GB2417567A

GB2417567A - Indicating charge state of a battery in a battery powered industrial vehicle

Info

Publication number: GB2417567A
Application number: GB0418902A
Authority: GB
Inventors: Rhys Owen Jones; Christopher John Lewis
Original assignee: Linde Material Handling UK Ltd
Current assignee: Linde Material Handling UK Ltd
Priority date: 2004-08-24
Filing date: 2004-08-24
Publication date: 2006-03-01
Anticipated expiration: 2024-08-24
Also published as: DE102005038648A1; GB0418902D0; GB2417567B

Abstract

A battery powered industrial truck includes means to determine and/or display at least two parameters indicative of the energy available in the battery. The parameters may be determined from measurement of the time profile of the battery voltage. They may be a measure of the charge which can be removed from the battery under ideal discharge conditions i.e. the open terminal or state of charge voltage, Vsoc and the charge which can be removed when the vehicle is in operation (under load) i.e. battery condition voltage VCON. Both parameters may be indicated to the vehicle driver on an illuminated segment bar chart display 1.

Description

24 1 7567 Industrial truck and method of operating an industrial truck

having at least one battery The invention relates to an industrial truck having at least one battery. The invention likewise relates to a method of operating an industrial truck having at least one battery. Industrial trucks of this type are used for goods transport within a company, preferably when using electrical traction and lifting drives where damaging exhaust gas emissions are to be avoided, particularly in internal spaces. As compared with vehicles with an internal combustion engine drive, however, reduced periods of use have to be tolerated, since recharging or replacement of the battery is substantially more complicated and more time-consuming then a tank filling operation. For this reason, frequent charging or replacement operations should be avoided, which may be achieved in particular by means of optimized utilization of the supply of energy stored in the battery. One of the preconditions for this is the most accurate possible knowledge of the current state of charge of the battery since, in the event of underestimating the charge available, excessively frequent charging operations are carried out, while in the case of overestimating the state of charge, the vehicle can fail unexpectedly, which signifies an even more substantially serious restriction to the period of use and reduces battery life.

In order to determine the supply of energy stored in the battery, various methods are known. One of these is to determine the state of charge by measuring the open terminal voltage of the battery. These two variables are approximately proportional in the case of a load-free battery. During operation of an industrial truck, however, various factors occur which can lead to distortion of the result: since the battery voltage decreases under load, in the case of a voltage-based indication of the state of charge, the energy content available will be underestimated, which leads to an unnecessarily early charging operation. However, as a result of lasting intensive use of the industrial truck, a nonuniform concentration distribution of the battery electrolyte may also arise, which leads to a reduced capacity. If this is not taken into account, the energy supply will be overestimated and a damaging low discharge of the battery can occur. Following a certain rest period, the distribution is normalized again and the full capacity is again available. A simple indication of the state of charge based on battery voltage is therefore subject to many uncertainties and for this reason is suitable only to a limited extent for use in an industrial truck.

Methods in which the charge content of the battery is determined by determining the currents flowing to and from the battery and integration of these measured values possess greater accuracy with regard to the supply of energy stored in the battery. In the event of a battery change, which is frequently performed in practice in industrial trucks, since the time needed for this is substantially shorter than for the charging of the battery, the usable amount of charge contained in the new battery is not known, however. This disadvantage can to some extent be circumvented by the storage of battery data on the battery or in a control device being performed, although this is relatively complex. To some extent, a combination of voltage and current measurement is also used, from time to time also taking the battery temperature into account, in order to determine the state of charge of the battery when it is installed in the industrial truck and to monitor the further removal of charge. However, all these methods have the disadvantage of being relatively complicated, since both comprehensive measurements and calculations are needed. Furthermore, - 3 using these methods it is not possible to register the reduction in the capacity because of nonuniform electrolyte concentration in the event of lasting heavy operation.

The methods known according to the prior art, for

industrial trucks having at least one battery, for calculating and displaying the electrical energy that can be taken from the battery are therefore either very complicated or supply only unreliable results, which either systematically overestimate or underestimate the true state, depending on the method, or else deviate in an unpredictable way, depending on the discharge conditions.

The invention is therefore based on the object of at least in preferred embodiments providing an industrial truck having at least one battery in which the residual capacity of the battery available under different operating and loading conditions can be monitored reliably with little effort for control tasks and/or for display to the operator.

Furthermore, the invention is based on the object of at least in preferred embodiments providing a method of operating an industrial truck having at least one battery with which the residual capacity of the battery available under different operating and loading conditions can be monitored reliably with little effort for control tasks and/or for display to the operator.

According to the invention, with regard to the industrial truck, at least in preferred embodiments this object is achieved in that means are provided for determining and/or displaying at least two different characteristic variables for characterizing the available electrical energy. - 4 -

Since two different characteristic variables for characterizing the available electrical energy are determined and/or displayed, it is possible, under different conditions, to use the respectively relevant characteristic variable, in order for example to operate a capacity display or to control the operation of the industrial truck. By means of determining the different

characteristic variables, a statement about the

uncertainty of the information is additionally possible.

