TW201807924A - Control method for battery parallel connection - Google Patents

Abstract

The invention provides a control method for battery parallel connection. for a load system having a first battery to connect in parallel a second battery via hot swapping way, the difference of state of charge of the first battery and the second battery is compared first and when the difference is smaller than a predetermined value, reduce the loading or charging current toward the first battery. Under the low loading or low current charging state, compare the difference of voltages between the first battery and the second battery. When the different of voltage is smaller than another predetermined value, the second battery is now connected in parallel to the load system before the load or charge current is restored.

Description

Control method for parallel connection of batteries

The invention relates to a control method of a battery circuit, in particular to a control method when a secondary battery is connected in parallel to a load system.

With the convenience of battery use and the wider and wider application field, using multiple batteries in parallel is one of the methods that can directly increase the battery capacity of the load system or increase the load's power usage. In the prior art, a battery originally outside the system is used. The battery referred to here is a rechargeable secondary battery. It is connected in parallel to a load system that already has a battery. The battery can be connected in a voltage determination manner, or the battery can be connected in a state of charge (SOC) determination.

When the entire load system is at rest, it is safest to attach another battery to the load system. However, the hot-swap operation method of directly connecting another battery to the load system under working conditions is an increasingly important requirement in the current environment. Because the battery in the load system may be working in high power and high current during the overlap process, if another battery is simply connected to the load system based on voltage or capacity alone, there is a high Risk from errors.

For example, only the battery voltage is used to determine whether another battery can be connected. When the battery in the load system is being pumped, the impedance in the battery or the line will cause a voltage drop, and in an environment with a voltage drop Then, rashly overlapping a new battery will produce a shunt effect, which will reduce the current draw of the battery in the load system, and the voltage drop of the battery will rise sharply, even exceeding the voltage difference between the two parallel batteries in the original setting. Safety value, which will harm the life of the battery cell and the safety of the use environment.

If only SOC (SOC is calculated by the battery itself, and SOC can be calculated from many functions and parameters) to judge, sometimes the actual voltage of the battery cannot be reflected immediately. For example, although the higher the SOC of a battery, the higher its voltage, but the voltage of the battery is not only related to the SOC, but also to the load the battery is subjected to. The higher the battery's load, the greater the voltage drop it immediately reflects. For example, when the SOC of both batteries is 50%, and one of the batteries is being pulled, the voltage of the battery that has been pulled down to a certain extent (you can know that this battery is not being unloaded) (The actual voltage will be higher), while the other battery is not connected yet and is at rest. If the SOC of these two batteries are both 50% and the battery in the static state is connected to the load system being pulled, it is obvious that the voltage between the two batteries is not the same. As a result, a surge current is generated in the battery switch, causing the switch to fail and be damaged.

In order to solve the aforementioned problems, the present invention discloses a control method for connecting a battery in parallel to a load system.

In one embodiment of the present invention, a control method for parallel connection of batteries is disclosed. A load system includes a load device and a first battery and a second battery connected in parallel with the load device. A first switch is connected in series between the first battery and the load device, and the second battery and the load are connected in series. A second switch is connected in series between the devices, and the control method includes the steps of comparing the first switch with the first switch turned on, the second switch not turned on, and the first battery being loaded by the load device with a high load. A first battery level of the first battery and a second battery level of the second battery; when the difference between the first battery level and the second battery level is less than a first preset value, the load device is The high load is reduced to a low load; in a state where the first battery is being pulled by the load device with the low load, a first voltage of the first battery and a second voltage of the second battery are compared; when When the difference between the first voltage and the second voltage is less than a second preset value, turning on the second switch, so that both the first battery and the second battery are pulled by the load device; and the load The counter increases from low load to a high load to the carrier of the first pumping cell and the second cell.

In the embodiment disclosed in the present invention, the high-load load is performed by the load device with a first current or a first power, and the low-load load is performed by the load device with a first current. Two currents or a second power is used for drawing, wherein the first current is greater than the second current and the first power is greater than the second power.

In the disclosed embodiment, when the second switch is turned on, the load device is raised from the low load to a maximum load to load the first battery and the second battery.

In the embodiment disclosed in the present invention, wherein the load device uses the high load to perform the load drawing with a first current or a first power, and the load device uses the maximum load to perform the load drawing with a first Three currents or a third power is used for drawing, wherein the third current is greater than the first current, and the third power is greater than the first power.

