Abstract:
The standby electric supply comprises an accumulator which comprises multiple interconnected blocks, a switching portion that conditionally connects the accumulator to a load or to a charging current supply and a measurement and control portion that produces measurement results for describing the state of the accumulator and that controls the switching portion on the basis of the measurement results produced. The measurement and control portion is arranged to measure, at an initial time, the initial value of the open cell voltage of each block from the accumulator when charged and to produce a threshold value from the measured block specific initial value of the open cell voltage. The measured open cell voltage at an observed time different from the initial time is compared with the threshold value. If the measured block specific value of the open cell voltage has reached the threshold value, the switching portion is controlled to connect the accumulator to the charging current supply.

Description:
This application is a § 371 U.S. national stage of PCT/FI00/00962 filed Nov. 3, 2000, which was published in English under PCT Article 21(2) on May 10, 2001, which in turn claims the benefit of Finnish application 19992396 filed Nov. 5, 1999. 
   TECHNICAL FIELD 
   The present invention relates generally to accumulator powered uninterrupted power supply systems. Particularly the present invention relates to the optimal controlling of charging and discharging of the accumulators used in such a system. 
   BACKGROUND 
   In many electrical appliances an uninterrupted power supply (UPS) system is used for securing the operation of the electrical appliance regardless of the disturbances in the power distribution network. The “uninterrupted power supply system” can also be abridged and referred to as the “standby electric supply”. 
     FIG. 1  presents a general, simple standby electric supply. The AC-supply line  101  connected to the power distribution network is connected to the rectifier units, the number of which can even be only one, but usually there are several units connected parallel to each other.  FIG. 1  presents particularly the rectifier units  102  and  103 . They produce a certain direct voltage which is supplied through the feed line  104  to the load i.e. to the device  105 , the power supply of which is desired to be secured. The feed line  104  has been connected through the switch  106  also to the accumulator unit  107 . The system functions so that during the normal operation of the power distribution network the rectifier units  102  and  103  supply electrical power both to the load  105  and to the accumulator unit  107 , in which case the accumulator unit remains in charged state. If a disturbance occurs in the power supply network, the accumulator unit starts to discharge to the load  105 , the electrical power supply of which is thus not disturbed. When the power distribution network returns to its normal operation, the rectifier units  102  and  103  start again to supply electrical energy both to the load and to the accumulator unit. Then the accumulator unit is recharged with the same amount of electrical charge, which was discharged from it during the disturbance. The switch  106  is opened only under special conditions e.g. when the disturbance lasts long, the accumulator unit has almost entirely discharged and the supplying power cannot be continued without the risk of damaging the accumulator unit. 
   When the accumulator unit is continuously connected to the charging voltage, this is called permanent charging. The voltage level of the permanent charging has to be selected accurately according to the recommendations of the accumulator manufacturer to ensure as long life as possible for the accumulator unit. It has been found out, however, that closed so called VRLA-accumulators (Valve Regulated Lead-Acid), which are used nowadays generally in accumulator units, tolerate poorly permanent charging compared with the traditional open i.e. flooded lead accumulators. This is thought to be caused by chemical phenomena inside the accumulators, caused by continuous overcharging. 
     FIG. 2  presents a more advanced so called standby construction, which is otherwise similar to the construction shown in  FIG. 1 , but a connection and charging device i.e. so called IBCM-module (Intelligent Battery Connection/Charge Module)  201  substitutes for the separating switch of the accumulator. It has a control connection to the accumulator unit  107  from it (presented by a narrow line in the figure) and it can take the charging energy of the accumulators either directly by bypassing the rectifiers from a connection by which the standby system is connected to the power distribution network (presented by a dashed line in the figure) or from the voltage supplied to load generated by the rectifiers. During the normal operation the IBCM-module  201  keeps the accumulator almost continuously separated from the direct voltage supplied by the rectifier units  102  and  103  i.e. from the feed line  104 . If the voltage in the feed line  104  falls below the threshold value e.g. because of the disturbance occurring in the power distribution network, the IBCM-module  201  connects the accumulator unit  107  to the feed line  104 , in which case the load  105  continues to get electric power. 
   The charge of the accumulator is discharged slowly by itself also when the accumulator is not connected anywhere. If the IBCM-module  201  detects that the charge of the accumulator  107  has under normal operation dropped below a certain threshold value, it connects the accumulator unit  107  either to the feed line or through a separate rectifier (not shown in  FIG. 2 ) to the power supply network, in which case the accumulator unit is fully charged relatively quickly. After this the IBCM-module  201  disconnects the accumulator unit again from the charging voltage. It has been assumed that using the IBCM-module can even double the life of the accumulator unit. 
   The problem in the system according to  FIG. 2  is finding the right control algorithm for the IBCM-module. A wrong algorithm can even lead to poorer operation of the system and that it wears out the accumulators more than the simple system according to  FIG. 1 . In addition, a wrong algorithm can contribute in increasing production costs e.g. if the components of the device must therefore be dimensioned for unnecessarily high current. 
   SUMMARY 
   An object of the present invention is to present a standby electric supply, which facilitates a long life time for an accumulator unit and which is economical to manufacture and which has good usability. In addition an object of the present invention is that the standby electric supply adapts to the variations in the properties of the components according to the production tolerances and to the changes in environment. Further an object of the invention is to present a method for controlling a standby electric supply so that the other objects mentioned above are attained. 
   The objects of the present invention are attained by presenting and making certain criteria for the starting and ending of the charging of the accumulator, of which the primary start charging criterium is based on the open cell voltage change monitored in blocks and the primary end charging criterium is based on the value of the charge current time derivate and on the value of voltage difference time derivate. 
   The standby electric supply according to the present invention comprises:
         an accumulator consisting of blocks   switching means for conditional connecting of the accumulator to the load or to the charging power source   measuring and controlling means for producing measurement results for describing accumulator status and for controlling the switching means on the basis of the measurement results produced.       