It is particularly advantageous if one of the two characteristic variables for characterizing the available electrical energy is a measure of the charge (state of charge, VsOc) which can be removed under approximately ideal discharge conditions. This provides an upper limit on the capacity which, even in the case of a prudent mode of operation, cannot be exceeded. In particular in the case of operation under low loading, for example light traction operation without high load movements and with sufficient rest periods, reliable information about the residual capacity is thus available.

It is, moreover, particularly advantageous if at least one of the at least two characteristic variables for characterizing the available electrical energy is a measure of the charge (battery condition, VcOn) which is available under current, possibly adverse, discharge conditions. The current battery condition is substantially determined by the discharge conditions and can lie considerably below the state of charge. Even in the case of an immediate changeover to ideal removal conditions, the full charge capacity of the battery, as characterized as state of charge, is not available. The information about the battery condition offers an estimate of the energy available even under unfavourable conditions and in this way reliably helps to avoid a damaging low discharge.

In an expedient development of the invention, the determination of at least one of the at least two characteristic variables for characterizing the available electrical energy may be performed exclusively from the time profile of the voltage present on the battery. Only one variable is monitored, which means that the technical outlay for acquiring and processing the measured values is low. A measurement of the battery voltage is normally provided in industrial trucks for other tasks, so that no additional measuring devices are required. Equipping older vehicles with means according to the invention is therefore also possible with little expenditure.

In a particularly expedient development, the determination of all the characteristic variables for characterizing the available electrical energy may be performed exclusively from the time profile of the voltage present on the battery. If all the characteristic variables are determined from the measurement of one variable, the technical outlay for acquiring and processing the measured values is reduced considerably. A measurement of the battery voltage is normally provided in industrial trucks for other tasks, so that no additional measuring devices are required.

Equipping older vehicles with means according to the invention is therefore also possible with little expenditure.

It is, moreover, advantageous if means are provided which automatically reduce the power which can be taken from the battery if the characteristic variable falls below at least one limiting value for the charge (VcOn) which can be removed under operationally typical, in particular current, discharge conditions. As a result, the functions - 6 of the industrial truck are no longer available to the full extent, which prolongs the operating time and, as a result of the lower loading, increases the available capacity. The operator is made aware of the low residual capacity and will move in good time to a charging station or replace the battery in order to produce full serviceability. Even completely switching off individual or all large electrical loads, such as traction and pump drives, is conceivable. A damaging low discharge of the battery is therefore avoided.

In a particularly advantageous refinement of the invention, only one indicating device is provided as means for displaying at least two characteristic variables for characterizing the available electrical energy. The display of the two variables in one indicating device makes it possible for the operator to register both variables with one glance. In addition, in this way a space-saving and simple construction of the industrial truck is achieved.

It is further of advantage if the means for displaying at least two characteristic variables for characterizing the available electrical energy have an indicating display in the form of a bar graph, which is preferably formed from segments. Displays of this type are familiar as a state-of-charge indication and trusted by operators. The legibility is very good. The display of two characteristic variables in a segmented bar graph is possible in a straightforward manner and indicates immediately to the operator that there is a difference between the variables.

With regard to the method, at least in preferred embodiments the object is achieved in that at least two different characteristic variables for characterizing the available electrical energy are determined. Since two different characteristic variables for characterizing the available electrical energy are determined, it is possible, under different conditions, to use the respectively relevant characteristic variable in order to control the operation of the industrial truck or in order to supply data to a driver information system. By means of determining the different characteristic variables, a statement about the uncertainty of the information is additionally possible.

At least one of the at least two characteristic variables for characterizing the available electrical energy is preferrably a measure of the charge (state of charge, VsOc) which is equivalent to the remaining charge if the battery is being discharged under approximately ideal discharge conditions. This variable is an upper limit on the capacity which, even in the case of a prudent mode of operation, cannot be exceeded. In particular in the case of operation under low loading, for example light traction operation without high load movements and with sufficient rest periods, reliable information about the residual capacity is thus available.

It is further expedient if at least one of the at least two characteristic variables for characterizing the available electrical energy is a measure of the charge (battery condition, VcOn) that can be removed under current, possibly adverse, discharge conditions. The current battery condition is substantially determined by the discharge conditions and can lie considerably below the state of charge. This provides an estimate of the available energy even under unfavourable conditions, which makes it possible to take measures against a damaging low discharge.

Preferably, the measured variable used for determining at least one of the at least two characteristic variables for characterizing the available electrical energy is only the time profile of the voltage present on the battery. The processing of an individual measured variable can also be managed well with simple means. A method which, for example at fixed time intervals, processes measured values processed for the battery voltage and uses the current measured value and values of the characteristic variables determined in the preceding step to calculate new values may be implemented simply in familiar electrical control devices for industrial trucks.