In the disclosed embodiment, the load system further includes a control unit that compares the first battery capacity of the first battery and the second battery capacity of the second battery, and compares the first battery capacity. The first voltage of the battery and the second voltage of the second battery. Wherein the control unit is constituted by a battery management system of the first battery and the second battery or is included in the load device, the control unit further controls the load device to reduce from the high load to the low load and control the load device From the low load to the high load.

In the disclosed embodiment, the control unit further turns on the second switch, so that both the first battery and the second battery are subject to load by the load device.

In the disclosed embodiment, the load system further includes a charging device, the first switch is connected in series between the first battery and the charging device, and the second switch is connected in series between the second battery and the charging device. And the control method further includes the steps of comparing the first battery with the first battery in a state where the first switch is on, the second switch is not on, and the first battery is charged by the charging device with a first current. A battery level and the second battery level of the second battery; when the difference between the first battery level and the second battery level is less than the first preset value, the charging device is changed from the first current to A second current charges the first battery; in a state where the first battery is charged by the charging device with the second current, the first voltage of the first battery and the second voltage of the second battery are compared ; When the difference between the first voltage and the second voltage is less than the second preset value, turning on the second switch, so that the first battery and the second battery are charged by the charging device; and Electrical means for changing the second current by the first current to the first battery and the second rechargeable battery. The first current is greater than the second current.

In the disclosed embodiment, when the second switch is turned on, the charging device is increased from the second current to a maximum current to charge the first battery and the second battery, wherein the maximum current Greater than the first current.

In the disclosed embodiment, the load system further includes a control unit that compares the first battery capacity of the first battery and the second battery capacity of the second battery, and compares the first battery capacity. The first voltage of the battery and the second voltage of the second battery.

In an embodiment disclosed by the present invention, wherein the control unit is composed of a battery management system of the first battery and the second battery, the control unit further controls the charging device to change from the first current to the second current And controlling the charging device to change from the second current to the first current.

In the disclosed embodiment, the control unit further turns on the second switch, so that the first battery and the second battery are both charged by the charging device.

With the control method for parallel connection of batteries disclosed in the present invention, during the operation of the load system, another battery can be connected in parallel in a hot-swap manner, so that the load system has longer battery life or more power output. And it can ensure that the gap between the power and voltage of two or more batteries connected in parallel with each other can be smoothly connected under safe conditions, which can greatly increase the switching life of the battery and prolong the service life of the battery cells to achieve safety. Purpose of connection and use.

Certain terms are used in the description and the scope of patent applications to refer to specific elements. Those of ordinary skill in the art will understand that manufacturers may use different terms to refer to the same component. The scope of this specification and the patent application does not use the difference in names as a way to distinguish components, but rather uses the difference in functions of components as a criterion for distinguishing components. "Inclusion" mentioned in the entire specification and patent application scope is an open-ended term, so it should be interpreted as "including but not limited to." In addition, the term "coupled" or "connected" includes any direct and indirect means of electrical or structural connection. Therefore, if a first device is described as being coupled / connected to a second device, it means that the first device may be electrically / structured to the second device directly, or indirectly electrically / structured to the second device through other devices or connection means.第二 装置。 The second device.

Please refer to FIG. 1. FIG. 1 is a functional block diagram of a load system to which the control method of the present invention is applied. The load system 1 uses a rechargeable battery that can be repeatedly charged and discharged as a power source, and the secondary battery therein can be replaced at any time when the system is operating or not operating. In a preferred embodiment, the load system 1 may be an electric bicycle or an electric motorcycle, but the present invention is not limited thereto. The load system 1 includes a first battery 10, a second battery 20 (the first battery 10 and the second battery 20 here represent secondary batteries that can be repeatedly charged and discharged), a first switch 30, and a second battery. The switch 40, a load device 50 and a charging device 60. In particular, the load device 50 includes all electric components and power output components in the load system 1, and is provided with necessary control units, mechanical structures, motors, and other devices. In addition, the charging device 60 in the load system 1 may be the same device under the same concept as the load device 50 (that is, for the load system 1, the present invention discusses the first battery 10 and the first battery 10 in the load system 1. The control of the load and charging behavior of the two batteries 20) may also be a charging device that is additionally connected to the load system 1. For convenience of description, in the following embodiments, the load device 50 and the charging device 60 are respectively depicted.

