cell includes a positive electrode including a positive electrode active material capable of inserting and extracting lithium ions, a negative electrode including a negative electrode active material capable of inserting and extracting lithium ions, and a non-aqueous electrolyte including a lithium salt. The assembled battery according to claim 8, which is a battery. The predetermined level of salt concentration is 1.0-1.5 mol / L,
The assembled battery according to claim 9, wherein the low level salt concentration is a concentration of 50 to 80% of the predetermined level of salt concentration and a salt concentration of 0.8 mol / L or less.

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