405 results about "Nickel–hydrogen battery" patented technology

Charging device 10 charges nickel-hydrogen batteries 58, while adjusting the current value so that the temperature follows the target temperature rise pattern. Therefore, nickel-hydrogen batteries that demonstrate a large temperature increase can be charged within a short time so that a high temperature is not attained. Furthermore, when lithium ion batteries 58 are charged, the current value is controlled so that the voltage follows the target voltage rise pattern. Therefore, lithium ion batteries can be charged at a potential no higher than the preset level.

A disk array device can carry out battery backup including a destage process of data by a battery mounted in a case body of the disk array device and can enhance instantaneous power failure durability to an instantaneous power failure. A logic box mounts a channel controlling unit, a disk controlling unit, a cache memory and a switch; and a hard disk box mounts a plurality of memory devices. A battery box mounts a nickel hydrogen battery for supplying a backup power source at the time of a power failure and is disposed at a lower portion of each case body including the logic box and the hard disk box. The nickel hydrogen battery of the battery box is cooled through natural convection of air created inside the case body by a fan for cooling each of the logic box and the hard disk box.

Charging device 10 charges nickel-hydrogen batteries 58, while adjusting the current value so that the temperature follows the target temperature rise pattern. Therefore, nickel-hydrogen batteries that demonstrate a large temperature increase can be charged within a short time so that a high temperature is not attained. Furthermore, when lithium ion batteries 58 are charged, the current value is controlled so that the voltage follows the target voltage rise pattern. Therefore, lithium ion batteries can be charged at a potential no higher than the preset level.

The invention discloses a method for surface modification of a hydrophobic polymer microporous membrane. The method comprises the steps of dissolving a pyrocatechol compound and a polyamine molecule in a solvent, and adjusting the mixture to be alkaline so as to obtain a modification solution; soaking the hydrophobic polymer microporous membrane in the modification solution to realize the surface modification of the hydrophobic polymer microporous membrane through reaction, wherein the structural formula of the pyrocatechol compound is shown by a formula I, and in the formula I, R is H, -CH3, -(CH2)nCOOH, -CHO or OH and n is an integer between 0 and 2. The microporous membrane modified by the method disclosed by the invention can be applied in the fields of diaphragms of positive and negative materials of lithium secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, polymer batteries and the like, as well as separation membranes including reverse osmose membranes, micro-filtration membranes, nano-filtration membranes, ultrafiltration membranes, electrodialysis membranes, gas separation membranes and the like to improve the performances of materials and devices.

The present invention relates to a charging equipment for nickel-hydrogen batteries, which comprises a power source management unit, a charging unit and a nickel-hydrogen battery connected with each other and is characterized in that the power source management unit is composed of a charging control module. Wherein, the charging unit is composed of a trickle charging control circuit, a constant current control circuit, a constant voltage control circuit, a charging mode control circuit and a charging circuit. The charging mode control module is connected with the charging control module. The trickle charging control circuit, the constant current control circuit and the constant voltage control circuit are connected in parallel and respectively connected with the charging circuit and the charging control module. The charging circuit is connected with the nickel-hydrogen battery. In addition, the charging control module controls the charging unit to select the constant current charging, the trickle charging or the constant voltage charging to charge the battery according to voltage of the nickel-hydrogen battery. The present invention can enhance nickel-hydrogen battery charging performance, prolong nickel- hydrogen service life and ensure battery safety.

The invention relates to a method for detecting nickel-hydrogen battery separator wet electric resistance and a device thereof. The main technical characteristics are: simulated battery separator materials are under the state of extrusion and stress in the battery system; multiaperture type electrodes similar to polar plate area and size in the battery system exercises certain pressure over a separator sample in the same area and is immerged into electrolyte to carry out the method for detecting battery separator wet electric resistance. The detection device comprises a lower electrode supporting mass, a stream guidance shallow groove, a multiaperture lower plate electrode, a multiaperture upper plate electrode, a stream guidance shallow groove, an upper electrode supporting mass, a pressing weight and a cylinder, which form a detection platform for holding the sample separator; a ring-shaped liquid storage tank where electrolyte is injected is arranged on the lower electrode supporting mass, the separator is immerged for wet electric resistance detection. The method and device of the invention feature simple and convenient operation, can quickly and accurately reflect quality of the battery separator, which is in favor of quality control.

