Abstract:
A method for charging a battery at a battery charger comprising connection means for connection to the terminals of a battery to be charged, means for detecting a voltage over the terminals of a connected battery, and control means. The method comprises the steps of initiating a burst cycle, wherein a plurality of consecutive voltage burst are applied to a connected battery to be charged, each burst successively lowering the internal resistance of the battery and initiating a charging cycle to charge the connected battery when said burst cycle has been terminated. Furthermore, a method for maintenance charging a battery at a battery charger including detecting a voltage over the connected battery; maintaining the voltage over the battery at a predetermined level for a predetermined period of time; monitoring a battery capacity parameter when said predetermined period of time has elapsed; and applying at least one voltage pulse if said parameter falls below a predetermined threshold level.

Description:
TECHNICAL AREA  
       [0001]     The present invention relates to a method for charging a battery having a high internal resistance due to sulphating during discharging of the battery and to a method of maintenance charging of a battery. The invention further relates to a computer readable medium comprising instructions for bringing a computer to perform such methods and to a battery charger.  
       BACKGROUND OF THE INVENTION  
       [0002]     During the discharging of a re-chargeable battery, lead-dioxide and sulphuric acid ions are converted to lead-sulphate at the positive plate or electrode of the battery cell and lead and sulphuric acid ions are converted to lead-sulphate at the negative plate or electrode. Correspondingly, lead-sulphate is converted to active material, i.e. lead-dioxide and sulphuric acid ions at the positive plate and lead and sulphuric acid ions at the negative plate, during the charging of the battery. However, this lead-sulphate may form a coating on the plates, which increases the internal resistance of the battery. If the battery has been discharged to a large extent, i.e. the amount of active material has decreased to very low level, which may occur if the battery has been unused for a long period of time, for example, during long-term storage (e.g. a battery of an engine of a boat where the engine has been unused during the winter), the internal resistance of the battery may increase to an extent that the battery cannot be charged using a normal charging cycle. This is due to the fact that the high internal resistance entails that the charging voltage rapidly rises to the normal maximum level, 2.3-2.5 V/cell, or to a total voltage of 14.4-14.9 V, even at small charging currents, which, in turn, entails to that the conversion of lead-sulphate to active material is prevented or that a very low amount of material is converted. For this reason, many conventional battery chargers fails in charging batteries being in this condition.  
         [0003]     A number of attempts have been made in order to overcome this severe problem of frequent occurrence with re-chargeable batteries. One example is chargers and methods at chargers that tries to solve this problem by delivering a very low charging current and thereby avoiding this rapid increase of charging voltage. However, this solution is impaired with the drawback that the charging period inevitably will become very long due to the low current. On the other hand, chargers and methods at chargers have been developed that allow a high voltage over the battery, in some applications even more than 20 V. This solution has instead the drawback that the battery has to be disconnected from the vehicle or the apparatus during the charging period, since such a high charging voltage may, for example, damage the electronics included in the vehicle or apparatus.  
         [0004]     One way to prevent the battery from discharging during long-term storage to the extent that a re-charging of the battery becomes difficult or impossible, is to keep the battery under maintenance charging during the storage period. Commonly, a maintenance charging procedure is performed in accordance with two different methods, float charging and pulse or hysteresis charging.  
         [0005]     According to the first method, float charging, the voltage over the battery is set to a lower level, typically 13.2-13.9 V, and the battery is held at a charge level of approximately 100%. This method suffers, however, from an enhanced water segregation, which negatively affects the duration of the battery. Further, the lead-sulphate content, in a valve regulated battery, can be enhanced at the negative electrode, which may increase the lead-sulphate coating of the electrode and thereby increase the internal resistance of the battery.  
         [0006]     In accordance with the second method, the voltage over the battery is monitored and if the voltage drops below a predetermined threshold level, typically 12.6-13 V, a voltage pulse is applied. Normally, the charge level of a battery subjected to such a maintenance charging method will be below 100%, i.e. the battery is not completely charged.  