It is particularly advantageous if, in order to determine all the characteristic variables for characterizing the available electrical energy, only the time profile of the voltage present on the battery is used. The processing of an individual measured variable in order to calculate all the characteristic variables reduces the effort for measured value acquisition and processing considerably.

The implementation of the method in conventional electrical control devices is made easier.

It is expedient if at least one of the characteristic variables for characterizing the available electrical energy can be represented as an electrical voltage, in particular in a voltage range between 1 volt and 3 volts.

Representation as a voltage facilitates a comparison with electrical measured variables, such as the voltage present on the battery or individual cells of the battery. The voltage range corresponds approximately to the voltage of an individual battery cell of a lead acid battery, which means that the comparison is possible in a straightforward manner even in the case of batteries with different numbers of cells and therefore a different output voltage. - 9 -

It is furthermore expedient if the characteristic variable being a measure of the charge which can be removed under approximately ideal discharge conditions (state of charge, VsOc) can be represented as an electrical voltage, preferably in a voltage range between 1.9 volts and 2.2 volts, especially in a voltage range between 1.989 volts and 2.14 volts. The voltage range corresponds approximately to the terminal voltage of an individual battery cell of a lead acid battery after a sufficient rest period.

If the battery condition VcOn falls below at least one limiting value Vcut, the power which can be taken from the battery is preferably reduced automatically.

As a result, the functions of the industrial truck are no longer available to the full extent, which prolongs the operating time and, as a result of the lower loading, increases the available capacity. The operator is made aware of the low residual capacity and will move in good time to a charging station or replace the battery in order to produce full serviceability. Even completely switching off individual or all large electrical loads, such as traction and lifting drives, is conceivable. A damaging low discharge of the battery is therefore avoided.

In an advantageous refinement of the invention, the limiting value for the battery condition characteristic variable VcOn which, if it is undershot, the power which can be taken from the battery is automatically reduced, preferably lies in a range from 1.989-2.004 volts. This voltage range corresponds to the individual cell voltage of a battery which is already considerably discharged but not yet in the low discharge range. -

It is expedient if, when the state-of-charge characteristic variable VsOc is greater than the voltage measured on the battery in the instantaneous measuring cycle and/or a comparative voltage assigned uniquely to this, in particular a calculated voltage of an individual battery cell yin, a smaller change is made in the state of charge VsOc than if VsOc is less than yin. If the state-of- charge characteristic variable VsOc is less than the battery voltage yin, then it may be assumed that a charging operation is taking place. During charging operations, the state of charge changes more slowly than during discharge operations, for which reason a smaller change is made in VsOc than if the battery is in the discharging state.

It is expedient that, if the battery voltage Vin is less than the stateof-charge characteristic variable VsOc but the state-of-charge variable VsOc is greater than a limiting value VfUll which corresponds to an approximately completely charged battery (i.e. more than 70% of maximum charge), a larger change is made in the state-of-charge characteristic variable VsOc than in normal operation. If Vin is less than VsOc' it may be assumed that a discharging operation is taking place. When the vehicle is switched on with a completely charged battery, because of the polarization effects within the battery during the charging operation, VsOc can lie above the value which corresponds to a full charge and in this way misrepresent a greater battery capacity. As a result of the larger change of VsOc as opposed to normal operation, faster adaptation of VsOc to the actual energy content is achieved.

It is also expedient that, if a rest state of the industrial truck is determined, a more rapid equalization of the state-of-charge characteristic variable Vsoc to the measured battery voltage Vin is made than in normal operation of the battery. In the rest state, battery recovery takes place. The state-of-charge characteristic variable VsOc reproduces this process better as a result of the more rapid adaptation.

If the battery voltage Vin falls below a limiting value, preferably at the level of 1.55 V, it is expedient if a smaller change is made in the state-of-charge characteristic variable VsOc than in normal operation.

Given a low state of charge of the battery, the internal resistance rises. Under a predefined load, the battery voltage decreases more considerably than under other conditions. This misrepresents an excessively low state of charge. In order to avoid this, VsOc may be equalized more slowly.

It is moreover of advantage if, if the battery voltage Vin exceeds a first limiting value Vhi, which lies above the voltage of an approximately fully charged battery VFU11 and preferably has a value from 2.1 V to 2.3 V, a smaller change is made in the state of charge VsOc than in normal operation. A high voltage of this type occurs during short-term charging of the battery, for example during regenerative braking and, in the case of direct conversion, would lead to an overestimate of the state of charge.

In an advantageous development of the invention, if the battery voltage Vin exceeds a second limiting value VGag, which is higher than the first limiting value Vhi, preferably a value at which effects occur, in particular gas formation, which are disadvantageous for the battery and/or the user and/or the industrial truck, the change in the state-of- charge characteristic variable Vsoc is again reduced as compared with the change made when the first limiting value Vhi is exceeded. When the battery voltage is excessively high, no more improvement to the - 12 state of charge takes place. However, such high voltages occur only briefly. By means of the slower adaptation of the state of charge to the battery voltage, such voltage peaks are prevented from falsifying the information.

