Method for heating a hydraulic fluid in an electrically operated machine

Abstract

The invention relates to a method for heating a hydraulic fluid in an electrically operated machine, wherein the machine comprises a battery (16) with several battery cells, wherein the method is carried out during a work process of the machine and comprises the following steps: - discharging a single battery cell for the work process; - dissipating the heat generated during the discharge of the single battery cell to the hydraulic fluid; - switching the discharge process to another battery cell when the state of charge of the currently discharged battery cell reaches a state of charge threshold; and - switching the discharge process to several battery cells when the hydraulic fluid has reached a target temperature.

Description

The invention relates to a method for heating a hydraulic fluid used by a working machine to an operating temperature.Many mobile work machines, such as excavators and similar machines, are equipped with internal combustion engines and are operated with diesel fuel. These are started before the start of the work and allowed to warm, so that a hydraulic fluid, in particular a hydraulic oil, is heated and the viscosity thereby decreases. In this case, the engine generates the waste heat used. The relatively low diesel prices and the rapid refueling enable the diesel engine to warm up and the use of the generated waste heat of the engine for heating the hydraulic oil in mobile work machines.At low ambient temperatures, for example around zero degrees Celsius, the heating of the oil to a temperature greater than 45° C. is absolutely necessary. In mobile work machines that are operated electrically, heating of the oil cannot be achieved by simply "running" the electric motor. As long as no further consumers are connected after the excavator has been started, the drive train is generally switched off for energy engineering reasons, similar to the start / stop system in electrically operated vehicles. When the work machine is not operating, is in standby mode, or is performing smaller functions, the current from the battery is so low that too little waste heat is generated. Moreover, a warm-up would mean that the battery is discharged and the energy required for this purpose is therefore no longer available for the working process.A charging process is substantially more time-consuming than refueling, which lowers the attractiveness of the mobile electric working machines. The object of the present invention is therefore to present a concept for the efficient heating of the hydraulic fluid of an electrically operated working machine, with which it is possible to eliminate or reduce the disadvantages of known systems.The object is achieved by the subject matters of the independent claims.Disclosure of the InventionAccording to a first aspect of the invention, this object is achieved by a method for warming up a hydraulic fluid in an electrically operated working machine. The work machine includes a battery having a plurality of battery cells. The method is executed during a working process of the working machine and comprises the following steps:discharging a single battery cell for the working process;removing the heat, which arises during the discharge of the battery cell, to the hydraulic fluid;switching the discharging process to another battery cell when the state of charge of the currently discharging battery cell reaches a state of charge threshold value,switching the discharging process to a parallel connection of a plurality of battery cells when the hydraulic fluid has reached a target temperature.In the context of this invention, the term "battery cell" can be understood as synonymous with the term "partial battery". It is less important how the individual battery cells or partial batteries are technically designed. Rather, the effect used for the invention is based on the fact that a battery generates waste heat under heavy load, which can be used. Modern battery systems generally consist of a plurality of battery cells or partial batteries which are connected in parallel or in series, depending on the intended use.Connecting a plurality of battery cells in parallel causes an increase in the total capacity of the battery and enables a higher current flow. The voltage across the battery cells is constant, however.A series connection of a plurality of battery cells, on the other hand, increases the voltage of the battery at a constant current intensity. The capacity of the individual cells is equal to the capacity of the battery.Depending on the working process to be performed and the associated requirements for voltage and current flow, the battery cells can be connected in series or in parallel. Smart controllers also enable a combined circuit in which multiple blocks of parallel-connected battery cells are connected in series or, conversely, multiple blocks of series-connected battery cells are connected in parallel.Within the scope of this invention, the "switching over of the discharging process to a plurality of battery cells" means in particular a switching over to an operation in which a plurality of battery cells can in principle be used. This describes the regulation or normal operation provided for the battery.This control mode is distinguished from the discharge of an individual battery cell in that an individual battery cell is intentionally subjected to a greater load in order to generate waste heat which is then supplied to the hydraulic fluid. The waste heat arises because the one battery cell supplies a high current, which then heats the battery cell.A work process of a work machine may include, for example, driving the work machine to its location or to its location or performing a function such as digging an excavator.Hydraulic fluid that can be used is, in particular, oils that are used in many working machines, such as excavators, trucks, dozers, truck mixers or other construction machines.If an individual battery cell is subjected to too heavy loading and the states of charge of the battery cells differ from one another too much, compensating currents can arise between the battery cells during the regulating operation of the battery, which can damage the latter. This is to be avoided with the present invention by monitoring the state of charge of the individual battery cells. If the state of charge of the discharging battery cell is too low and reaches or falls below a state of charge threshold value, the battery cell is changed and another battery cell is discharged to execute the