The invention relates to a portable wireless charger, particularly to the secondary batteries of the nickel-hydrogen batteries and nickel-cadmium batteries having the parallel-connected separated detection charging mode and series-connected combined discharging mode to achieve highest efficiency of power release. The present invention also provide a DC TO AC output control unit coupled with the power storing unit and converting a discharging current into an output power with a predetermined voltage level and a wireless power transmitter having stable high efficiency of the output power. Whereby the present invention provides the slim portable wireless charging platform for charging mobile devices with built-in wireless inductive receiver without electrical contacts to enhance ease of use and safety effect.

One problem with a sealed type nickel-metal hydride battery is that the high-rate discharge capability is inferior to that of a nickel-cadmium storage battery, because of a slow transfer rate of charges to the surface of a hydrogen storing alloy that is a negative electrode. Another problem is that the use of an alloy having excellent life characteristics takes much time for initial activation of battery characteristics. The invention provides a solution to the aforesaid problems by the provision of a sealed type nickel-metal hydride battery (1) improved in high-rate discharge capability and charge-discharge cycle characteristics. To this end, the invention is characterized by locating a 50 nm to 400 nm thick nickel-rich layer (11) on the surface of a hydrogen storing alloy powder, and locating the nickel-rich layer (11) as well on the surface of cracks (12) that open at the surface of alloy, and more preferably setting the mass saturation magnetization of the alloy powder at 2.5 to 9 emu / g and the content of magnetic nickel at 0.5 to 1.9 mmol per gram of the hydrogen storing alloy powder. A succession of hydrogen absorption step, alkali treatment step, product removal step, hydrogen desorption step and partial oxidization step by air are applied to the hydrogen storing alloy powder to obtain alloy powder, which is then used to obtain a battery having the aforesaid features. The invention is effectively applied to corrosion-resistant hydrogen storing alloys containing Er, Y and Yb.

To present an electric railway power-supply system not requiring vast area for installation, excellent in rapid charge-discharge characteristic, and low in manufacturing cost. At a substation 9 for electric railway having a transformer 3 for receiving power from an alternating-current power line 2, a rectifying device 4 connected to the transformer 3, and a feeder line 5 connected to the rectifying device 4, a nickel hydrogen battery 8 is provided as a direct-current supply facility, and the nickel hydrogen battery 8 is directly coupled to the feeder line 5.

The invention provides a hydrogen storage alloy. The alloy has the compositions shown in formula La(1-x)YxNiaCobMncBd, wherein B is one or a plurality of aluminum, iron, copper, tin, titanium, chromium and vanadium; x, a, b, c and d respectively represents the mole fractions of Y, Ni, Co, Mn and B; moreover, x is more than or equal to 0.05 and less than or equal to 0.5, a is more than or equal to 2.5 and less than or equal to 3.5, b is more than or equal to 0 and less than or equal to 0.5, c is more than or equal to 0.05 and less than or equal to 1.0, d is more than or equal to 0 and less than or equal to 1.0 and the sum of a, b, c and d is more than or equal to 3.0 and less than or equal to 4.0. In the invention, the specific capacity of the hydrogen storage alloy is much higher than that of the hydrogen storage alloy in the prior art. In addition, the initial capacity of a nickel-hydrogen battery made by the hydrogen storage alloy is also increased, and the cycle performance of the nickel-hydrogen battery is substantially improved.

The invention discloses a quickly charging device and a method of nickel-hydrogen battery. The device comprises a switch circuit, an adapter, a thermistancce clinging to the surface of the battery, a switch control circuit which is arranged in a micro-processor and connected with the connected digital analog conversion module and the switch circuit. When in charging, the voltage value of the battery is compared with the preset voltage boundary value; if the voltage of the battery is smaller than the voltage boundary value, the switch circuit is controlled to switch to constant voltage mode for quickly charging up the battery; if not, the switch circuit is controlled to switch impulse mode to conduct supplementary charging. When in quick charging, the battery temperature is compared with the preset temperature boundary value; if the battery temperature is higher than the preset temperature boundary value, supplementary charging is conducted; if the battery temperature is lower than the temperature boundary value, quick charging is conducted. The invention overcomes the occurrence of debounce voltage in measuring voltage at low cost, enhances the precision of the measure voltage value of the battery, and improves the control sensitivity.