         [0007]     Thus, it is a difficult problem to find a method at a charger that provide a fast, reliable, and safe charging of a discharged battery, independently of the levels of active material of the battery, i.e. independently of the internal resistance of the battery.  
         [0008]     Further, there is a problem to find a method and a charger that provide a maintenance charging that keeps the battery at a capacity level of 100% or near 100% at the same time as the enhancement of the water segregation is prevented and the lead-sulphate content is kept a low level.  
       SHORT DESCRIPTION OF THE INVENTION  
       [0009]     An object of the present invention is to provide a method for charging batteries in a fast reliable, and safe way, independently of the levels of active material of the battery.  
         [0010]     Another object of the present invention is to provide an improved method for maintenance charging of a battery, in particular during long-term storage of the battery.  
         [0011]     These and other objects are achieved according to the present invention by providing methods and chargers having the features defined in the independent claims. Preferred embodiments are defined in the dependent claims.  
         [0012]     According to a first aspect of the present invention, there is provided a method of charging a battery at a battery charger comprising connection means for connection to the terminals of a battery to be charged, and control means. The method is characterised in that it comprises the steps of: initiating a burst cycle, wherein a plurality of consecutive voltage bursts are applied to a connected battery to be charged, each burst delivering an amount of charge to the battery and thereby successively lowering the internal resistance of the battery; and initiating a charging cycle to charge the connected battery when said burst cycle has been terminated.  
         [0013]     According to a second aspect of the present invention, there is provided a method of maintenance charging a battery at a battery charger comprising connection means for connection to the terminals of a battery to be charged, means for detecting a voltage over a connected battery, and control means. The method according to the second aspect is characterised by the steps of: detecting a voltage over the connected battery; maintaining the voltage over the battery at a predetermined level for a predetermined period of time; monitoring a battery capacity parameter when said predetermined period of time has elapsed; and applying at least one voltage pulse if said parameter drops below a predetermined threshold level.  
         [0014]     According to third aspect of the present invention, there is provided a computer readable medium comprising instructions for bringing a computer to perform a method according to the first or the second aspect.  
         [0015]     According to further aspect of the invention, there is provided a battery charger comprising connection means for connection to the terminals of a battery to be charged, means for detecting a voltage over a connected battery, and control means. The charger is characterised in that said control means is connected to the means for detecting, and is arranged to execute the methods according to the first or the second aspect.  
         [0016]     The solution according to the first aspect of the present invention provides several advantages over the existing solutions. The method of charging of a battery, in accordance to the present invention, having a high internal resistance due to sulphating during discharging of the battery is on one hand more rapid compared to the low-current charging method, and, on the other hand, more user-friendly compared to the high-voltage charging method. In comparison with the low-current charging method, the method according to the present invention is significantly more rapid since it uses the conventional current strength and utilises the rise time of the voltage over the battery to successively increase the charge of the battery until that the battery is susceptible for a normal charging cycle. Accordingly, each short interval between connecting and disconnecting of the voltage is used to convert a small amount of active material. When using high-voltage charging method, the battery has to be disconnected from the vehicle or apparatus due to the risk of damaging the electronics. Hence, in comparison with the high-voltage charging method, the method according to the present invention is more user-friendly since the battery can remain connected to the vehicle or the apparatus during the charging since a moderate voltage is used for the charging, preferably 14.4-14.9 V.  
         [0017]     Thus, the second aspect of the present invention is based on the insight of combining float charging and hysteres charging during maintenance charging of a battery. By applying float charging only during an initial, carefully selected period and by initiating hysteres charging after the predetermined period of float charging, the battery can be placed in maintenance charging during a very long period of time, i.e. several months, with a very low water-segregation and at a very high capacity level.  