working process and generate waste heat. This ensures that the states of charge of the battery cells do not differ too much.In one embodiment, the method further comprises the step of monitoring the state of charge of the discharging battery cell.The discharging battery cell is the battery cell used to execute the working process and generate the waste heat.The selective discharge of an individual battery cell of a battery of a working machine generates waste heat which is used according to the present invention to heat the hydraulic fluid. This achieves the object of the invention.In one embodiment, the battery cells are changed through rotationally when the discharging process is switched over to another battery cell.For example, a battery cell 1 is discharged first. If the state of charge of the battery cell 1 falls below the state of charge threshold value, a switch is made to a battery cell 2 so that the latter is now discharged. If a further battery cell 3 is available, a switch is made from battery cell 2 to battery cell 3 if the state of charge of battery cell 2 falls below the state of charge threshold value, etc.In this way, it is ensured that the battery cells are discharged as uniformly as possible in succession and large compensation currents are avoided.In one embodiment, the state of charge threshold is a percent value of the average state of charge of all battery cells. The state-of-charge threshold value may be, in particular, 95%, 98% or 99% of the average state of charge of the battery cells. In an alternative embodiment, the state of charge threshold is a percentage of a selection of battery cells. In a further embodiment, the state-of-charge threshold value is calculated from the states of charge of the battery cells, wherein the states of charge of the battery cells which have recently been discharged are calculated with low weighting factors and the states of charge of the battery cells which have not been discharged for a longer time are calculated with a high weighting factor.Advantageously, the discharge of an individual battery cell is improved because the load on the battery cells is distributed to the batteries which have the highest state of charge. This minimizes the formation of compensation currents in the control mode.In one embodiment, the working machine further comprises a heating element, wherein the heating element is operated when the working machine is in a standby mode or the discharge current from the battery cell falls below a minimum current.Not all processes that a work machine executes require the same power. Some processes require little, others require more energy. For example, lifting an empty bucket requires less energy than lifting a full bucket.If the current from the battery cell is too low to generate significant heat that can be used to heat the hydraulic fluid, then the turning on of a heating element can speed up the heating. The heating element itself gives off heat. On the other hand, it must be supplied with power to generate the heat. The current flow required for the heating element in turn ensures increased loading of the battery cell and thus an increase in the waste heat. Thus, the effect underlying this invention is also exploited when using a heating element.In a further aspect, the invention relates to an electrically operated working machine. The working machine comprises a hydraulic working system which is operated with a hydraulic fluid, a battery having a plurality of battery cells, a means for dissipating heat of the battery cells to the hydraulic fluid and a means for switching over a discharging process from one battery cell to another battery cell if the state of charge of the currently discharging battery cell reaches a state of charge threshold value, and a means for switching over the discharging process to a plurality of battery cells if the hydraulic fluid has reached a target temperature.The temperature of the hydraulic fluid can be determined in particular with a thermometer or a suitable sensor for temperature determination. Other properties of the hydraulic fluid, such as viscosity or volume, also change with temperature. The change of these properties can likewise be used to determine the temperature of the hydraulic fluid at least indirectly.The means for removing heat of the battery cells to the hydraulic fluid can comprise, for example, a heat exchanger which transfers the heat to the hydraulic fluid with a suitable heat transport medium. Alternatively or additionally, the line of hydraulic fluid may be laid through the battery and past the battery cells, such that the line leads into direct contact with the battery cells.The means for switching over the discharging process to another battery cell comprises in particular a means, preferably an onboard computer of the working machine configured for this purpose, in order to monitor the state of charge of the battery cells. Preferably, the means for monitoring the state of charge of the battery cells is further configured to determine the state of charge threshold value as a percentage of the states of charge of the battery cells.In a further aspect, the invention relates to a computer program having program code for carrying out a method as described above when the computer program is executed on a computer.In a further aspect, the invention relates to a computer-readable data carrier having program code of a computer program for carrying out a method as described above when the computer program is executed on a computer.In summary, it can be stated that the present invention specifies a method for warming up a hydraulic fluid in an electrically operated working machine and a corresponding working machine.The described embodiments and developments can be combined with one another as desired.Further possible embodiments, developments and implementations of the invention also include combinations of features of the invention described above or below with respect to the exemplary embodiments, which combinations are not explicitly mentioned.Brief Description of the DrawingsThe accompanying drawings are intended to provide a further understanding of the embodiments of the invention. They illustrate embodiments and, in conjunction with the description, serve to explain principles and concepts of the invention.Other embodiments and many of the advantages mentioned are evident with reference to the drawings. The elements of the drawings shown are not necessarily shown true to scale with respect to one another.The following are shown: FIG. 1 schematically shows the sequence of a method for