The invention discloses a comprehensive recovery method for a nickel-hydrogen waste battery. The method comprises the following steps of: crushing the nickel-hydrogen waste battery and sieving the crushed battery by magnetic separation; preparing leaching solution which contains sulfuric acid and oxidant; throwing undersize powder into the leaching solution, increasing the temperature to 50 to 100 DEG C and leaching the mixture for 1 to 3 hours; performing solid-liquid separation, adjusting the pH value of filtrate to be 2 to 5 and adding water soluble sulfate to precipitate rare earth elements therein; and performing the solid-liquid separation and adding an extraction agent into the filtrate to remove impurities to obtain nickel and cobalt-containing sulfate solution. The comprehensive recovery method for the nickel-hydrogen waste battery can effectively recover various valuable metal elements in various nickel-hydrogen batteries. The extracted and purified nickel and cobalt-containing sulfate solution can be directly applied to the production of positive material spherical nickel hydroxide of the nickel-hydrogen battery. The recovery method has the advantages of low energy consumption in the recovery process, short route and good recovery benefit.

The invention discloses rare earth-magnesium-nickel system heterogeneous hydrogen storage alloys used for nickel-hydrogen batteries and a preparing method thereof. The alloys are characterized in that: the general structure formula of the alloys is LnMgNi<x>Y<y>Z<z>, wherein the Ln is at least one element selected from rare earth elements; the Y is at least one element selected from a group comprising Co, Mn, Al, Cu, Nb, V, Fe, Zn, Ti, Cr, As, Ga, Mo, Sn, In, W, Si, B and P; the Z is at least one element selected from a group comprising Ag, Sr, Ge and Au; the a is less than 1 and not less than 0.5; the b is less than 0.5 and more than 0; the x is more than 2.5 and less than 4.5; the y is more than 0 and less than 2; the z is more than 0 and not more than 1; an AB5 phase accounts for 10-90%; an A2B7 phase accounts for 10-90%; and the ratio of other impurity phases is not more than 30%. B side elements are replaced by the Ag, the Sr, the Ge, the Au, and the like, alloy components and an annealing process are optimized, and the ratio of the AB5 phase to the A2B7 phase in the product is adjusted, thus increasing the discharge capacity of the hydrogen storage alloy, prolonging the cycle lifetime of the hydrogen storage alloy, and improving self-discharge and low-temperature properties.

The present invention discloses a nickel-hydrogen battery charging equipment, which comprises a micro-processing unit and a charging unit. Wherein, the micro-processing unit is composed of a battery voltage sampling module and a charging control module. The charging control module controls the charging unit according to voltage information collected by the battery voltage sampling module. The charging unit is composed of a constant current control circuit, a constant voltage control circuit and a trickle control circuit connected in parallel, a charging circuit and a charging mode control circuit. The charging mode control circuit controls the constant current control circuit, the constant voltage control circuit and the tickle control circuit to select constant current, constant voltage and trickle charging mode according to a control signal of the charging control module. Based on the charging circuit, the nickel-hydrogen battery is charged. The present invention also discloses a nickel-hydrogen battery charging method and a terminal applying the charging equipment and the method above.

The invention discloses a Nickel-hydrogen battery pack heat dissipation system used in a hybrid motor vehicle. A reasonable battery interval and a suitable inclination angle are first designed to realize the vertical flow of air; and an upper layer battery and a lower layer battery are arranged at a certain interval; the lower layer battery is at the central line of the upper layer battery and the upper layer battery is at the central line of the lower layer battery, which causes the air not to flow into the upper side flow channel of the battery earlier; secondly, a thermal resistor is added on the battery surface based on the improvement of the structure; the thermal resistor is wrapped on the outside of a the battery module; the required wrapping thickness of each module is determined based on the heat dissipation effect; the surface of the battery with low temperature and good heat dissipation effect is thicker and the surface of the battery with high temperature and poor heat dissipation effect is thinner, thus the heat exchange between the battery and the air is reduced, and the local battery temperature is improved, and furthermore the temperature homogeneity of the whole battery pack is improved. The invention relates to a method that takes the temperature homogeneity of the module as priority by compromising the heat dissipation strength of the battery.