The first battery 10 and the second battery 20 are connected in parallel on the line with the load device 50 and the charging device 60, respectively, and a first switch 30 is connected in series between the first battery 10 and the load device 50 and the charging device 60, and the second battery 20 and A second switch 40 is connected in series between the load device 50 and the charging device 60. The first battery 10 or the second battery 20 can be directly plugged into the load system 1. When the load system 1 is operating, as long as any one of the batteries supplies power to the load system 1, the other battery can also be directly connected to the load system 1. To complete the hot swap action. In an actual embodiment, the second battery 20 may be mounted on the load system 1 in advance, or may be additionally added during the operation of the load system 1 (for example, during the riding of an electric bicycle). In FIG. 1 (and the following figure), a state in which the first battery 10 can be unloaded / charged is shown in FIG. 1 (and the first battery 10 is already loaded in the load system 1 and the first switch 30 is turned on). The battery 10 is connected to the load system 1 (shown below) to describe a control method performed by additionally connecting the second battery 20 to the load system 1.

Please refer to FIG. 2. FIG. 2 is a flowchart of a discharge operation in the control method 100 for battery parallel connection disclosed in the present invention. The steps of the control method 100 are described as follows:

Step 110: In a state where the first switch 30 is on and the second switch 40 is not on, the first battery 10 is connected to the load device 50, the second battery 20 is not connected to the load device 50, and the load device 50 is at a high level. The load draws the first battery 10;

Step 120: Compare a first battery level of the first battery 30 and a second battery level of the second battery 40.

Step 130: When the difference between the first battery power and the second battery power is less than a first preset value, the load device 50 reduces the high load to a low load to continue to load the first battery 10;

Step 140: Compare a first voltage of the first battery 10 with a second voltage of the second battery 20 in a state where the first battery 10 is being loaded by the load device 50 with the low load;

Step 150: when the difference between the first voltage and the second voltage is less than a second preset value, turning on the second switch 40, so that both the first battery 10 and the second battery 20 are pulled by the load device 50;

Step 160: The load device 50 is raised from the low load to the high load to pull off the first battery 10 and the second battery 20.

Please refer to FIG. 3. FIG. 3 is a flowchart of a charging operation in the control method 100 for battery parallel connection disclosed in the present invention. The steps are described as follows:

Step 210: In a state where the first switch 30 is on and the second switch 40 is not on, the first battery 10 is connected to the charging device 60, the second battery 20 is not connected to the charging device 60, and the charging device 60 is A current charges the first battery 10;

Step 220: Compare the first battery capacity of the first battery 30 and the second battery capacity of the second battery 40;

Step 230: When the difference between the first battery capacity and the second battery capacity is less than the first preset value, the charging device 60 changes the first current to a second current to continue charging the first battery 10;

Step 240: In a state where the first battery 10 is charged by the charging device 60 with the second current, compare the first voltage of the first battery 10 and the second voltage of the second battery 20;

Step 250: when the difference between the first voltage and the second voltage is less than the second preset value, turning on the second switch 40, so that both the first battery 10 and the second battery 20 are charged by the charging device 60;

Step 260: The charging device 60 changes from the second current to the first current to continue charging the first battery 10 and the second battery 20.

Please refer to FIG. 4A to FIG. 4D and FIG. 2, where FIGS. 4A to 4D are schematic diagrams of specific embodiments of the discharge operation in the control method of the battery parallel connection of the present invention. In FIG. 4A and step 110, the first switch 30 is turned on and the second switch 40 is not turned on, that is, the load system 1 is powered by the first battery 10 to the load device 50 first, while the second battery 20 is mounted In the load system 1, but the load device 50 is not connected (the second switch 40 is not turned on), so equivalently in the load system 1, the second battery 20 has not been connected in parallel with the first battery 10. In the state shown in FIG. 4A, the load device 50 loads the first battery 10 with the high load. The so-called high load and the low load mentioned later can be expressed by the magnitude of the current or the power. For example, in the embodiment of the present invention, the high load can be represented by a first current to the first battery. 10 loads, and a low load may indicate that the first battery 10 is loaded with a second current, and the first current is greater than the second current. In addition, a high load may indicate that the first battery 10 is being loaded with a first power, and a low load may be expressed that the first battery 10 is being loaded with a second power, and the first power is greater than the second power . In addition, in the load system 1, the magnitude of the current (power) of a low load or a high load is determined by the load device 50. In practice, the load system 1 may mount the first battery 10 and the second battery 20 in advance, or may only mount the first battery 10 in advance, and when the load system 1 is operating (that is, the first battery 10 supplies power to the load device 50) Time), the second battery 20 is mounted on the load system 1 in a hot-swap manner, and since the second switch 40 between the second battery 20 and the load device 50 has not been turned on, the second battery 20 at this time is not connected. The load device 50 is not connected.