warming up a hydraulic fluid; FIG. 2 shows a work machine in the form of a wheel loader according to one embodiment.In the figures of the drawings, identical reference numerals designate identical or functionally identical elements, components or components, unless indicated to the contrary.FIG. 1 schematically shows the sequence of a method for warming up a hydraulic fluid in an electrically operated working machine.The method begins with step S 10, for example when the work machine is started. Since the work machine requires a warm hydraulic fluid for performing its tasks, the energy for its first tasks is provided by only one battery cell in step S 12. This generates heat which is dissipated to the hydraulic fluid in parallel in step S 14.In carrying out the method according to this embodiment, two parameters must be monitored: the state of charge of the discharging battery cell and the temperature of the hydraulic fluid.The state of charge of the discharging battery cell is checked in step S 16. If the state of charge reaches a state of charge threshold value, for example 95% of the average state of charge of all battery cells, the battery is switched over in step S 18, with the result that a different battery cell is now discharged. This ensures that a battery cell is not excessively discharged and thus harmful compensation currents are produced during the regulating operation of the battery.If the state of charge of the discharging battery cell has not reached the state of charge threshold value in step S 16, the method continues with the monitoring of the temperature of the hydraulic fluid in step S 20.The temperature can be determined directly with a thermometer or indirectly, for example via a pressure sensor in the hydraulic line. If the hydraulic fluid has not yet reached the necessary or desired temperature, the target temperature, the operation starts from the beginning at step S10. When the hydraulic fluid has reached the necessary target temperature, the battery switches over to the regulating operation in which all battery cells are available to the working machine in any circuit configuration, preferably in a parallel circuit.In one embodiment, the hydraulic systems of the working machine are only available when the hydraulic fluid has reached the target temperature. Thus, the optimum working performance of the hydraulic systems can be ensured. The working steps which the working machine can carry out up to this point in time can then be limited, for example, to moving or driving the working machine, preferably to its place of use.FIG. 2 shows a work machine having a hydraulic work system. The illustrated work machine is a wheel loader 10 having a loader arm 12 to which a bucket is attached at the front. The loading arm 12 can be moved, in particular raised and lowered, via a hydraulic system 14. If the pressure in the lines of the hydraulic system 14 is not sufficiently high, the loader arm 12 cannot be moved with the bucket.The pressure in a hydraulic line can be regulated by means of the temperature of the hydraulic fluid contained therein. Therefore, it is important to regulate the temperature of the hydraulic fluid. In the case of fossil-powered working machines, this task is taken over by the engine and the waste heat generated therewith.However, the wheel loader 10 shown here is electrically operated. It has an electric motor which is supplied with electrical energy by a battery 16 having a plurality of battery cells. However, an electric motor does not generate sufficient waste heat to heat a hydraulic fluid. This is a problem, especially on cold days, for example in winter.Furthermore, loading a single battery cell is problematic because it can thereby discharge strongly. If the battery cell is then connected in parallel with the other, fully charged battery cells during the actual working operation, compensation currents arise between the battery cells, which can damage the cells.In the embodiment shown here, the invention can be realized in such a way that a single battery cell of the battery 16 is used to drive the wheel loader 10 to its place of use on a construction site. For this purpose, the wheel loader has a chassis 18, which is driven by an electric motor, which in turn draws the necessary electrical energy from the individual battery cell.When the battery cell has reached a certain state of charge and the hydraulic fluid has not yet been warmed up to its target temperature, the battery 16 switches over to another battery cell, so that it is now discharged. These steps are repeated until the hydraulic fluid has reached the target temperature. Preferably, the battery cells are rotated through in this process, so that the one-sided load is distributed stepwise over all battery cells and the differences in the states of charge are kept to a minimum.The wheel loader 10 further comprises a second hydraulic system 20 which serves for steering the wheel loader 10. The hydraulic fluid in the hydraulic system 14 of the loader arm 12 may be heated during travel of the wheel loader 10 to its location of use. However, without a functioning steering system, the wheel loader 10 can only move to a limited extent. That is, the hydraulic system 20 for steering must be operational in front of the hydraulic system 14 of the loader arm 12.Of course, the hydraulic fluid of the hydraulic system 20 may be heated prior to the hydraulic fluid of the hydraulic system 14. For this purpose, the hydraulic fluids should each have their own circuit. The hydraulic fluid of the hydraulic system 20 can, however, be heated by means of a consumer, in particular a heating element. A heating element loads the battery 16 of the wheel loader 10 and thus generates heat. If the same method as described above is used for the operation of the battery 16, additional heat is also released by the load of the individual battery cell, which heat can be used for heating the hydraulic fluid of the hydraulic system 20. In particular, it is then advantageous if the hydraulic systems 14 and 20 are separated from one another. The operation of a heating element is to be restricted to a minimum. When the hydraulic systems 14 and 20 are disconnected, only the hydraulic fluid of the hydraulic system 20 needs to be heated until the steering is operational and the wheel loader 10 is ready to drive. The hydraulic fluid of the hydraulic system 14 can then be heated during the travel of the wheel loader 10 to the location of use solely by the waste heat from the battery 16, which then generates sufficient waste heat as a result of the travel.