         [0018]     Accordingly, the solution in accordance with the second aspect of the invention provides several advantages over the existing solutions. One advantage is that the battery is held at a near maximum charge level, i.e. almost at 100% of the maximum charge of the battery, and can during the maintenance charging be used for applications where full capacity is required. Another advantage is that the life span of the battery is prolonged since the water segregation, which negatively affects the life span of the battery, is minimized.  
         [0019]     An additional advantage is that the risk of an enhanced lead-sulphate content at the negative electrode, which may increase the lead-sulphate coating of the electrode, is significantly reduced when using the method for maintenance charging a battery according to the present invention.  
         [0020]     As realized by the person skilled in the art, the methods of the present invention, as well as preferred embodiments thereof, are suitable to realize as a computer program or a computer readable medium, preferably within the contents of a battery charger.  
         [0021]     These and other advantages with, and aspects of, the present invention will become apparent from the following detailed description and from the accompanying drawings. 
     
    
     SHORT DESCRIPTION OF THE DRAWINGS  
       [0022]     In the following description of an embodiment of the invention, reference will be made to the accompanying drawings of which:  
         [0023]      FIG. 1  shows schematically the design of an embodiment of a battery charger suitable for use with the present invention;  
         [0024]      FIG. 2   a  shows schematically graphs over voltage/current vs. time and charge vs. time, respectively, during a normal charging cycle of a battery;  
         [0025]      FIG. 2   b  shows schematically graphs over voltage/current vs. time and charge vs. time, respectively, during a charging cycle of a battery having an increased internal voltage;  
         [0026]      FIG. 2   c  shows schematically graphs over voltage/current vs. time and charge vs. time, respectively, during a charging cycle of a battery having an increased internal voltage using the method for charging a battery in accordance with the present invention;  
         [0027]      FIG. 3  shows schematically an embodiment of a method of charging battery having an high internal voltage;  
         [0028]      FIG. 4  shows schematically a graph over voltage/current vs. time during maintenance charging of a battery using the method for maintenance charging of a battery in accordance with the present invention; and  
         [0029]      FIG. 5  shows schematically an embodiment of a method for maintenance charging of a battery in accordance with the present invention;  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     In the following there will be discussed preferred embodiments of the methods according to the present invention.  
         [0031]     With reference first to  FIG. 1 , a schematic diagram of an embodiment of a battery charger suitable for use with the present invention will be shown, as will be discussed below the invention is susceptible of embodiments in many different forms. Since the use and structure of a battery charger are well known to the man skilled in the art only parts or components being relevant for the invention will be described in detail.  
         [0032]     The battery charger shown in  FIG. 1  is a primary switched charger comprising in a known manner a DC power circuit  8  connectable to the mains. The DC power circuit  8  comprises, in turn, a diode bridge (not shown), a smoothing capacitor (not shown). Further, the charger comprises a high frequency transformer  14  having a primary winding  14   a  connected to the DC power circuit  8  and a secondary winding  14   b . The smoothing capacitor stores energy as a high DC voltage. The transformer transforms the high voltage to a charging voltage. A control unit  16  comprising, inter alia, an electronic switch (not shown), like a field effect transistor FET, arranged to the DC power circuit and the transformer capable of chopping up the DC power from the DC power circuit into pulses, and controlling and modulating the signal. Furthermore, the control unit  16  comprises modulation circuitry (not shown) arranged for the modulation of the signal.  
         [0033]     On the output side of the high frequency transformer  14  are two lines, positive  18  and negative  20 , provided with means to connect to a battery  21 . A rectifying element (not shown), such as a diode, is arranged to the positive line, and a smoothing capacitor (not shown) is arranged between the positive and negative line.  
