Power supply control method and power supply control device

Abstract

The invention discloses a power supply control method and a power supply control device, and relates to the technical field of communication station power supply, and the power supply control method comprises the steps: S10, judging whether a rectifier is connected with commercial power or not; s20, if yes, controlling a rectifier to convert commercial power into direct current to charge a lead-acid battery and supply power to a load, and controlling a DC / DC converter to convert the direct current and then charge a lithium battery; and S30, adjusting the actual output current of the DC / DC converter according to a preset relation curve between the working efficiency of the rectifier and the charging current of the lithium battery, so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relation curve. According to the power supply control method provided by the embodiment of the invention, when the rectifier supplies power to the load and charges the lithium battery and the lead-acid battery, the rectifier can be located at the optimal working efficiency point as much as possible, and the energy utilization rate of the communication station is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power supply for communication stations, and in particular to a power supply control method and a power supply control device. Background Art

[0002] At present, backup batteries play an irreplaceable role in communication stations. With the large-scale construction of 4G / 5G base stations, lead-acid batteries and lithium batteries are mixed in the backup batteries of existing communication stations.

[0003] In the prior art, when the battery capacity of lead-acid batteries and lithium batteries is insufficient, after the communication station is connected to the AC power, on the one hand, the AC power is converted into DC power through a rectifier to power the load of the communication station (mostly communication equipment), and on the other hand, the lithium batteries and lead-acid batteries are charged to keep the lithium batteries and lead-acid batteries as fully charged as possible.

[0004] However, when existing communication stations supply power to loads and charge lithium batteries and lead-acid batteries after rectification through rectifiers, since the rectifiers are usually redundantly configured, the rectifier working efficiency (working efficiency = output power / input power) is not high, resulting in low energy utilization. Summary of the invention

[0005] The embodiments of the present invention provide a power control method and a power control device to solve the technical problem in the related art that when the existing communication station supplies power to the load and charges the lithium battery and the lead-acid battery after rectification by the rectifier, the rectifier has low working efficiency, resulting in low energy utilization.

[0006] In a first aspect, a power supply control method is provided, comprising the following steps:

[0007] Determine whether the rectifier is connected to the mains;

[0008] If yes, control the rectifier to convert the AC power into DC power to charge the lead-acid battery and supply power to the load, and control the DC / DC converter to convert the DC power to charge the lithium battery;

[0009] According to the preset relationship curve between the rectifier working efficiency and the lithium battery charging current, the actual output current of the DC / DC converter is adjusted so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve.

[0010] In some embodiments, the actual output current of the DC / DC converter is adjusted according to a preset relationship curve between the working efficiency of the rectifier and the charging current of the lithium battery so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve, including:

[0011] When the actual output current of the DC / DC converter is less than the lithium battery charging current corresponding to the theoretical optimal working efficiency point, increase the actual output current of the DC / DC converter;

[0012] When the actual output current of the DC / DC converter is greater than the lithium battery charging current corresponding to the theoretical optimal working efficiency point, the actual output current of the DC / DC converter is reduced.

[0013] In some embodiments, the step of determining whether the rectifier is connected to the mains power includes:

[0014] If not, the lithium battery is controlled to supply power to the DC bus bar through the DC / DC converter.

[0015] In some embodiments, if not, controlling the lithium battery to supply power to the DC bus bar through the DC / DC converter includes:

[0016] Get the battery capacity of the lithium battery;

[0017] Determine whether the battery capacity of the lithium battery is less than a preset capacity value;

[0018] If so, the DC / DC converter is controlled to stop working, so that the lead-acid battery supplies power to the DC bus bar.

[0019] In some embodiments, if not, controlling the lithium battery to supply power to the DC bus bar through the DC / DC converter includes:

[0020] The actual output voltage of the DC / DC converter is made greater than the actual output voltage of the lead-acid battery by a preset voltage difference.

[0021] In a second aspect, a power supply control device is provided, comprising: a rectifier, a lead-acid battery, a lithium battery, a DC / DC converter, and a controller;

[0022] The rectifier and the lead-acid battery are both connected to the DC bus bar, and the DC / DC converter is connected to the lithium battery and the DC bus bar;

[0023] The controller is connected to the rectifier and the DC / DC converter, and is configured as follows:

[0024] Determining whether the rectifier is connected to the mains;

[0025] If yes, control the rectifier to convert the AC power into DC power to charge the lead-acid battery and supply power to the load, and control the DC / DC converter to convert the DC power to charge the lithium battery;

[0026] According to the preset relationship curve between the working efficiency of the rectifier and the charging current of the lithium battery, the actual output current of the DC / DC converter is adjusted so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve.