The invention relates to an energy flow controller for fuel battery electric motor car, and the controlling system is composed of pedal, entire controller ECU, fuel battery engine, nickel hydrogen battery package and driving system. The invention is to implement reasonable distribution of energy source between fuel battery and fuel storage battery and enhance and optimize power performance of the whole car. Its character: there is also an energy flow controller connected between the ECU and the driving system, where the energy flow controller is a module in the ECU, its input end is connected with the output end of a fuzzy logic controller (FLC), and its output end is connected with the input end of an electric motor controller in the driving system. It synthesizes advanced techniques in the fields of CAN field bus, fuzzy control, communication, etc.

The present invention provides a method for charging a NiMH battery, comprising estimating a state of charge of the battery (100), determining an end of charging time based on the estimated state of charge (200), applying a constant charging current with a rate in the range from 0.9 C to 2.1 C to the battery until a threshold voltage is reached (300), upon reaching the threshold voltage, applying a constant charging voltage substantially equal to the threshold voltage to the battery (400) and continuing applying the constant charging voltage to the battery until the end of charging time is reached (500). Furthermore, a battery charger adapted to perform such a method as well as a system comprising such a battery charger and a hearing device powered by a NiMH battery are given.

The present invention discloses a low-temperature nickel-hydrogen battery, which comprises a positive electrode, a negative electrode and an electrolyte, wherein active materials of the positive electrode comprise nickel hydroxide, cobalt-coated spherical nickel hydroxide, and yttrium oxide, a weight ratio of the nickel hydroxide to the cobalt-coated spherical nickel hydroxide to the yttrium oxide is 100:5-15:1-3, a substrate of the positive electrode is foam nickel, and the edge of the foam nickel has a compaction white edge with a width of 2-3 mm. The present invention further discloses a preparation method for the low-temperature nickel-hydrogen battery. The low-temperature nickel-hydrogen battery can maintain high charging efficiency under a low temperature environment, such that electrolyte solidification inside the nickel-hydrogen battery under a low temperature environment can be avoided, and a use temperature range of the nickel-hydrogen battery is expanded. In addition, characteristics of simple preparation and low cost are provided.

The invention relates to a hydrogen storage alloy, an electrode for a Ni-MH battery, a secondary battery and a method for preparing the hydrogen storage alloy. The chemical composition of the hydrogenstorage alloy is represented by the general formula La(3.0-3.2)x CexZrySm (1- (4.0~4.2)x-y)NizCouMnvAlw, wherein x, y, z, u, v, w are molar ratios; 0.14 <= x <= 0.17; 0.02 <= y <= 0 .03; 4.60 <= z +u + v + w <= 5.33; 0.10 <= u <= 0.20; 0.25 <= v <= 0.30; and 0.30 <= w <= 0.40. The overcharge performance of the electrode material is satisfied by fixing a ratio of lanthanum (La) to cerium (Ce) to3.0 - 3.2. A large number of samarium (Sm) elements on a side A are replaced, namely, the ratio of Sm atoms accounts for 25.6 to 42% of the side A to overcome a decrease in service life caused by lowcobalt (Co). The equilibrium pressure is adjusted by changing the ratios of Sm and La to Ce in order to satisfy the charge and discharge dynamics performance of the electrode material. The nucleationrate of a solidification process is increased by adding zirconium (Zr) having an atomic ratio of 2 to 3% relative to the elements at the side A to the elements at the side A. The Ni-MH battery anode material obtained by using the hydrogen storage alloy has high overcharge resistance, high rate discharge performance and good cycle stability.

The invention relates to a method for full-automatically preparing a positive plate of a nickel hydrogen battery, which comprises the processes of automatically discharging, automatically feeding and welding a nickel plate, prepressing, gluing, feeding powder, rolling, detecting thickness, softening, automatically welding a nickel wire, automatically rubberizing, automatically splitting, and automatically weighing. The whole process of manufacturing and processing the positive plate is full-automatic, so that the consistency of the positive plate, which is a key factor influencing the consistency of the battery, can be improved; and the method has the advantages of shortening the manufacturing time, saving labor cost, reducing the influence of personal factors, contributing to tracing theproblems of products, along with wide industrial application prospect.