         [0034]     Control means  29 , which will be described in more detail below, for controlling the charging process and the maintenance process of the battery charger is connected to the control unit  16  and to a measuring and amplifying circuitry  30  for measuring, detecting and feeding-back of voltage/current. Furthermore, the measuring and amplifying circuitry  30  for measuring, detecting and feeding-back of voltage/current is connected with the positive line  18  and the negative line  20 . A feed-back line  32  is provided from the measuring and amplifying circuitry  30  to a modulation circuitry included in the control unit  16  for modulation. The details of the modulation circuitry will not be described in detail here, because it do not form part of the present invention and its function and design is well known to the man skilled in the art. Preferably, the signal is modulated using pulse width modulation (PWM). Of course, the present invention can be used with a number of other modulation methods, for example, pulse-position modulation (PPM) or pulse frequency modulation (PFM). In such cases, any necessary modifications of the circuits of the current device of the present invention in order to adapt the current device to the modulation method used are easily performed by the skilled man and are therefore not described herein. The measuring and amplifying circuitry  30  is also connected to the control means for transferring information regarding, for example, the voltage at the terminals of the battery.  
         [0035]     Optionally, the control means  29  comprises means for obtaining voltage information of the battery and/or the current delivered to the battery connected to the measuring and amplifying circuitry  30  for measuring, detecting and feeding-back of voltage/current. As an alternative, the control means  29  can obtain the voltage information of the battery and/or the current directly from the measuring and amplifying circuitry  30  for measuring, detecting and feeding-back of voltage/current. Furthermore, the control means  29  is arranged for bringing the control unit  16  to act or respond to control commands based upon the obtained information and memory means (not shown). The memory means may comprise a non-volatile memory chip (e.g. an EEPROM or FLASH memory chip) which is capable of storing data. The details of the control means will not be described in further detail here, because the functions and design of its parts are well known to the man skilled in the art.  
         [0036]     Of course, there are a number of conceivable designs of the control means, for example, the control means can be realized by means of a processor including, inter alia, programmable instructions for executing the methods according to the present invention  
         [0037]     Above, a primary switched charger has been described, but the methods according to the present invention can easily be implemented in other battery chargers, such as linear chargers.  
         [0038]     With reference to  FIG. 2   a , graphs over voltage/current vs. time and charge vs. time, respectively, during a “normal” charging cycle of a battery will be shown. The lines  50  and  51  indicate the voltage and the current, respectively, during a charging cycle of a battery having a “normal” internal resistance, and the line  52  indicates the charge build-up or increase, i.e. the conversion of active material, during the same charging cycle. As can be seen, the voltage  50  rises smoothly and the current  51  is almost constant when the charging is initiated, and the charge is, correspondingly, build-up in a smooth manner. Although the line  51  indicates that the current is almost constant or, in fact, slightly rising, the current may fall during this initial period of time, indicated by t 1 , in certain cases. The first period of time is denoted as the bulk period and during this period the charging voltage rises to the charge level, which normally is approximately 14.0-14.9 V, and the charge contained in the battery is successively build-up. Subsequently, when the voltage has increased to the charge level, the absorption period, indicated by t 2 , is initiated during which the voltage is held at this level. During this period the build-up of the charge is continued. In order to bring the charge to 100%, or almost to 100%, of the theoretical battery capacity, a voltage boost may be applied, as indicated by t 3 , during which the voltage is increased to, for example, approximately 16 V. Thereafter, the battery is nearly fully charged and the maintenance period is initiated, indicated by t 4 . Preferably, the period t 1  has a length within a range from about lh to about 20 h, the period t 2  has a length within a range from about 1 h to about 40 h, the period t 3  has a length within a range from about 15 min. to about 6 h, and the period t 4  is unlimited.  
         [0039]     As a comparison, graphs over voltage/current vs. time and charge vs. time, respectively, during a charging cycle of a battery having an increased internal voltage is shown in  FIG. 2   b . The lines  53  and  54  indicate the voltage and current, respectively, during the charging cycle, and the line  55  indicates the charge build-up. As can be seen, the rise time of the voltage, see line  53 , is very short when the charging voltage is applied to a battery having an increased internal voltage. In other words, the lapse of the increase of the voltage is almost instantaneous due to the high internal voltage. In fact, the rise time will be of the order of a few hundreds of microseconds. Accordingly, the voltage  53  rises rapidly to the maximum level of the battery and, correspondingly, the current  52  falls rapidly, which entails that a very low amount of charge is delivered to the battery, as indicated by the line  55 . Thus, as can be seen in  FIG. 2   b , this rapid process caused by the high internal resistance makes it difficult or even impossible to charge the battery using a conventional battery charger.  