In the state of FIG. 4A and step 110, then in step 120, a first battery power of the first battery 30 and a second battery power of the second battery 40 are continuously obtained and compared. Please refer to FIG. 1. The load system 1 further includes a control unit, which can be a unified central control unit for the entire system, or all control tasks can be performed by the load device 50 and the control elements of each battery. For example, the load device 50 itself includes a control unit 52, and the first battery 10 also has a battery management system (BMS) 12 and the second battery 20 also has a battery management system (BMS). twenty two. Here, the first battery power (ROC) and the second battery power (ROC) are obtained by the battery management systems 12, 22 in the first battery 10 and the second battery 20, and then the battery management system 12 Step 22 is executed to compare the difference between the first battery capacity and the second battery capacity, and when the difference between the first battery capacity and the second battery capacity is less than a first preset value, The communication signal or the input / output (I / O) signal notifies the load system 1 to reduce the load to the low load, and continues to load the first battery 10, as shown in step 130 and FIG. 4B. The first battery capacity and a first voltage also continue to decrease at a relatively slow rate. In other embodiments, the control unit 52 of the load device 50 may also obtain the battery power status of the respective batteries from the battery management systems 12 and 22 in the first battery 10 and the second battery 20, and then the control unit 52 may compare the battery power. When the difference between the battery levels is less than the first preset value, the load is automatically reduced to the low load.

As shown in FIG. 140 and FIG. 4C, in a state where the first battery 10 is continuously loaded by the load device 50 at the low load, the first voltage of the first battery 10 and the second battery 20 are continuously obtained and compared.一 second voltage. Similarly, the first voltage and the second voltage here can be obtained by the battery management systems 12, 22 in the first battery 10 and the second battery 20, and then the battery management system 12, 22 performs step 150. Comparing the gap between the first voltage and the second voltage, and when the gap between the first voltage and the second voltage is less than a second preset value, the battery management system 12 of the first battery 10 The battery management system 22 of the second battery 20 is notified by a communication signal or an input / output (I / O) signal, and the second switch 40 is controlled to be turned on, so that the second battery 20 is connected to the load device 50, so that in the load system 1, The second battery 20 is connected in parallel to the first battery 10, so that both the first battery 10 and the second battery 20 are pulled by the load device 50. In particular, at this time, the load device 50 still loads the first battery 10 and the first battery 10 with a low load. Two batteries 20 load. In other embodiments, the control unit 52 of the load device 50 may also obtain the voltage status of the respective batteries from the battery management systems 12 and 22 in the first battery 10 and the second battery 20, and then the control unit 52 compares the battery voltages. When the difference between the battery voltages is less than the second preset value, the second switch 40 is turned on or the battery management system 22 notifying the second battery 20 is turned on.

Finally, as shown in step 160 and FIG. 4D, when the second battery 20 is connected in parallel to the load system 1 in a state close to the power and voltage of the first battery 10, the control unit of the load system 1 (as described above, can be It is a unified central control unit or a cooperative operation of the control elements of each device.) The load device 50 is controlled to increase the load from the low load to the high load to load the first battery 10 and the second battery 20. In particular, since two batteries have been connected in parallel in the load system 1 in FIG. 4D, in addition to the implementation described in step 160 and FIG. 4D, the load device 50 in the load system 1 can also be Greater output power to load the two batteries, that is, after the second switch 40 is turned on, the load device 50 is further increased from the low load to a maximum load to load the first battery 10 and the second battery 20 The load is drawn, where the maximum load can be expressed as a load on the first battery 10 and the second battery 20 with a third current, and the third current is greater than the first current with a high load. In addition, the maximum load may also be expressed as a third power that is used to load the first battery 10 and the second battery 20, and the third power is greater than the first power.

Please refer to FIG. 5A to FIG. 5D and FIG. 3, wherein FIG. 5A to FIG. 5D are schematic diagrams of specific embodiments of the charging operation in the control method of the battery parallel connection of the present invention. The control method of the present invention charges batteries connected in parallel in a similar manner. In FIG. 5A and step 210, the first switch 30 is turned on and the second switch 40 is not turned on, that is, the charging device 60 charges the first battery 10 first, and the second battery 20 is not connected to the charging device 60. . Also in the state of FIG. 5A, the charging device 60 charges the first battery 10 with the first current. The so-called first current and the second current mentioned later represent high-current charging and low-current charging, that is, the first current is larger than the second current. In other embodiments, different charging behaviors of the battery may be indicated by high power or low power.