Claims

Method for warming up a hydraulic fluid in an electrically operated working machine, wherein the working machine comprises a battery (16) having a plurality of battery cells, wherein the method is carried out during a working process of the working machine and comprises the following steps: - discharging a single battery cell for the working process; - removing the heat which arises during the discharging of the single battery cell to the hydraulic fluid; - switching the discharging process to another battery cell if the state of charge of the currently discharging battery cell reaches a state of charge threshold value; and - switching the discharging process to a plurality of battery cells if the hydraulic fluid has reached a target temperature.Method according to Claim 1, wherein the battery cells are changed through rotationally when the discharging process is switched over to another battery cell.The method of any preceding claim, wherein the state of charge threshold is a percent value of the average state of charge of all battery cells or selected battery cells.Method according to one of the preceding claims, wherein the working machine further comprises a heating element for heating the hydraulic fluid and wherein the heating element is operated when the working machine is in a standby mode or the discharge current from the battery cell falls below a minimum current.An electrically operated working machine comprising: - a hydraulic working system (14, 20) operated with a hydraulic fluid; - a battery (16) having a plurality of battery cells; - means for dissipating heat of the battery cells to the hydraulic fluid; - means for switching a discharging process from one battery cell to another battery cell when the state of charge of the currently discharging battery cell reaches a state of charge threshold; and - means for switching the discharging process to a plurality of battery cells when the hydraulic fluid has reached a target temperature.A computer program comprising program code for carrying out a method according to any one of claims 1 to 4 when said computer program is executed on a computer.Computer-readable data medium with program code of a computer program for carrying out a method according to one of Claims 1 to 4 when the computer program is executed on a computer.

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