The adaptation of the battery condition characteristic variable VcOn is preferably performed as a function of the level of the voltage Vin measured on the battery. Voltage changes of equal magnitude in the battery voltage Vin at different levels of the battery voltage Vin can correspond to differently intense loading of the battery.

It is likewise advantageous if, as Vin falls, the battery condition characteristic variable VcOn is reduced increasingly more sharply with a change of equal magnitude in the battery voltage yin. As a result, account is preferably taken of the fact that the battery condition is substantially more adversely affected at low battery voltages, that is to say following intense loading of the battery, than at higher battery voltages.

It is preferable that, when a rest state of the industrial truck is determined and the battery condition characteristic variable VcOn lies below the state-of charge characteristic variable VsOc, the battery condition characteristic variable VcOn rises. This takes account of the fact that the battery recovers in rest phases and the battery condition therefore improves.

Further advantages and details of the invention will be explained in more detail using the exemplary embodiment illustrated in the schematic figures, in which: Figure 1 shows a typical profile for individual cell voltage and state of charge of a battery during a discharge cycle with constant discharge current, - 13 Figure 2 shows a typical profile for individual cell voltage, state of charge and battery condition of a battery during a cycle of use which is close to reality of an industrial truck with alternating stress, Figure 3 shows a schematic representation of an indicating device according to the invention.

Figure 1 shows a typical profile of individual cell voltage Vin and stateof-charge voltage VsOc for a discharge cycle of a battery with constant discharge current.

In industrial trucks, use is normally made of lead-acid batteries, which are assembled from individual cells, in order to achieve the typical nominal voltages, for example 24 V, 48 V or 72 V. From the total voltage VBat measured on the battery and the number of cells, the battery voltage Vin of an individual cell can be calculated, and thus a universal comparative variable can be determined.

The state-of-charge voltage VsOc is a voltage which corresponds to the open terminal voltage of the battery after a sufficiently long rest period. The open terminal voltage is proportional to the state of charge of the battery, that is to say the amount of energy which can be taken under optimum discharge conditions, the state of charge of the battery in turn being directly proportional to time, given a constant discharge current. The state of charge is one of the characteristic variables which, according to the invention, are used for characterizing the available electrical energy. - 14

For most applications optimum discharge conditions can be defined as the conditions which have to be applied in order to obtain the complete charge according to the battery capacity as quoted by the battery manufacturer.

In the case of lead acid batteries for use in industrial trucks these conditions usually are given as a 5 hour constant current discharge cycle.

During the charge cycle, the charge voltage V1 is applied, which is switched off one hour before the start. The battery voltage Vin falls to the value V2. This value is the rest voltage of the fully charged battery and is a reference variable for the calculation of the state of charge.

During the removal of power, the battery voltage Vin falls steeply in accordance with the internal resistance of the battery. After an initial overawing, the battery voltage falls with an almost constant slope over a wide range of the discharging operation. When a voltage value V3 is reached at which the battery is already largely discharged, the internal resistance rises strongly, as a result the voltage falls more quickly and ultimately reaches the final value V4. After the end of the removal of current, the battery voltage rises again and reaches the value V5, which corresponds to a completely discharged battery.

In the event of an interruption to the discharge cycle before the complete discharge of the battery, the voltage Vin rises in a similar way to that at the end of the discharge cycle shown and, after a certain time, reaches a specific rest voltage VsOci'. If this profile can be modeled by suitable calculations, every point (i) on the battery voltage curve can be assigned a voltage VsOci, to which the battery voltage Vin would rise after a sufficient rest period.

Given a constant discharge current, the calculation of the state-ofcharge voltage vsoc and therefore the state of charge is relatively simple over a wide range, since the recovery curve for all the times would have approximately the same profile as that shown at the end of the discharge cycle, and thus the voltage difference between battery voltage and state-of-charge voltage depends only on the level of loading and the internal resistance of the battery. Since the internal resistance is under the given conditions a function of the amount of charge taken from the battery this difference can be caclulated easily.

Only at the start (tB) of the discharging operation and at the end (tE) is no direct relationship given by a deviation of the battery voltage Vin from the ideal linear profile, that is to say an addition of a constant value to the measured battery voltage according to the method described above would supply a false statement about the state of charge for these ranges. Under real operating situations, a constant discharge current is normally not given, for which reason, depending on the operating state of the battery, which can be determined by using the measured battery voltage, corrections can be made, which are explained in the following text.

Because the battery voltage is measured at fixed times, it is possible, in addition to the voltage itself, to determine its change over the course of time as well. A simple exemplary embodiment of the calculation of a state-of-charge voltage VsOcn' consists in adding a correction value to the state-of-charge voltage Vsocn-1' determined in the previous measuring cycle (n-1), and also a value which consists of a difference, weighted with a factor, between the state-of-charge voltage Vsocn-l' determined in the previous measuring cycle (n-1) and the - 16 battery voltage Vin(n'. If the battery is in the rest state, that is to say the battery voltage Vin does not change, then the state-of-charge voltage VsOc remains at the same level as the battery voltage yin. The influence of the different operating states is ta ken into account by the weighting factor and the correction value.