In the state of FIG. 5A and step 210, then in step 220, the battery management systems 12, 22 in the first battery 10 and the second battery 20 continuously obtain and compare the first battery power of the first battery 30 and A gap between the second battery capacity of the second battery 40 and a communication signal or an output (I / I) when the gap between the first battery capacity and the second battery capacity is less than the first preset value O) The signal notifies the load system 1 to continue to charge the first battery 10 with a smaller second current, as shown in step 230 and FIG. 5B. At this time, the first battery capacity of the first battery 10 and the first battery 10 The voltage also continues to rise at a relatively slow rate. In other embodiments, the central control unit of the load system 1 may also obtain the battery power status of the respective batteries from the battery management systems 12 and 22 in the first battery 10 and the second battery 20, and the central control unit may compare the battery power. When the difference between the battery levels is less than the first preset value, the battery is automatically charged with a smaller second current.

As shown in step 240 and FIG. 5C, in a state where the first battery 10 is continuously charged by the charging device 60 with the second current, the first voltage of the first battery 10 and the second battery 20 are continuously obtained and compared. This second voltage. Similarly, after the voltages of the first battery 10 and the second battery 20 are obtained by the battery management systems 12, 22, the battery management system 12, 22 executes step 250 to compare the difference between the battery voltages and the battery voltages. When the gap is less than the second preset value, the battery management system 12 of the first battery 10 notifies the battery management system 22 of the second battery 20 with a communication signal or an input / output (I / O) signal, and controls the second switch 40 to be turned on. The second battery 20 is connected to the charging device 60 so that in the load system 1, the second battery 20 is connected in parallel to the first battery 10, so that both the first battery 10 and the second battery 20 are charged by the charging device 60. Specifically, at this time, the charging device 60 still charges the first battery 10 and the second battery 20 with a second current (low current). In other embodiments, the charging device 60 may also compare the difference between the battery voltages after the battery management systems 12 and 22 in the first battery 10 and the second battery 20 obtain the voltage status of the respective batteries. And when the difference between the battery voltages is less than the second preset value, the second switch 40 is turned on or the battery management system 22 notifying the second battery 20 is turned on.

Finally, as shown in step 260 and FIG. 5D, when the second battery 20 is connected in parallel with the load system 1 in a state close to the power and voltage of the first battery 10, the control unit of the load system 1 (as described above, can be It is a unified central control unit or the cooperative operation of the control elements of each device.) The charging device 60 is controlled to increase from the second current (or low power) to the first current (or high power) to the first battery 10 and The second battery 20 is charged. In particular, since two batteries have been connected in parallel in the load system 1 in FIG. 5D, in addition to the implementation described in step 260 and FIG. 5D, the charging device 60 in the load system 1 may also be More charging power to charge the two batteries, that is, after the second switch 40 is turned on, the charging device 60 further increases from the second current to a maximum current to charge the first battery 10 and the second battery 20 Charging, wherein the maximum current is greater than the first current.

In the method for controlling parallel connection of batteries disclosed in the present invention, in a load system that already has a first battery, when a second battery is to be connected in parallel in a hot-swap manner, the first battery and the second battery are first compared. The battery power gap of the battery. When the battery power gap is less than a preset value, the system reduces the load on the first battery or the charging current. Then in the case of low load or low current charging, compare the voltage difference between the first battery and the second battery. When the voltage difference is less than another preset value, the second battery can be connected in parallel to the load system, and Restore the original load or charging current, make the load system have longer battery life or more power output, and can ensure that the gap between the power and voltage between two or more batteries connected in parallel with each other is smooth under safe conditions Overlap can greatly increase the switching life of the battery and prolong the service life of the battery cell, achieving the purpose of safe connection and use. The above description is only a preferred embodiment of the present invention, and all equivalent changes and modifications made in accordance with the scope of patent application of the present invention shall fall within the scope of the present invention.

