JP2007089363A - Secondary battery charging method and apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To fully charge all cells of a battery pack while suppressing an increase in the maximum value of battery temperature.
In a charging method of a secondary battery for charging an assembled battery in which a plurality of secondary battery cells are connected in series, the secondary battery is charged until the secondary battery is fully charged, and for a short period of time immediately after the completion of charging. A preparatory discharge is performed for the discharge current.
[Selection] Figure 2

Description

  The present invention relates to a secondary battery charging method and apparatus, and more particularly, to a battery pack charging method and apparatus in which a large number of secondary batteries are connected in series or in parallel.

  A battery pack in which a large number of secondary batteries are connected in series or in parallel is widely used as a power source for various machines. In recent years, battery capacity and density have been rapidly increasing. For example, when it becomes a large-capacity battery pack used for a power source of an electric vehicle or the like, 100 or more batteries (hereinafter referred to as cells) are connected in series.

  The problem with this type of battery pack is that the self-discharge rate of each cell varies. Since this self-discharge rate varies, a contrivance is required for the charging method in order to fully charge all the cells.

The above problems will be described in more detail with reference to FIGS.
FIG. 4A shows the charging rate of each cell, taking as an example a battery pack consisting of 20 cells. When a new battery pack is fully charged, the charging rate of each cell is 100%.
If the battery pack is left unattended or is used as a power source and is repeatedly charged and discharged without being fully charged, the charging rate may eventually become as shown in FIG. FIG. 4B shows a state in which the charging rate of each cell is approximately half, 50%. However, since the individual cells constituting the battery pack have a difference in self-discharge rate, The cells have slightly different charge rates. FIG. 4 (c) shows the charging rate of each cell when the assembled battery is further discharged from the state of FIG. 4 (b) until reaching a predetermined discharge final voltage. In this case, some cells are in an overdischarged state.

  Next, when the battery packs having variations in the self-discharge rate of each cell as described above are charged with a charge rate of 100%, the result is as shown in FIG. That is, a cell that has already had a low charging rate at the time when the variation occurs is not fully charged.

  Thus, in order to avoid the disadvantage that the cells constituting the battery pack are not fully charged due to the variation in the self-discharge rate, as shown in FIG. , Charging a little extra until all cells are fully charged. In this case, each cell is in a state where the charging rate exceeds 100%.

  In the conventional method of charging more cells until all the cells are fully charged, the battery temperature increases first. If the temperature of the battery is too high, self-discharge will be accelerated and the electric energy charged to full charge will be consumed wastefully, and the chemical reaction will proceed inside the battery and the temperature will continue to rise. The final temperature rises, and the battery deteriorates and adversely affects the life.

  In this regard, large-capacity batteries used for power sources such as electric vehicles may have a heat dissipation condition or be dealt with by a cooling system. However, as with small and medium-capacity batteries used for scooters, robots, etc. In particular, it is particularly problematic in the case of batteries that do not incorporate a cooling system.

  Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art, suppress the rise of the battery temperature peak, and charge the secondary battery so that all the cells of the battery pack can be fully charged. To provide an apparatus.

  In order to achieve the above object, a charging method according to the present invention is a secondary battery charging method for charging a battery pack in which a plurality of secondary battery cells are connected in series or in parallel, and the secondary battery is fully charged. And a discharge step of performing a short-time preparatory discharge with a predetermined discharge current immediately after the completion of charging.

  Further, a charging device according to the present invention includes a charging circuit for charging the secondary battery, a charging circuit for charging the assembled battery in which a plurality of secondary battery cells are connected in series or in parallel, a predetermined circuit, A pre-discharge circuit for performing short-time pre-discharge with the discharge current, a switch for opening and closing the charging circuit and the pre-discharge circuit, and charging the battery according to a predetermined operation sequence after the battery is fully charged. And a control means for opening the circuit and closing the preparatory discharge circuit at a predetermined timing to execute the preparatory discharge.