The invention relates to a diaphragm for batteries, in particular to a novel nickel-hydrogen battery diaphragm. The novel nickel-hydrogen battery diaphragm comprises a diaphragm base fabric and sulfonic acid group. The sulfonic acid group is uniformly grafted on the surface of the diaphragm base fabric by sulfonation. The novel nickel-hydrogen battery diaphragm further comprises an acrylic acid monomer. The acrylic acid monomer is uniformly grafted on the surface of the diaphragm base fabric through a chemical grafting reaction. The diaphragm base fabric is prepared from polyolefin sheath-core type fiber or polypropylene fiber or polyolefin sheath-core type fiber and polypropylene fiber. The diaphragm base fabric is shaped by a wet process paper-making technology or a dry process carding technology and is reinforced by spunlace process. The novel nickel-hydrogen battery diaphragm disclosed by the invention has the characteristics of high chemical stability, strong oxidation resistance and high strength. The diaphragm has permanent hydrophilicity and liquid maintaining performance, improves ionic migration capacity and ion exchange capacity, effectively reduces internal resistance, improves large current discharging capacity and has excellent charge maintaining capacity.

The diaphragm is non-woven fabric of polypropylene backing material with hydrophilic group. Raw fibers are hot melted composite fiber with dual constituents in low melting point and superfine ultra short vinylon fiber; or the said composite fiber, superfine ultra short vinylon fiber, and superfine glass microfiber. Being in 60-90% in raw fibers, the said composite fiber includes inner core of polypropylene, and cortical layer of polyethylene. Through graft polymerization, the said hydrophilic group combines to non-woven fabric of polypropylene backing material. The fabricating method includes steps: hydrophilic preprocessing for raw fibers; making mesh; heat forming; hot rolling; grafting modification. Features are: simple method, permanent hydrophilic capability for keeping moisture, antioxidation, alkali proof, high tensile strength, and small discharge rate. The disclosed diaphragm is adaptive to technical requirement of diaphragm in nickel hydrogen battery.

A nickel-hydrogen battery pack heat removal system for hybrid vehicle is provided. The system includes a battery box, a power battery pack (8), a battery support frame (9), a support bar (5), an air inlet (1), an air outlet (6) and a fan (7). Batteries in the power battery pack (8) are aligned in parallel in two layers one above the other in the battery support frame (9) of the battery box, and the support bar (5) and the battery support frame (9) are fixed on the upper and lower soleplates respectively of the battery box. The air inlet (1) is set at one end of the battery box, the air outlet (6) is set at the other end of the battery box, and the fan (7) is fitted at the air outlet (6). The system further includes conduction plates (2) in the shape of circular arcs, said conduction plates (2) being fitted on the opposing faces of the two upper and lower batteries which are closest to the air inlet (1).

The invention discloses a flexible photovoltaic integrated power supply system. The power supply system comprises a physical power supply, a chemical power supply and a power supply controller. Solar energy is converted into electric energy through the physical power supply. A flexible thin film solar battery module is adopted and comprises an amorphous silicon thin film solar battery module, a microcrystalline silicon thin film solar battery module, an amorphous silicon-germanium thin film solar battery module, and a laminated solar battery module consisting of the amorphous silicon thin film solar battery module, the microcrystalline silicon thin film solar battery module and the amorphous silicon-germanium thin film solar battery module, or a copper-indium-gallium-selenium thin film solar battery module. The chemical power supply adopts a storage battery component which comprises a nickel-cadmium battery module, a nickel-hydrogen battery module, a lithium ion battery module, an alkaline-manganese charging battery module or a lead-acid storage battery module. The power supply controller is a solar power supply single output controller with the model of V-ST10, is respectively connected with the physical power supply, the chemical power supply and the load, and is used for regulating and controlling the physical power supply and the chemical power supply. The flexible photovoltaic integrated power supply system provided by the invention has the advantages of good flexibility, light weight, high intensity, good environment tolerance and the like.