1‧‧‧ load system
10‧‧‧ first battery
12,22‧‧‧Battery Management System
20‧‧‧ secondary battery
30‧‧‧First switch
40‧‧‧Second switch
50‧‧‧ load device
52‧‧‧Control unit
60‧‧‧Charging device
100‧‧‧Control method
110 ~ 160,210 ~ 260‧‧‧step

FIG. 1 is a functional block diagram of a load system to which the control method of the present invention is applied. FIG. 2 is a flowchart of a discharge operation in a method for controlling parallel connection of batteries disclosed in the present invention. FIG. 3 is a flowchart of a charging operation in the method for controlling parallel connection of batteries disclosed in the present invention. FIG. 4A to FIG. 4D are schematic diagrams of specific embodiments related to discharge work in the control method for battery parallel connection of the present invention. FIG. 5A to FIG. 5D are schematic diagrams of a specific embodiment of a charging operation in the control method for battery parallel connection of the present invention.

100‧‧‧Control method

110 ~ 160‧‧‧step

Claims (14)

A control method for parallel connection of batteries. A load system includes a load device, a first battery and a second battery connected in parallel with the load device. A first battery is connected in series between the first battery and the load device. Switch, a second switch is connected in series between the second battery and the load device, and the control method includes the steps of: the first switch is turned on, the second switch is not turned on, and the first battery is subjected to a high level by the load device In a state of load shedding, a first battery capacity of the first battery and a second battery capacity of the second battery are compared; when the difference between the first battery capacity and the second battery capacity is less than a first At a preset value, the load device is reduced from the high load to a low load; in a state where the first battery is being pulled by the load device with the low load, a first voltage of the first battery and the first battery are compared. A second voltage of the two batteries; when the difference between the first voltage and the second voltage is less than a second preset value, turning on the second switch, so that the first battery and the second battery are both Subject to load by the load device; and the load device is raised from the low load to the high load to load the first battery and the second battery. The control method according to claim 1, wherein the load device performs the pumping with the high load at a first current or a first power, and the load device performs the pumping with the low load at a first Two currents or a second power is used for drawing, wherein the first current is greater than the second current and the first power is greater than the second power. The control method according to claim 1, wherein when the second switch is turned on, the load device is raised from the low load to a maximum load to load the first battery and the second battery. The control method according to claim 3, wherein the load device performs the pumping with the high load at a first current or a first power, and the load device performs the pumping with the maximum load at a first Three currents or a third power is used for drawing, wherein the third current is greater than the first current, and the third power is greater than the first power. The control method according to claim 1, wherein the load system further includes a control unit, the control unit compares the first battery capacity of the first battery with the second battery capacity of the second battery, and compares the first battery The first voltage of the battery and the second voltage of the second battery. The control method according to claim 5, wherein the control unit is composed of a battery management system of the first battery and the second battery, and the control unit further controls the load device to decrease from the high load to the low load and control The load device is raised from the low load to the high load. The control method according to claim 6, wherein the control unit further turns on the second switch, so that the first battery and the second battery are both loaded by the load device. The control method according to claim 5, wherein the control unit is included in the load device, the control unit further controls the load device to decrease from the high load to the low load and control the load device to rise from the low load to That high load. The control method according to claim 8, wherein the control unit further turns on the second switch, so that the first battery and the second battery are both loaded by the load device. The control method according to claim 1, wherein the load system further includes a charging device, the first switch is connected in series between the first battery and the charging device, and the second battery is connected in series between the second battery and the charging device. The control method further includes the steps of: comparing the first battery with the first battery in a state where the first switch is turned on, the second switch is not turned on, and the first battery is charged by the charging device with a first current. A battery level and the second battery level of the second battery; when the difference between the first battery level and the second battery level is less than the first preset value, the charging device is changed from the first current to A second current charges the first battery; in a state where the first battery is charged by the charging device with the second current, comparing the first voltage of the first battery and the second voltage of the second battery ; When the difference between the first voltage and the second voltage is less than the second preset value, turning on the second switch, so that the first battery and the second battery are charged by the charging device; and Charging means for changing the second current from the first current to the first battery and the second rechargeable battery; wherein the first current is greater than the second current. The control method according to claim 10, wherein when the second switch is turned on, the charging device is increased from the second current to a maximum current to charge the first battery and the second battery, wherein the maximum current Greater than the first current. The control method according to claim 10, wherein the load system further includes a control unit that compares the first battery power of the first battery and the second battery power of the second battery, and compares the first battery The first voltage of the battery and the second voltage of the second battery. The control method according to claim 12, wherein the control unit comprises a battery management system of the first battery and the second battery, and the control unit further controls the charging device to change from the first current to the second current And controlling the charging device to change from the second current to the first current. The control method according to claim 13, wherein the control unit further turns on the second switch, so that the first battery and the second battery are both charged by the charging device.