         [0040]     Turning now to  FIG. 2   c , graphs over voltage/current vs. time and charge vs. time, respectively, during a charging cycle of a battery having an increased internal voltage using the method for charging a battery in accordance with the present invention will be shown. The lines  56  and  57  indicate the voltage and the current, respectively, during the charging cycle, and the line  58  indicates the charge build-up during the charging cycle. In accordance with the method for charging a battery having a high internal resistance due to sulphating during discharging of the battery according to the present invention, a number of consecutive voltage pulses or bursts  56   a  having a duration of t 5 , t 6 , and t 7 , respectively, are applied to the battery. In  FIG. 2   c  only three pulses  56   a  are delivered, but it should be seen only as exemplifying and in a practical application it may be necessary to deliver a great number of pulses  56   a  in order to make the battery susceptible for a normal charging procedure. As can be seen in  FIG. 2   c , the charge, as indicated with line  58  of the battery successively increases with each delivered pulse, and, eventually, the charge of the battery plates has been sufficiently large, i.e. a sufficient amount of charge has been converted, that a normal charging cycle can be applied, indicated by the period of time t 8 . Concurrently with the decrease of internal resistance of the battery the rise times of the pulses also decreases and the duration of the pulses successively grows longer. Thus, t 5  is shorter than t 6 , which, in turn, is shorter that t 7 . Each of the periods t 5 -t 7  can have a length in a range from about 50 ms up to several seconds. Moreover, an offset time having a predetermined length can be introduced between consecutive voltage bursts, i.e. an interval period of time between two consecutive voltage bursts. As an alternative, the intervals can have successively increasing or decreasing lengths. It should be noted that the  FIGS. 2   a - 2   c  only are schematic and that, for example, that the axis indicating voltage, current and charge not are according to scale.  
         [0041]     With reference now to  FIG. 3 , an embodiment of a method of charging battery having an high internal voltage will be described. At step  60 , a battery is connected to a battery charger, for example, the charger shown in  FIG. 1 . Then, at step  62 , the charging cycle is initiated, but due to a high internal resistance of the electrode plates of the battery, the voltage over the battery increases rapidly, which is sensed by control means  29  by means of the measuring and detecting circuitry  30 . This triggers the control means  29  to activate a burst cycle period or, in other words, a sequence of consecutive voltage bursts, at step  64 , as indicated by  56   a  in  FIG. 2   c . In fact, each burst is a start of a “normal” charging followed by a disconnection of the voltage when the current has fallen below a predetermined level. Preferably, this predetermined level is approximately 0.5 A. Thereby, a small amount of charge is delivered to the battery, i.e. a small amount of active material is converted, during each burst. Consequently, the amount of converted active material grows successively. Subsequently, at step  68 , it is sensed that the amount of converted material has grown to the extent that a normal charging cycle can be applied and the control means  29  initiates a “normal” charging cycle. This is indicated by the fact that the current does not fall below the predetermined level following the applying of the charging voltage.  
