James & Wells ref: 46566AU/62 AN LED FLOODLIGHT WITH REPLACEABLE BATTERY PACK TECHNICAL FIELD The present utility model relates to a lamp, and in particular to an LED floodlight with a replaceable battery pack, which Is mainly applicable In locations and events such as vehicle 5 repair plants, personal outdoor vehicle maintenance and repair, camping, outdoor lighting and so on. BACKGROUND OF THE UTILITY MODEL The LED work lamp is a lamp that uses a light emitting diode as a light source. The LED work lamp has about a quarter of the power consumption of an ordinary lamp at the same 0 brightness, has a service life of up to ten years, and Is easier to carry and use compared to a conventional lamp. Thus LED work lamps have been widely used, for example in maintenance shops, processing shops, construction sites, and so on. Further, with the popularity of the family car and gradually increasing use of household appliances, the LED work lamp is increasingly used In the home. Existing similar LED floodlights are equipped 5 with a power cord, and input of 120V or 220V is used as the working power supply, so that they can only be used in places with a power grid. Thus, existing LED floodlights are very inconvenient to use. Our company has already applied for and been granted a patent in China in respect of the series of rechargeable LED floodlight products produced by our company, with patent 0 number 201220139511.8. Such a rechargeable LED floodlight has a more compact structure and is easy to carry due to its small size, but there are also some disadvantages. The lithium battery has a limited life and Is costly, for example the battery would become ineffective after 500 charges and discharges. However, the LED lamp Itself has a long life. Therefore, when the lithium battery can no longer be used, the whole LED lamp is also unable to be further 5 used, and the user must then purchase a new product or replace the lithium battery inside the LED lamp through complicated dismantling. Thus the undamaged LED lamp body part would be wasted, or significant manual labour spent if the lithium battery is replaced. SUMMARY OF THE UTILITY MODEL In order to overcome the disadvantages in the prior art, an object of the present utility model 0 is to provide an LED floodlight with a replaceable battery pack that has a simple and practical structural design, is convenient to use and carry, has a high degree of safety, has a longer life, is compatible with various kinds of batteries, where both the battery and the battery pack are replaceable, and possesses a bullt-in recharging function as well as a replaceable power output port. 5 The following technical solutions are adopted by the present utility model in order to solve the above-described problems. The LED floodlight with replaceable battery pack includes a lamp body and a battery pack. The battery pack is located on the back surface of the lamp body. The back surface of the lamp body is provided with a sliding plate, and the sliding plate is provided with a cavity therein. The bottom portion of the battery pack is provided with a a sliding seat plate. Electrical contacts are provided both in the cavity and on the sliding seat plate and matched with each other, The electrical contacts In the cavity or on the sliding seat plate are elastic electrical contacts. The cavity and the sliding seat plate move in unison when sliding. Thus the structural design of the present utility model is more practical, and the electrical contacts of the lamp body and the battery pack are not likely to be mismatched, 5 thereby having good contact performance, being safer to use, with longer life. In addition, a plurality of cavities may be provided In the sliding plate depending on different wattages of 1 James & Wells ref: 46566AU/62 the lamp body. For example, one cavity may be provided for 5w and 1Ow; two c vities may be provided for 30w, 40w and 50w; and one or two cavities may be provided for 20w. The battery pack in the present utility model is an ordinary battery pack. The ordinary battery pack is further provided with an ordinary battery, a battery holder, a battery box and a battery 5 cover. The ordinary battery is installed in the battery holder. The battery holder is installed in the battery box. The battery cover Is installed on the battery box. The sliding seat plate and the battery box are fixedly connected, The ordinary battery is a dry battery or a nickel hydrogen battery. Such a structure of the ordinary battery pack makes it convenient to install and replace the dry battery or nickel hydrogen battery. The user may select various kinds of 0 batteries in accordance with his/her different requirements. The battery pack in the present utility model is a lithium battery pack. The lithium battery pack is further provided with a lithium battery, a battery box and a battery cover The lithium battery is installed in the battery box, and the battery cover is located on the battery box. The sliding seat plate and the battery box are fixedly connected. Such a structure of battery pack 5 makes it convenient to install and replace the lithium battery. The battery box in the present utility model is provided with a charger slot therein. A waterproof plug is provided on the charger slot. The waterproof plug is connected to the battery box via a soft connecting body. The charger slot is designed in such a manner that the battery pack may be detached as a whole to recharge the nickel hydrogen battery or the 0 lithium battery when the battery dies, without the need to take the battery out and place it in a separate charger for recharging. Thus, the operation Is easy. The design of the waterproof plug prevents water from easily getting into the charger slot, which protects the battery pack and results in safer use. The design of the charger slot can be matched to any other kind of charger such as an AC/DC adapter, a 12/24V car charger, a solar charger or magnetic-field 5 charger with a DC output port, and so on. The battery box in the present utility model Is further provided with a control panel thereon. The control panel is provided with a control switch and a power level indicator light thereon. The power level Indicator light may show the remaining power level, so that the user may arrange to replace the battery at an appropriate time during use and make the relevant ) preparations, thereby avoiding interrupted operation as a result of suddenly having no power. Pressing the control switch for a short time controls the tum-on and turn-off state of the lamp, and can also control colour variations. Pressing the control switch for an extended time adjusts the brightness of the lamp, and the control switch can then be released when the desired brightness is achieved. The battery box in the present utility model is further provided with a power supply output port thereon. As such, the battery pack may serve as an emergency power supply alone. Depending on the requirements of the user, the battery box may be provided with a plurality of output ports (for example, USB output ports and the like), thereby being capable of charging electronic devices such as a mobile phone. A seal ring is provided at the periphery of the elastic electrical contact in the pre nt utility model. The seal ring is generally made of wear resistant rubber material, so as t prevent water or steam from entering, thereby functioning to protect the electrical contacts, the lamp body and the battery pack. Thus the service life of the whole lamp may be prolonged. In the present utility model, the two side edges of the cavity and the sliding seat late that engage with each other are of a matched segmental-staggered concavo-convex tructure. Such a matched segmental-staggered concavo-convex structure makes it easier to take out the battery pack. Specifically, the battery pack slides slightly, such that the sliding seat plate and the cavity are offset. This enables the battery pack to be removed. 