For example, if the battery voltage Vin falls slowly, which corresponds to the gentlest possible discharge, then the state-of-charge voltage VsOc remains virtually identical to the battery voltage yin. In the event of a sudden drop in the battery voltage yin, the state-of- charge voltage VsOc follows the battery voltage Vin with a certain delay, which depends on the weighting factor.

The discharging operation, in which none of the conditions mentioned below is relevant, is designated normal operation. In this operating mode, the battery voltage Vin(n' typically lies in a range between 1.2 V and 2.14 V, is less than the state-of-charge voltage Vsoc(n-l' determined in the preceding measuring cycle (n-1) and falls at a rate which is not excessive.

If the battery voltage Vin(n, in the current measuring cycle (n) lies above the state-of-charge voltage Vsoc(n-1' determined in the preceding measuring cycle (n-l), a charging operation is to be assumed. In this case, a smaller weighting factor should be chosen than if the battery voltage Vin(n' is lower than the state-of-charge voltage VsOc(n-l'' that is to say, with a change of equal magnitude in the battery voltage yin, the new value of the state-of-charge voltage VsOc(n' differs less from the preceding VsOcn-l' during a charging operation than during a discharging operation.

After the battery has been charged, the voltage is initially somewhat higher than corresponds to the fully - 17 charged state. The cause of this is polarization effects within the battery. This results in an overestimate of the state of charge. If Vintn' is less than Vsocn-l' from the preceding measuring cycle (n-l), that is to say no charging operationis detected, but VsOcn-l, from the preceding measuring cycle (n-l) is greater than a limiting value VfUll, which corresponds to an approximately fully charged battery, the correction factor will therefore be chosen in such a way that a larger change is made in VsOc than if VsOcn-l, is less than Vfull. As a result, the fastest adaptation possible of VsOc to the actual conditions is achieved.

In the rest state of the battery, that is to say when the electrical loads of the industrial truck are switched off, the battery voltage Vin approaches the ideal open terminal voltage corresponding to the state of charge.

This procedure takes place relatively quickly and therefore, given a weighting factor which is unchanged as compared with normal charging or discharging operation, can lead to an estimate of the state of charge which is wrong from time to time. Therefore, if Vin lies below Vsoc and if the battery voltage Vin remains approximately constant over a specific time period, the weighting factor is changed in such a way that a more rapid adaptation of VsOc to Vin is made than in normal discharging operation. In order to determine whether there is a rest state, a time period of about 15 seconds, during which the change of Vin does not exceed a fixed limiting value, has proven to be expedient in the case of industrial trucks.

In the case of a low state of charge of a battery, the internal resistance increases. Under the same loading, this leads to a sharper drop in the battery voltage Vin than at higher states of charge. In order to avoid an erroneous calculation of the state of charge here, the - 18 correction factor when the battery voltage Vin falls below a limiting value Vrif is changed with the effect that, with a change of equal magnitude in the battery voltage yin, a smaller change is made in the state-of-change voltage VsOc than under normal discharge conditions. The state-of-charge voltage VsOc in the example shown in Figure 1 with a constant discharge current does not follow the battery voltage Vin but falls further with a constant slope. In practice, a limiting value of 1. 55 V for Vrif has proven to be advantageous.

In particular during regenerative braking, that is to say when the drive motors of the industrial truck are operated as a generator during a braking operation and feed electrical energy back into the battery, Vin can exceed the nominal voltage of the battery considerably without the state of charge rising to the same extent.

In order to the greatest possible extent to prevent distortion of the state-of-charge value, if a limiting value Vhi for Vin is exceeded, the weighting factor is changed in such a way that equalization takes place more slowly than in normal operation.

If the battery voltage exceeds a further limiting value Vga5, which corresponds to the gassing voltage, the weighting factor is changed once more, so that the equalization is retarded once more. This prevents the state of charge rising excessively sharply in the event of short-term voltage peaks in the battery voltage Vin and leading to an overestimate of the energy reserves.

In Figure 2, a typical profile for the characteristic variables individual cell voltage yin, state-of-charge voltage VsOc and battery condition voltage VCon which are relevant during the discharge of a battery of an industrial truck in a cycle of use which is close to reality is shown. State of charge and battery condition - 19 are the two characteristic variables which are determined with the method according to the invention and are displayed in an industrial truck according to the invention in an indicating device shown in Figure 3.

The state of charge is calculated as already outlined and merely characterizes the energy which can be taken under ideal conditions, the influence in particular of short- term effects, such as the sharp fall in the battery voltage under high stress, deliberately being suppressed.

However, under unfavourable operating conditions, in particular with a high power consumption, this can lead to the state-of-charge information overestimating the energy reserve available in the current operating state.