[0027] In some embodiments, the actual output current of the DC / DC converter is adjusted according to a preset relationship curve between the working efficiency of the rectifier and the charging current of the lithium battery so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve, including:

[0028] When the actual output current of the DC / DC converter is less than the lithium battery charging current corresponding to the theoretical optimal working efficiency point, increasing the actual output current of the DC / DC converter;

[0029] When the actual output current of the DC / DC converter is greater than the lithium battery charging current corresponding to the theoretical optimal working efficiency point, the actual output current of the DC / DC converter is reduced.

[0030] In some embodiments, after determining whether the rectifier is connected to the mains, the following steps are performed:

[0031] If not, the lithium battery is controlled to supply power to the DC bus bar through the DC / DC converter.

[0032] In some embodiments, if not, controlling the lithium battery to supply power to the DC bus bar through the DC / DC converter includes:

[0033] Obtaining the battery capacity of the lithium battery;

[0034] Determining whether the battery capacity of the lithium battery is less than a preset capacity value;

[0035] If so, the DC / DC converter is controlled to stop working, so that the lead-acid battery supplies power to the DC bus bar.

[0036] In some embodiments, if not, controlling the lithium battery to supply power to the DC bus bar through the DC / DC converter includes:

[0037] The actual output voltage of the DC / DC converter is made greater than the actual output voltage of the lead-acid battery by a preset voltage difference.

[0038] The beneficial effects brought about by the technical solution provided by the present invention include:

[0039] The embodiment of the present invention provides a power control method and a power control device. The power control method first determines whether the rectifier is connected to the mains, and then when the rectifier is connected to the mains, controls the rectifier to convert the mains into direct current to charge the lead-acid battery and supply power to the load, and controls the DC / DC converter to charge the lithium battery after converting the direct current, and finally adjusts the actual output current of the DC / DC converter according to the preset relationship curve between the rectifier working efficiency and the lithium battery charging current, so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve. That is, the power control method of the embodiment of the present invention can make the rectifier at the optimal working efficiency point as much as possible when the rectifier is supplying power to the load and charging the lithium battery and the lead-acid battery, thereby improving the energy utilization rate of the communication station. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0041] Figure 1 A flow chart of a power control method provided by an embodiment of the present invention;

[0042] Figure 2 The embodiment of the present invention provides Figure 1 Flow chart for implementing step S30;

[0043] Figure 3 A curve showing the relationship between the working efficiency of the rectifier and the charging current of the lithium battery provided in the embodiment of the present invention;

[0044] Figure 4 Another flow chart of a power control method provided by an embodiment of the present invention;

[0045] Figure 5 The embodiment of the present invention provides Figure 4 Flow chart for implementing step S40;

[0046] Figure 6 A schematic diagram of the structure of a power supply control device provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0047] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0048] The embodiment of the present invention provides a power control method, which can solve the technical problem that when the existing communication station supplies power to the load and charges the lithium battery and the lead-acid battery after rectification by the rectifier, the rectifier has low working efficiency, resulting in low energy utilization.

[0049] See also Figure 1 As shown, an embodiment of the present invention provides a power supply control method, comprising the following steps:

[0050] Step S10, determining whether the rectifier is connected to the mains.

[0051] Step S20: If yes, control the rectifier to convert the AC power into DC power to charge the lead-acid battery and supply power to the load, and control the DC / DC converter to convert the DC power to charge the lithium battery.

[0052] Step S30, according to the preset relationship curve between the rectifier working efficiency and the lithium battery charging current, the actual output current of the DC / DC converter is adjusted so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve.

[0053] Specifically, see Figure 2 As shown, the actual output current of the DC / DC converter is adjusted according to the preset relationship curve between the rectifier working efficiency and the lithium battery charging current, so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve, including:

[0054] Step S301, when the actual output current of the DC / DC converter is less than the lithium battery charging current corresponding to the theoretical optimal working efficiency point, the actual output current of the DC / DC converter is increased.

[0055] Step S302, when the actual output current of the DC / DC converter is greater than the lithium battery charging current corresponding to the theoretical optimal working efficiency point, the actual output current of the DC / DC converter is reduced.