         [0042]     Turning now to  FIG. 4 , a graph over voltage/current vs. time during maintenance charging of a battery using the method for maintenance charging of a battery in accordance with the present invention will be shown. The lines  80  and  82  indicates the voltage and current, respectively, during the maintenance charging in accordance with an embodiment of the method for maintenance charging of a battery according to the present invention. During a first period of time t 9 , when the battery is put at maintenance charging after that the normal charging period has been completed, the voltage over the battery is held at a predetermined voltage level, preferably about 13.2-13.9 V. Further, the voltage level of the battery is approximately at 100% during this period, about 97-98%. This voltage level is maintained during a predetermined period of time, t 9 , and if the battery is not used during this period, the pulse or hystereses state, t 10 , will be initiated. This implies that if the voltage falls below a predetermined threshold value or level of battery capacity parameter, which parameter preferably is the voltage indicated by v 0  in  FIG. 4 , one or more voltage pulses will be applied to or delivered to the battery in order to raise the voltage over this predetermined threshold level v 0 , indicated by a pulse having a duration of t 11 . The length of t 11  depends of, inter alia, the size of the charger, the type of the battery and the condition of the battery. Generally, the period t 11  is about 2-15 minutes and the voltage level is preferably 12.6-13 V. The current and voltage during the period t 11  are, in principle, similar to a “normal” charging cycle, i.e. first, a constant or almost constant current with a rising voltage and, subsequently, a constant voltage with a falling current. The constant current part is either short, or almost non-existent. In an alternative embodiment a normal charging cycle, see  FIG. 2   a , is initiated if the voltage falls below the predetermined threshold level of the battery capacity parameter.  
         [0043]     In addition, a function can be implemented that lay down conditions for that the current is not allowed to rise above a threshold value, typically 200 mA at a 100 Ah battery. This function is useful, inter alia, in a back-up system. In this case the charger may remain in a float state since a current above this threshold indicates that the device or application connected to the battery constantly draws current of the battery.  
         [0044]     With reference to  FIG. 5 , an embodiment of a method for maintenance charging of a battery in accordance with the present invention will be described. At step  90 , the maintenance charging is initiated by the control means  29  and the voltage over the battery is detected and locked to or held at preferably 13.2-13.9 V. This can initiated either manually by an user of the battery charger or automatically. The period of maintenance charging or float is set or predetermined to, for example, ten days. During this float period the charge of the battery is held at approximately 100% of the capacity level and the battery can be used in application where maximum capacity is required. If the battery is not used during this float period, window charging will be initiated, i.e. a pulse or hysteres period, at step  92 . If the battery is used during this period, the charger returns to the float mode. Then, at step  94 , the voltage over the battery is monitored or detected and when or if the voltage falls below a predetermined threshold level or value of a battery capacity indicating parameter, which parameter in this embodiment is the voltage, one or more voltage pulses or burst will be applied, at step  96 , in order to raise the voltage over the battery above the predetermined threshold level of the battery capacity parameter. As discussed above, the length of these pulses depends of, inter alia, the size of the charger, the type of the battery and the condition of the battery. Generally, a period is about 2-15 minutes and the voltage level is preferably 12.6-13 V. As alternative, the pulses can be applied during a predetermined period of time. This window charging period is maintained until the battery is used or that is turned of manually by a user of the battery charger. The skilled man in art realizes that there are a number of conceivable alternatives to the above described method for maintenance charging of a battery. For example, the battery capacity parameter can be the current instead of the voltage. The modifications required to the circuits of the charger in order to adapt it to current monitoring are easily carried out by a man skilled in the art. Furthermore, a normal charging cycle, see  FIG. 2   a , can be initiated when the level of battery capacity parameter falls below the predetermined threshold level instead of window charging.  
         [0045]     Although specific embodiments have been shown and described herein for purposes of illustration and exemplification, it is understood by those of ordinary skill in the art that the specific embodiments shown and described may be substituted for a wide variety of alternative and/or equivalent implementations without departing from the scope of the invention. Those of ordinary skill in the art will readily appreciate that the present invention could be implemented in a wide variety of embodiments, including hardware and software implementations, or combinations thereof. As an example, many of the functions described above may be obtained and carried out by suitable software comprised in a micro-chip or the like data carrier. This application is intended to cover any adaptations or variations of the preferred embodiments discussed herein. Consequently, the present invention is defined by the wording of the appended claims and equivalents thereof.