In order to take such effects into account, according to the invention a further characteristic variable, what is known as the battery condition, is used. The battery condition is thus a characteristic variable which is used to characterize the instantaneous state of the battery.

The calculation of the battery condition is carried out, for example, by a method according to the principle outlined above for the state of charge, but weighting factor and correction value for the battery condition generally assuming different values from those for the calculation of the state of charge. In addition, in the case of the battery condition, the weighting factor is chosen as a function of the operating conditions and, as an output variable, a battery condition voltage VcOn is generated, which can simply be compared with further variables in the manner already outlined. In the following text, special operating states and their effects on the battery condition voltage Vcon will be outlined.

If Vine, is higher than the battery condition voltage VcOnn-l' determined in the previous measuring cycle (n-1), - 20 it is to be assumed that the battery is recovering from a stress, that is to say, for example, is being charged up or else the loading has been reduced. If, on the other hand, Viny is less than VcOnn-l'' the battery is under stress. In the latter case, a higher weighting factor will be chosen, that is to say changes to VCOn will be made more rapidly than during recovery phases. This corresponds to the different behaviour of the battery during recovery and stress, that is to say recovery takes place more slowly than a worsening of state under stress.

As opposed to the state of charge, in the case of the battery condition it is generally not possible to detect unambiguously from the comparison of battery condition voltage VcOn and battery voltage Vin whether a charging or discharging operation is taking place, since the battery condition does not reproduce the upper limit of the available charge and therefore, even during operation with power being taken, the battery voltage Vin can briefly be higher than the battery condition voltage VcOn.

As the battery voltage Vin decreases, the battery reacts increasingly more sensitively to stress, that is to say, with the same power consumption, at a low battery voltage Vin the battery condition becomes considerably worse than at higher battery voltages. In order to take account of this fact, four threshold values are fixed (vSl=l.5 V, Vs2= 1.4 V, Vs3= 1.3 V, VS4=1.2 V), at which, if they are undershot, in each case the weighting factor is increased, that is to say a battery voltage change with the same magnitude leads to a larger change of VCon if Vin lies below one of the threshold values than if the voltage Vin lies above this.

If VcOn falls below a defined limiting value Vcut, typically 1.989 V in the case of lead-acid batteries with liquid electrolyte at the 5hdischarge-rate, the power - 21 discharge from the battery will be limited. Only when a further value Von lying slightly above Vcut (2.004 V in the case of liquid electrolyte) is exceeded again will the full power be made available.

If, in the manner described above, a rest period is detected, the weighting factor is changed in such a way that a more rapid elevation in the battery condition voltage VcOn will be made than during normal operation.

The more rapid recovery of the polarization voltage of a load-free battery is therefore taken into account.

The time profile of the characteristic variables calculated in accordance with the method outlined for a typical discharge cycle is shown in Figure 2.

If the industrial truck, for example a tractor, is moving freely without a load, then the battery voltage Vin falls relatively slowly, starting from virtually complete charge at the time to' and reaches a first relative minimum at the time t1. The loads are then turned off briefly, for example in order to couple up a trailer, and in the rest phase the battery voltage Vin rises again in order to fall again from the time t2 with renewed loading.

As long as the loading is not excessively high, there is no great difference between battery condition voltage VcOn and state-of-charge voltage VsOc. If a higher current is taken from the battery, since the industrial truck is transporting a large load, for example, such as between the times t2 and t3, the battery condition falls considerably below the state of charge. During a relatively long operating pause, such as occurs from the time t3, the battery voltage Vin initially rises rapidly again and then, in a slower rise, reaches approximately the value of the state-of-charge voltage VsOc or the battery condition voltage VcOn. The rapid rise occurs virtually immediately when the load is switched off, - 22 since no more current flows and therefore no more current is dropped across the internal resistance of the battery either. The subsequent slower rise takes place on account of the homogenizing electrolyte distribution, which has the effect of a rise in the cell voltage.

After a number of load changes, at t4 a relatively long rest pause follows, in which battery voltage yin, state- of-charge voltage vsoc and battery condition voltage Vcon equalize. Then, after t5, a great deal of power is taken from the battery, the battery voltage Vin falls sharply and, in addition, the battery condition voltage VcOn falls considerably below the state-of-charge voltage VsOc. At to, the power taken is reduced and the battery voltage Vin rises again, while the battery condition voltage VcOn remains approximately constant. The state-of-charge voltage VsOc falls only slightly. After a brief renewed removal of higher power at t7, which effects a further drop in the battery condition voltage VcOn' the vehicle is switched off and the battery voltage Vin rises steeply.

The state-of-charge voltage VsOc remains virtually constant over the course of the brief power removal, since in this case only little energy is drawn from the battery and therefore the state of charge remains approximately constant. From to, the vehicle is operated only with little stress, for example by using secondary loads, or switched off completely, so that the battery can recover again. The battery voltage Vin at this time virtually corresponds to the battery condition voltage VcOn and the differences can be attributed substantially to fluctuations on account of the calculation method.