[0056] See also Figure 6As shown, the power supply of the communication station includes a rectifier, a lead-acid battery, a lithium battery, a DC / DC converter, and a controller. One end of the rectifier is connected to the mains, and the other end is connected to the DC bus bar, and the DC bus bar is connected to the load. The lead-acid battery is connected to the DC bus bar, the DC / DC converter is connected to the lithium battery and the DC bus bar, and the controller is connected to the rectifier and the DC / DC converter.

[0057] When the controller determines that the rectifier is connected to AC mains, the rectifier operates under the control of the controller to output DC power, and the DC power supplies power to the load of the DC bus bar, charges the lead-acid battery, and charges the lithium battery after the DC power is converted by the DC / DC converter. The output current value of the rectifier is I0, the branch current of the lead-acid battery is I1, the branch current of the lithium battery is I2, and the load current is I3, I0=I1+I2+I3. Among them, the load can be the communication equipment of the communication station.

[0058] The controller can control the magnitude of the output current of the DC / DC converter to the lithium battery. In actual applications, it is found that the output current I0 of the rectifier has the same change trend as the output current I2 of the DC / DC converter. When I2 increases, I0 increases; similarly, when I2 decreases, I0 decreases. Even if the branch current I1 and the load current I3 of the lead-acid battery have slight fluctuations, it will not change the change trend of I0 with I2. Since the rectifier working efficiency is correlated with the output current I0 of the rectifier, the rectifier working efficiency σ is also correlated with the output current I2 of the DC / DC converter (i.e., the lithium battery charging current I). The relationship curve between the rectifier working efficiency σ and the lithium battery charging current I can be obtained by pre-calibration, as shown in FIG. Figure 3 As shown, Figure 3 When the relationship curve is at point 2, the rectifier working efficiency σ is the maximum value, and the charging current of the lithium battery is a certain value I'; when the actual output current of the DC / DC converter corresponds to point 1, the actual output current of the DC / DC converter is less than the charging current I' of the lithium battery corresponding to the theoretical optimal working efficiency point, and the controller adjusts the DC / DC converter to increase its actual output current, so that point 1 moves to the right close to point 2, that is, the rectifier working efficiency increases. When the actual output current of the DC / DC converter corresponds to point 3, the controller adjusts the DC / DC converter to reduce its actual output current, so that point 3 moves to the left close to point 2, that is, the rectifier working efficiency increases, and the rectifier working efficiency can be placed at the optimal working efficiency point 2.

[0059] It should be noted that a current collector can be set between the lead-acid battery branch, the lithium battery branch, and the DC busbar and the load to feed back the currents of the three branches to the controller in real time, so as to facilitate the controller to make logical judgments. In addition, assuming that the battery capacity of the lead-acid battery is C1, the branch current of the lead-acid battery is I1, which is generally 0.1C1.

[0060] The power control method in the embodiment of the present invention first determines whether the rectifier is connected to the mains, and then when the rectifier is connected to the mains, controls the rectifier to convert the mains into direct current to charge the lead-acid battery and supply power to the load, and controls the DC / DC converter to charge the lithium battery after converting the direct current, and finally adjusts the actual output current of the DC / DC converter according to the preset relationship curve between the rectifier working efficiency and the lithium battery charging current, so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve. That is, the power control method in the embodiment of the present invention can make the rectifier at the optimal working efficiency point as much as possible when the rectifier is supplying power to the load and charging the lithium battery and the lead-acid battery, thereby improving the energy utilization rate of the communication station.

[0061] As an optional implementation, in one embodiment of the invention, see Figure 4 As shown, after determining whether the rectifier is connected to the mains, the method includes:

[0062] Step S40: If not, control the lithium battery to supply power to the DC bus bar through the DC / DC converter.

[0063] In the embodiment of the present invention, when there is no mains access, the lithium battery is preferentially used to provide power to the load, thereby fully utilizing the long cycle life of the lithium battery.

[0064] As an optional implementation, in one embodiment of the invention, see Figure 5 As shown, if not, controlling the lithium battery to supply power to the DC bus bar through the DC / DC converter includes:

[0065] Step S401, obtaining the battery capacity of the lithium battery, and determining whether the battery capacity of the lithium battery is less than a preset capacity value.

[0066] Step S402: If yes, control the DC / DC converter to stop working, so that the lead-acid battery supplies power to the DC bus bar.

[0067] In the embodiment of the present invention, when the battery capacity of the lithium battery is insufficient, the lead-acid battery is used as a second backup discharge, thereby reasonably extending the life of the lead-acid battery.