  According to the present invention, by performing a preparatory discharge immediately after the completion of charging, the oxygen gas generated in the positive electrode during the charging process is absorbed into the positive electrode by the discharge, and as a result, recombines with hydrogen to generate water. Since the reaction is suppressed, an increase in battery temperature peak can be suppressed. For this reason, all the cells of the battery pack can be fully charged evenly without any problem due to temperature rise.

Hereinafter, an embodiment of a charging method and apparatus for a secondary battery according to the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a circuit diagram of a secondary battery charging apparatus according to the present embodiment. In FIG. 1, reference numeral 10 indicates a battery pack of a secondary battery to be charged. The cells constituting the battery pack 10 are nickel-metal hydride batteries, and many cells are connected in series. Reference numeral 12 denotes a power supply unit that supplies power necessary for charging. Reference numeral 14 is a controller that controls the sequence of charge / discharge circuit switching. This controller switches the first switch 16 and the second switch 18 using transistors on and off according to a sequence programmed in advance. In this case, when the battery pack 10 is charged, the first switch 16 is turned on and the second switch 18 is turned off to form a charging circuit. As will be described later, when preparatory discharge of the battery pack 10 is performed after charging is completed, the first switch 16 is turned off and the second switch 18 is turned on to switch to the preparatory discharge circuit.

  Next, FIG. 2 is a time chart showing the relationship between the change in voltage and temperature of the battery pack 10 during the charging method according to the present embodiment and the switching operation between charging and discharging.

In the time chart of FIG. 2, T0 indicates a point in time when charging of the battery pack 10 is started. At the same time as the first switch 16 is turned on, the second switch 18 is turned off and charging is started. T1 indicates a point in time when the charging of the battery pack 10 is completed. As the charging progresses, the battery temperature and voltage increase, and the degree of increase increases as the battery approaches full charge. At the time T1 when charging is completed, the controller 14 detects that the voltage satisfies a predetermined termination condition, turns off the first switch 16 to open the charging circuit, stops charging, and simultaneously with stopping charging, 2 The switch 18 is turned on to close the preparatory discharge circuit, and a short preparatory discharge is performed. In this embodiment, the discharge current of the preparatory discharge is passed for 90 seconds at a 2-hour rate.

As described above, by performing a small amount of preparatory discharge immediately after the completion of charging, the temperature increase after charging can be suppressed as follows.
In FIG. 2A, a curve 100 indicated by a thick line indicates a change in battery temperature when the preliminary discharge according to the present embodiment is performed, and a curve 102 indicated by a thin line indicates the battery temperature when the preliminary discharge is not performed. Showing change.

  The change in the battery temperature shows a characteristic that the temperature rapidly rises as the charging is completed, and continues to increase even after the time point T1 when the charging is completed to reach the maximum value.

  Even after charging is complete, the battery temperature continues to rise because the chemical reaction that results from the combination of oxygen generated at the positive electrode and hydrogen at the negative electrode inside the battery to become water continues after charging is complete. It is considered that the battery temperature rises due to reaction heat. And what is lost by the temperature rise is a part of the energy stored in the battery by charging.

  On the other hand, it can be seen that the maximum value of the battery temperature when the preparatory discharge is performed after the completion of charging is greatly reduced by performing the preparatory discharge. In the case of this embodiment, even if charging was performed under the same conditions except for the preparatory discharge, it was possible to confirm the effect of reducing the peak temperature by about 4 ° C. by performing the preparatory discharge.

It is considered that there is the following relationship between the effect of performing the preliminary discharge and the effect of lowering the maximum battery temperature.
In FIG. 2A, when preparatory discharge is not performed, if ΔT is the difference between the maximum temperature and the battery temperature immediately after the completion of charging, the thermal energy of (ΔT × battery heat capacity C) is equal to the battery temperature after charging. Lost by rising.

  On the other hand, when preparatory discharge is performed, oxygen gas generated from the positive electrode during the charging process is absorbed by the positive electrode by the discharge, and as a result, the chemical reaction that recombines with hydrogen and generates water is suppressed. It is predicted. It is thought that the maximum temperature of the battery temperature has decreased by the amount of recombination of oxygen and hydrogen suppressed from the time of completion of charging. In other words, oxygen gas, which is the cause of heat generation, can be absorbed by performing a preparatory discharge with a discharge current that is less than a considerable amount of heat energy that should be lost immediately after the completion of charging, so it should be lost as heat. It is thought that the temperature reduction effect is obtained by using energy effectively.