Battery voltage yin, battery condition voltage Vcon and state-of-charge voltage VsOc equalize again.

Figure 3 shows a display (1) for state of charge and battery condition of an industrial truck according to the invention. The battery characteristic variables are - 23 displayed in the form of a 10-stage bar graph, the scaling ranging from 0% to 100% of the usable battery capacity and being graduated in steps of 10%. The usable battery capacity corresponds to a range from 100% to 20% of the actual battery capacity, since a discharge below 20% of the actual battery capacity already acts as a damaging low discharge.

When the battery is fully charged, only the topmost display segment (2) is illuminated, which signals a value of more than 90% both for the state of charge and also for the battery condition. This state is shown in Figure 3a. As long as the values of state of charge and battery condition correspond to a single display segment, only the latter is active.

If the industrial truck is in operation for a longer time or if it is subjected to higher stress, then the values of battery condition and state of charge differ so considerably that these no longer lie within the range of a single display segment. As shown in Figure 3b, a further segment (3) then becomes active, so that the upper segment (4) indicates the state of charge and the lower segment (3) indicates the battery condition. Since the battery condition is always lower than the state of charge, the allocation of the segments is unambiguous.

The operator detects that there is a difference between the two characteristic variables and how large this is.

As Figure 3c illustrates, with further loading the battery condition falls more quickly than the state of charge. By using the battery condition display (1), the operator can estimate how large the energy reserve available under current operating conditions is. If this is not taken into account and the vehicle continues to be highly stressed, the battery condition display falls, as Figure 3d shows, until it finally reaches the lowest - 24 segment (5), that is to say to a value of less than 10% usable battery capacity. This segment (5) is preferably emphasized by means of a red colouring, in order to make the operator aware of the critical state. In order to avoid a low discharge, the power discharge from the battery must now be reduced. This can also be carried out by an automatic circuit which limits the power discharge from the battery, for example even by individual loads being switched off.

If the industrial truck is switched off or loaded less, the battery recovers again. The battery condition display rises again into higher segments (6), approaches the segment (7) which corresponds to the state of charge.

This state is illustrated in Figure Be. Given a sufficiently long recovery time, both characteristic variables are again represented by one segment. During further operation, the display for both characteristic variables falls again; given low stress, battery condition and state of charge are approximately identical and only one segment is visible; given higher stress, splitting into two segments (8, 9) can take place again, as illustrated in Figure 3f. If both characteristic variables have fallen to a value below 10%, the lower segment (5) points to the exhaustion of the battery by means of regular flashing, and at the same time the measures which are also performed for the battery condition on its own when the 10% mark is undershot are carried out. This state is shown in Figure 3f.

Replacement or recharging of the battery is absolutely necessary.

In the display (1) of the exemplary embodiment, as is usual in industrial trucks according to the prior art, the ability to detect critical states of charge is made easier, for example by colouring the segments, preferably - 25 in the colours green, yellow and red for high, medium and low states of charge and battery conditions.

In addition to the form of the display (1) shown in the exemplary embodiment, other devices according to the prior art are also suitable for displaying state of charge and battery condition, for example pointer instruments, digital displays, acoustic indications or else combinations thereof. The display shown in the exemplary embodiment has the advantage of displaying both characteristic variables only when these in fact also have considerably different values. In addition, a space-saving construction and good clarity is therefore achieved. Displays of this type are frequently already present in industrial trucks and are driven by the electrical control system of the vehicle. A modification to the method according to the invention can then be made by only simple changes, for example to the control program, without having to make structural changes to the industrial truck. - 26