[0068] As an optional implementation, in an embodiment of the invention, if not, controlling the lithium battery to supply power to the DC bus bar through the DC / DC converter includes:

[0069] The actual output voltage of the DC / DC converter is made greater than the actual output voltage of the lead-acid battery by a preset voltage difference.

[0070] Specifically, in the embodiment of the present invention, when there is no mains access, the actual output voltage of the DC / DC converter is greater than the actual output voltage of the lead-acid battery by a preset voltage difference ΔU. At this time, the lithium battery is in a discharge state, and the lead-acid battery is in a non-charge and non-discharge state. Usually, ΔU can be 0.5V to 1V, which can ensure that the lithium battery is in a discharge state and the lead-acid battery is in a non-charge and non-discharge state. As shown above, when the battery capacity of the lithium battery is lower than the preset capacity value, the DC / DC converter stops working, and the lead-acid battery discharges to supply the load. At this time, the lithium battery is in a non-charge and non-discharge state.

[0071] See also Figure 6 As shown, an embodiment of the present invention further provides a power supply control device, including: a rectifier, a lead-acid battery, a lithium battery, a DC / DC converter, and a controller.

[0072] The rectifier and the lead-acid battery are both connected to the DC bus bar, and the DC / DC converter is connected to the lithium battery and the DC bus bar.

[0073] The controller is connected to the rectifier and the DC / DC converter, and is configured as follows:

[0074] Determining whether the rectifier is connected to the mains;

[0075] If yes, control the rectifier to convert the AC power into DC power to charge the lead-acid battery and supply power to the load, and control the DC / DC converter to convert the DC power to charge the lithium battery;

[0076] According to the preset relationship curve between the working efficiency of the rectifier and the charging current of the lithium battery, the actual output current of the DC / DC converter is adjusted so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve.

[0077] The power control device in the embodiment of the present invention is provided with a rectifier, a lithium battery, a DC / DC converter, and a controller. The controller first determines whether the rectifier is connected to the mains, and then when the rectifier is connected to the mains, controls the rectifier to convert the mains into direct current to charge the lead-acid battery and supply power to the load, and controls the DC / DC converter to charge the lithium battery after converting the direct current. Finally, according to the preset relationship curve between the working efficiency of the rectifier and the charging current of the lithium battery, the actual output current of the DC / DC converter is adjusted so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve. That is, the power control device in the embodiment of the present invention can make the rectifier at the optimal working efficiency point as much as possible when the rectifier supplies power to the load and charges the lithium battery and the lead-acid battery, thereby improving the energy utilization rate of the communication station.

[0078] As an optional implementation mode, in an embodiment of the invention, adjusting the actual output current of the DC / DC converter according to a preset relationship curve between the working efficiency of the rectifier and the charging current of the lithium battery so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve includes:

[0079] When the actual output current of the DC / DC converter is less than the lithium battery charging current corresponding to the theoretical optimal working efficiency point, increasing the actual output current of the DC / DC converter;

[0080] When the actual output current of the DC / DC converter is greater than the lithium battery charging current corresponding to the theoretical optimal working efficiency point, the actual output current of the DC / DC converter is reduced.

[0081] As an optional implementation manner, in an embodiment of the invention, after determining whether the rectifier is connected to the mains, the method includes:

[0082] If not, the lithium battery is controlled to supply power to the DC bus bar through the DC / DC converter. In the embodiment of the present invention, when there is no mains access, the lithium battery is preferentially used to provide power to the load, making full use of the long cycle life of the lithium battery.

[0083] As an optional implementation, in an embodiment of the invention, if not, controlling the lithium battery to supply power to the DC bus bar through the DC / DC converter includes:

[0084] Obtaining the battery capacity of the lithium battery;

[0085] Determining whether the battery capacity of the lithium battery is less than a preset capacity value;

[0086] If so, the DC / DC converter is controlled to stop working, so that the lead-acid battery supplies power to the DC bus bar.

[0087] In the embodiment of the present invention, when the battery capacity of the lithium battery is insufficient, the lead-acid battery is used as a second backup discharge, thereby reasonably extending the life of the lead-acid battery.

[0088] As an optional implementation, in an embodiment of the invention, if not, controlling the lithium battery to supply power to the DC bus bar through the DC / DC converter includes:

[0089] The actual output voltage of the DC / DC converter is made greater than the actual output voltage of the lead-acid battery by a preset voltage difference.