  Further, in view of the action of such a preparatory discharge, it is preferable that the preparatory discharge is executed immediately after the completion of charging until the battery temperature reaches the maximum value.

  As described above, according to the charging method of the present embodiment, since the battery temperature rise after the completion of charging can be suppressed, even if charging a little more, each cell is evenly distributed without losing the battery life as a result. Can be charged. In addition, there is a great effect in shortening the waiting time for cooling.

  By the way, there is a concern that the voltage may decrease as a result of preparatory discharge after the completion of charging, as a result of consuming electric energy of the battery.

  However, as shown in FIG. 2B, the decrease in the open-circuit voltage after the end of charging is faster in the case of no preparatory discharge (curve 106) than in the case of performing preparatory discharge (curve 104). I understand that. This shows that the electrical energy stored by charging is nothing but the energy that should be lost in vain due to the temperature rise, and is used to suppress the rise in battery temperature by performing a preparatory discharge. ing.

  Moreover, there is a concern that the discharge capacity of the battery may be reduced by performing the preparatory discharge as compared to the case without the preparatory discharge. However, the opposite is actually the opposite. FIG. 3 is a graph showing discharge characteristics of the charged battery pack 10. In FIG. 3, when the preparatory discharge is performed is represented by a thick curve, and when the preparatory discharge is not represented by a thin line, it can be seen that the discharge characteristics are slightly higher for the battery that has undergone the preparatory discharge.

  As described above, the present invention has been described with reference to the embodiment applied to an assembled battery in which a large number of secondary battery cells are connected in series. However, the present invention is similarly applied to an assembled battery in which secondary battery cells are connected in parallel. Of course, it can be applied.

The circuit diagram which shows the charging circuit which implements the charging method of the secondary battery by this invention. 4 is a time chart showing the relationship between charging / discharging switching timing, battery temperature change, and voltage change in the secondary battery charging method according to the present invention. The graph which shows the change of the discharge capacity of the battery charged by the charging method of the secondary battery by this invention. The graph which shows the dispersion | variation in the self-discharge rate in the conventional secondary battery. The graph which shows the dispersion | variation in a charge rate when it overcharges in order to cope with the dispersion | variation in the self-discharge rate of a secondary battery.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Battery pack 12 Power supply unit 14 Controller 16 1st switch 18 2nd switch

Claims (5)

In a secondary battery charging method for charging an assembled battery in which a plurality of secondary battery cells are connected in series or in parallel,
Charging the secondary battery until it is fully charged;
A method for charging a secondary battery, comprising a discharging step of performing a short time preparatory discharge with a predetermined discharge current immediately after completion of charging.   2. The method of charging a secondary battery according to claim 1, wherein the preparatory discharge is performed immediately after completion of charging and before the battery temperature reaches a maximum peak value.   2. The method of charging a secondary battery according to claim 1, wherein a discharge current of the preparatory discharge is equal to or less than an amount of energy lost by recombination of oxygen and hydrogen after the completion of charging.   4. The secondary battery according to claim 1, wherein the secondary battery is a secondary battery that suppresses an increase in internal pressure by recombining oxygen generated in the battery with hydrogen of the negative electrode. 5. To charge the secondary battery. In a secondary battery charger for charging an assembled battery in which a plurality of secondary battery cells are connected in series or in parallel,
A charging circuit for charging the secondary battery;
A preparatory discharge circuit for performing a short preparatory discharge at a predetermined discharge current;
A switch for opening and closing the charging circuit and the preparatory discharging circuit;
After the battery is charged to full charge, according to a predetermined operation sequence, the charging circuit is opened and the preparatory discharge circuit is closed at a predetermined timing to execute preparatory discharge;
A charging device for a secondary battery, comprising:

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