2 James & Wells ref: 46566AU/62 In the present utility model, an opening is provided at the entrance of the cavity, and an elastic pull snap is provided on the sliding seat plate, with the elastic pull snap being engaged in the opening. Engagement between the elastic pull snap and the opening enables the lamp body and the battery pack to be more securely connected, so hat the 5 cavity and the sliding seat plate cannot be easily offset which would otherwise c use poor contact between the electrical contacts. When the battery pack is to be taken ot, this may be achieved by slightly pulling the elastic pull snap. In the present utility model, the battery cover is provided with a bayonet thereon, and the battery box is provided with a slot therein, with the bayonet being engaged with the slot. The 0 design of the bayonet and the slot enables the battery cover and the battery box to be connected firmly. That is, the battery cover can not easily detach, which prevents the battery or the battery holder in the battery box from falling out, avoids a broken circuit caused by poor contact, and further makes it more convenient to open the cover for replacing the battery. 5 The ordinary battery pack in the present utility model includes an ordinary battery pack without a charger slot and an ordinary battery pack with a charger slot. The ordinary battery pack without a charger slot is designed such that the ordinary battery, the battery box and the battery cover are detachable. Thus, the dry battery or the nickel hydrogen battery may be conveniently replaced. 0 The ordinary battery pack with the charger slot according to the present utility model Is also designed such that the ordinary battery, the battery box and the battery cover are detachable. In addition to using a dry battery or a nickel hydrogen battery, the lamp body may also be directly powered with an AC power source by connecting the AC/DC adapter into the charger slot. Alternatively, It Is possible to recharge the nickel hydrogen battery in 5 the battery holder by using an AC power source, and it Is possible to recharge and illuminate at the same time. The dry battery is cheaper, and will meet the needs of users who do not use the lamp often. For example, it can be used for emergency lighting by buying and Installing a dry battery. If the user prefers not to use a dry battery, a standard nickel hydrogen battery may be bought and Installed to provide a rechargeable floodlight. 0 The lithium battery pack in the present utility model may also include a lithium battery pack without a charger slot and a lithium battery pack with a charger slot. The lithium battery pack without the charger slot is designed such that the lithium battery, the battery box and the battery cover are detachable. When the power level of the lithium battery runs down, a fresh lithium battery may be installed instead, or the removed lithium battery may be fully 5 recharged using a charger, and remounted for further use. When the lithium battery fails, a new lithium battery may be Installed for continued use. The lithium battery pack with the charger slot according to the present utility model is generally designed such that the.lithium battery, the battery box and the battery cover are of one integrated structure. When the power level of the lithium battery runs down, this may be D recharged directly through the charger slot, and it is possible to recharge and Illuminate at the same time. When the lithium battery fails, the user may directly power the lamp body through the charger slot. The lithium battery pack with the charger slot according to the present utility model may also be designed such that the lithium battery, the battery box and the battery cover are 5 detachable. When the power level of the lithium battery runs down, a fresh lithium battery may be installed instead, Alternatively, the lithium battery may be fully recharged using a charger, and remounted for further use; or may be recharged directly through the charger slot, thereby charging and lighting at the same time. When the lithium battery fails, a fresh 3 James & Wells ref: 46566AU/62 lithium battery may be installed for continued use, or the lamp body may be directly powered through the charger slot. Compared with the prior art, the present utility model has the following advantages and effects: The ordinary battery pack and the lithium battery pack may be replaced at will and 5 be matched with lamp bodies of different wattages, so as to meet the requirements of different users. The structural design is practical, safe, securely connected, convenient to use, reduces resource waste and manual costs, and has wider application. DESCRIPTION OF THE DRAWINGS Fig. 1 is a rear structural schematic view of a first embodiment using two battery packs (the 0 battery packs are disengaged). Fig. 2 is a rear structural schematic view of a second embodiment using one lithium battery pack according to the present utility model. Fig. 3 is another perspective structural schematic view of Fig. 2. Fig. 4 is front perspective structural schematic view of the second embodiment using one 5 lithium battery pack according to the present utility model. Description of reference numerals: 1. lamp body, 2. sliding plate, 3. cavity, 4. elastic electrical contact, 5. seal ring, 6. opening, 7. ordinary battery pack, 7a. lithium battery pack, 8. sliding seat plate, 9. nickel hydrogen battery, 10. battery holder, 11. ordinary battery box, 11 a. lithium battery box, 12. ordinary battery cover, 12a. lithium battery cover, 13. control D panel, 14. control switch, 15. power level indicator light, 16. elastic pull snap, 17. Charger slot, 18. waterproof plug, 19. bayonet, 20. slot, In addition, our company recently launched an LED floodlight with a replaceable battery pack, in which the lamp body and the battery pack are connected by a screw mechanism. The thread of this structure can wear out easily, so that the electrical contacts of the lamp 5 body and the battery pack easily become misaligned, causing poor contact, in addition, the battery pack is generally designed into a square shape. Thus if the provision of two battery packs is desired, large space in the design may be required. 4