[0090] In the description of the present invention, it should be noted that the terms "upper", "lower", etc. indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific position, be constructed and operated in a specific position, and therefore cannot be understood as a limitation on the present invention. Unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be a connection between the two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to the specific circumstances.

[0091] It should be noted that, in the present invention, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0092] The above description is only a specific embodiment of the present invention, so that those skilled in the art can understand or implement the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but should conform to the widest scope consistent with the principles and novel features of the present invention.

Claims

1. A power control method, characterized in that: The following steps are involved: Determine whether the rectifier is connected to the mains; If yes, control the rectifier to convert the AC power into DC power to charge the lead-acid battery and supply power to the load, and control the DC / DC converter to convert the DC power to charge the lithium battery; According to the preset relationship curve between the rectifier working efficiency and the lithium battery charging current, the actual output current of the DC / DC converter is adjusted so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve.

2. The power control method according to claim 1, characterized in that: The method of adjusting the actual output current of the DC / DC converter according to the preset relationship curve between the working efficiency of the rectifier and the charging current of the lithium battery so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve includes: When the actual output current of the DC / DC converter is less than the lithium battery charging current corresponding to the theoretical optimal working efficiency point, increase the actual output current of the DC / DC converter; When the actual output current of the DC / DC converter is greater than the lithium battery charging current corresponding to the theoretical optimal working efficiency point, the actual output current of the DC / DC converter is reduced.

3. The power control method according to claim 1, characterized in that: After determining whether the rectifier is connected to the mains, the method includes: If not, the lithium battery is controlled to supply power to the DC bus bar through the DC / DC converter.

4. The power control method according to claim 3, characterized in that: If not, controlling the lithium battery to supply power to the DC bus bar through the DC / DC converter includes: Obtain the battery capacity of the lithium battery, and determine whether the battery capacity of the lithium battery is less than a preset capacity value; If so, the DC / DC converter is controlled to stop working, so that the lead-acid battery supplies power to the DC bus bar.

5. The power control method according to claim 3, characterized in that: If not, controlling the lithium battery to supply power to the DC bus bar through the DC / DC converter includes: The actual output voltage of the DC / DC converter is made greater than the actual output voltage of the lead-acid battery by a preset voltage difference.

6. A power supply control device, characterized in that: include: Rectifier, lead-acid battery, lithium battery, DC / DC converter, controller; The rectifier and the lead-acid battery are both connected to the DC bus bar, and the DC / DC converter is connected to the lithium battery and the DC bus bar; The controller is connected to the rectifier and the DC / DC converter, and is configured as follows: Determining whether the rectifier is connected to the mains; If yes, control the rectifier to convert the AC power into DC power to charge the lead-acid battery and supply power to the load, and control the DC / DC converter to convert the DC power to charge the lithium battery; According to the preset relationship curve between the working efficiency of the rectifier and the charging current of the lithium battery, the actual output current of the DC / DC converter is adjusted so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve.

7. The power supply control device according to claim 6, characterized in that: The step of adjusting the actual output current of the DC / DC converter according to a preset relationship curve between the working efficiency of the rectifier and the charging current of the lithium battery so that the actual working efficiency of the rectifier reaches the theoretical optimal working efficiency point in the relationship curve includes: When the actual output current of the DC / DC converter is less than the lithium battery charging current corresponding to the theoretical optimal working efficiency point, increasing the actual output current of the DC / DC converter; When the actual output current of the DC / DC converter is greater than the lithium battery charging current corresponding to the theoretical optimal working efficiency point, the actual output current of the DC / DC converter is reduced.

8. The power supply control device according to claim 6, characterized in that: After determining whether the rectifier is connected to the mains, the method includes: If not, the lithium battery is controlled to supply power to the DC bus bar through the DC / DC converter.

9. The power supply control device according to claim 8, characterized in that: If not, controlling the lithium battery to supply power to the DC bus bar through the DC / DC converter includes: Obtaining the battery capacity of the lithium battery, and determining whether the battery capacity of the lithium battery is less than a preset capacity value; If so, the DC / DC converter is controlled to stop working, so that the lead-acid battery supplies power to the DC bus bar.

10. The power supply control device according to claim 8, characterized in that: If not, controlling the lithium battery to supply power to the DC bus bar through the DC / DC converter includes: The actual output voltage of the DC / DC converter is made greater than the actual output voltage of the lead-acid battery by a preset voltage difference.

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