The invention discloses a manufacturing method of a nickel-hydrogen battery. The manufacturing method comprises a positive plate manufacturing process, a negative plate manufacturing process, a rolling process, a placement process, an electrolyte injection process and a seal process. A diaphragm is arranged between a positive plate and a negative plate. A turnup edge of the positive plate stretches out of the upper end of the diaphragm and a raised part of a negative plate stretches out of the lower end of the diaphragm so that the plates are rolled together to form a cell. The upper end of the cell is connected to a battery cap by a current collector disc. The nickel-hydrogen battery comprises the positive plate, the negative plate, the diaphragm sandwiched between the positive plate and the negative plate, the battery cap and a battery case. The turnup edge of the positive plate stretches out of the upper end of the diaphragm and is bent by pressing to reach to the upper end surface of the cell. The upper end of the cell is electrically connected to the battery cap by the current collector disc. The manufacturing method is convenient for operation and batch production. The nickel-hydrogen battery has high charging efficiency, good heavy current discharge performances and excellent dynamic performances.

The invention relates to a wall-climbing robot which comprises a caterpillar tank chassis, wherein two sets of adsorption devices are arranged on a bottom plate of the caterpillar tank chassis, each set of adsorption device comprises a sucker type electromagnet and a vacuum sucker, and two sets of adsorption devices are arranged in an inverted triangular way; a micro vacuum pump and a battery box are respectively arranged at the left part and the right part in the caterpillar tank chassis; each vacuum sucker is connected with the micro vacuum pump; two sides of the caterpillar tank chassis are respectively connected with motor-driven caterpillar tracks; the width of the caterpillar tank chassis is 179mm, and the caterpillar tank chassis is made of ABS engineering plastics; three nickel-hydrogen batteries with capacity of 2300mAh and 1.2V are placed in the battery box; the volume of the micro vacuum pump is 55mm*16mm*36mm, the vacuum pressure is greater than or equal to 45kp, the air flow rate per minute is greater than or equal to 2.5L, and the working voltage is 5V; and the suction force of each sucker type electromagnet is 5kG. The invention has the characteristics of good universality, strong adsorptivity, small volume, light quality, large carrying capacity, convenient transportation and the like.

The invention discloses a method for preparing high purity nano cobalt hydroxide, belonging to the technical field of battery energy material preparation. Homogeneous phase precipitation method is adopted to synthesize cobalt hydroxide, including that: cobaltous sulphate aqueous solution, sodium hydroxide aqueous solution, cobalt ion chelating agent aqueous solution and aqueous solution of reagent A which are in certain concentration are respectively, sequentially and continuously added into a reactor to be stirred according to flow ratio of 1: 0.68-0.74: 0.048-0.054: 0.058-0.064, reaction is taken place, appropriate amount of additive aqueous solution is added, and ageing, solid-liquid separation, washing and drying are carried out, thus obtaining the high purity nano cobalt hydroxide product. The product not only has high purity, regular shape, small grain size and uniform distribution, but also has high specific area, higher bulk density and good liquidity, dispersity and workability, thus being beneficial to separate processing or uniformly coating cobalt hydroxide on the surface of nickel hydroxide particle by adopting chemical plating, so that combination property of nickel-hydrogen battery is improved.

A segmented nickel hydrogen battery system includes a hydrogen storage segment (130) and a battery segment (120) in fluid communication with the storage segment. The battery segment includes a plurality of electrochemical cells each having a current collector plate (104) and a plastic seal component (104) provided about the peripheral edge of the collector plate. The plastic seal component may be secured to the collector plate using a variety of methods, but is preferably injection-molded about the collector plate edge. The collector plate / seal segment subassemblies may then be stacked and the seal components bonded together to form an integral seal. The electrodes and separator are placed between the collector plates before bonding. Preferably, the electrodes and separator are formed as a bipolar cell construction.

The invention discloses a method for directly growing a carbon nanotube on a foamed nickel substrate so as to prepare battery electrodes. The method comprises the following steps: (1) performing chemical and ultrasonic cleaning by taking foamed nickel as a catalysis and substrate material for chemical vapor deposition reaction of the carbon nanotube; and (2) setting a CVD reaction device, directly performing the chemical vapor deposition growth of the carbon nanotube on the foamed nickel substrate in the CVD reaction device, the flow ratio between a hydrocarbon gas and a carrier gas is (10-40sccm):(100-400sccm) when the growth temperature is at 600-900DEG C, the growth pressure is at 1-760Torr, and the growth time lasts for 1-30min. The method has the advantages that the significant role in enhancing the performance of a battery device can be achieved, the specific surface area and the electrochemical activity of the CNT-foamed nickel substrate can be obviously improved, and the cycle life of the CNT-foamed nickel substrate nickel-hydrogen battery can be obviously prolonged.