Claims

CLAIMS: 1. Industrial truck having at least one battery, wherein means are

provided for determining and/or displaying at least two different characteristic variables for characterizing the available electrical energy.
2. Industrial truck according to Claim 1, wherein at least one of the at least two characteristic variables for characterizing the available electrical energy is a measure of the charge (state of charge, VsOc) which can be removed under approximately ideal discharge conditions.
3. Industrial truck according to Claim 1 or 2, wherein at least one of the at least two characteristic variables for characterizing the available electrical energy is a measure of the charge (battery condition, VcOn) which can be removed under current, possibly adverse, discharge conditions.
4. Industrial truck according to one of Claims 1 to 3, wherein the determination of at least one of the at least two characteristic variables for characterizing the available electrical energy (Vsoc' VcOn) can be performed exclusively from the time profile of the voltage present on the battery.
5. Industrial truck according to one of Claims 1 to 4, wherein the determination of all the characteristic variables for characterizing the available electrical energy (VsOc' VcOn) can be performed exclusively from the time profile of the voltage present on the battery.
6. Industrial truck according to one of Claims 1 to 5, wherein means are provided which automatically - 27 reduce the power which can be taken from the battery if the characteristic variable falls below (Vcut) at least one limiting value for the charge (VcOn) which can be removed under operationally typical, in particular current, discharge conditions.
7. Industrial truck according to one of Claims 1 to 6, wherein only one indicating device is provided as means for displaying at least two characteristic variables for characterizing the available electrical energy.
8. Industrial truck according to one of Claims 1 to 7, wherein the means for displaying at least two characteristic variables for characterizing the available electrical energy have an indicating display in the form of a bar graph, which is preferably formed from 10 segments.
9. Method of operating an industrial truck, wherein at least two different characteristic variables for characterizing the available electrical energy are determined.
10. Method according to Claim 9, wherein at least one of the at least two characteristic variables for characterizing the available electrical energy is a measure of the charge (state of charge, VsOc) which can be removed under approximately ideal discharge conditions.
11. Method according to Claim 9 or 10, wherein at least one of the at least two characteristic variables for characterizing the available electrical energy is a measure of the available charge (battery condition, VcOn) which is available under operationally typical, in particular current, discharge conditions. - 28
12. Method according to one of Claims 9 to 11, wherein the measured variable used for determining at least one of the at least two characteristic variables for characterizing the available electrical energy is only the time profile of the voltage present on the battery.
13. Method according to one of Claims 9 to 12, wherein, in order to determine all the characteristic variables for characterizing the available electrical energy, only the time profile of the voltage present on the battery is used.
14. Method according to one of Claims 9 to 13, wherein at least one of the characteristic variables for characterizing the available electrical energy can be represented as an electrical voltage, in particular in a voltage range between 1 volt and 3 volts
15. Method according to Claim 13, wherein the characteristic variable being a measure of the charge which can be removed under approximately ideal discharge conditions (state of charge, Vsoc) can be represented as an electrical voltage, preferably in a voltage range between 1.9 volts and 2.2 volts, especially in a voltage range between 1.939 volts and 2.14 volts.
16. Method according to one of Claims 9 to 15, wherein, if the battery condition characteristic variable (VcOn) falls below at least one limiting value, the power which can be taken from the battery is reduced automatically.
17. Method according to Claim 16, wherein the limiting value for the battery condition characteristic variable (VcOn) which, if it is undershot, the power - 29 which can be taken from the battery is automatically reduced, lies in a range from 1.94-1.957 volts.
18. Method according to one of Claims 9 to 17, wherein, when the state-ofcharge characteristic variable (VsOc) is greater than the voltage measured on the battery in the instantaneous measuring cycle and/or a comparative voltage assigned uniquely to this, in particular a calculated voltage of an individual battery cell (Vin), a smaller change is made in the state-of-charge characteristic variable (VsOc) than if the state-of-charge characteristic variable (Vsoc) is less than the battery voltage (Vin).
19. Method according to one of Claims 9 to 18, wherein, if the battery voltage (Vin) is less than the state of-charge characteristic variable (VsOc) but the state-of-charge variable (VsOc) is greater than a limiting value (VfUll) which corresponds to an approximately completely charged battery, a larger change is made in the state-of-charge characteristic variable (VsOc) than in normal operation.
20. Method according to one of Claims 9 to 19, wherein, if a rest state of the industrial truck is determined, a more rapid equalization of the state of-charge characteristic variable (VsOc) to the measured battery voltage (Vin) is made than in charging or discharging operation of the battery.
21. Method according to one of Claims 9 to 20, wherein, if the battery voltage (Vin) falls below a limiting value (Vrif), preferably at the level of 1.55 V, a smaller change is made in the state-of-charge characteristic variable (Vsoc) than in normal operation. -
22. Method according to one of Claims 9 to 21, wherein, if the battery voltage (Vin) exceeds a first limiting value (Vhi), which lies above the voltage of an approximately fully charged battery (Vail) and preferably has a value from 2.1 V to 2.3 V, a smaller change is made in the state-ofcharge characteristic variable (VsOc) than in normal operation.
23. Method according to Claim 22, wherein, if the battery voltage (Vin) exceeds a second limiting value (VGas), which is higher than the first limiting value (Vhi), preferably a value at which effects occur, in particular gas formation, which are disadvantageous for the battery and/or the user and/or the industrial truck, the change in the state-of-charge characteristic variable (VsOc) is again reduced as compared with the change made when the first limiting value (Vhf) is exceeded.
24. Method according to one of Claims 9 to 23, wherein the adaptation of the battery condition characteristic variable (VcOn) is performed as a function of the level of the voltage (Vin) measured on the battery.
25. Method according to one of Claims 9 to 24, wherein as the battery voltage (Vin) falls, the battery condition characteristic variable (VcOn) is reduced 3 0 increasingly more sharply with a change of equal magnitude in the battery voltage (Vin).
26. Method according to one of Claims of 9 to 25, wherein, when a rest state of the industrial truck is determined and the battery condition characteristic variable (VcOn) lies below the state of-charge characteristic variable (VsOc), the battery condition characteristic variable (VcOn) rises. - 31
27. Industrial truck operated with a method according to one of Claims 9 to 26.
28. Industrial truck substantially as herein described with reference to the accompanying figures.
29. A method of operating an industrial truck substantially as herein described with reference to the accompanying figures.