   According to the first embodiment of the invention it is characterized in that the measurement and control means have been arranged:
         to measure the block specific initial value of the open cell voltage of the charged accumulator   to produce a certain threshold value from the measured initial open cell voltage of a block   to compare the measured value of the open cell voltage of the block with the threshold value   as a response to the observation according to which the measured value of the open cell voltage of the block has reached the threshold value, to control the switching means so that they connect the accumulator to a certain charging current source.       

   According to the second embodiment of the invention the invention is characterized in that the measurement and control means have been arranged:
         to measure the value of the charging current change in relation to time   to measure the value of the voltage difference change between the blocks in relation to time   as a response to the observation according to which the value of the charging current change in relation to time has returned from its negative limit value to essentially zero and the value of the voltage difference change of the blocks relation to time has returned from its positive limit value to essentially zero, to control the switching means so that they disconnect the accumulator from a certain charging current source.       

   The invention relates also to a method characterized in that according to the first embodiment of the invention it consists of phases, in which
         the block specific initial value of the open cell voltage of the charged accumulator is measured   a certain threshold value from the open cell voltage of a block is produced   the measured value of the open cell voltage of a block is compared with the threshold value   as a response to the observation according to which the measured value of the open cell voltage of the block has reached the threshold value, a charging current is connected to the accumulator.       

   According to the second embodiment of the invention the invention is characterized in that it consists of phases, in which
         the value of the charging current change in relation to time is measured   the value of voltage difference change between the blocks in relation to time is measured   as a response to the observation according to which the value of the charging current change in relation to time has returned from its positive limit value to essentially zero the accumulator is disconnected from the charging current.       

   A VRLA-accumulator, which is also called a string, as it is known comprises group cells i.e. monoblocks, which further consist of cells. There can be several VRLA accumulators so that they usually are connected parallel to each other, in that case it is called the accumulator unit. In the system according to the present invention the open cell voltage is monitored most beneficially in each block separately. In addition, the charging current and temperature of the accumulator are monitored. A minimum value has been defined for the open cell voltage of the monoblock, which corresponds to the open cell voltage in case the capacity of the monoblock has decreased, when the accumulator has discharged, to a certain minimum level. The minimum value takes into count the initial level of the open cell voltage when the earlier charging has ended, it also takes into count the temperature change compared with the moment the earlier charging has ended and the maximum amount the capacity of the monoblock is allowed to decrease before the following charging period must be started at the latest. The charging is started when the open cell voltage of a certain monoblock reaches its minimum value or at the latest when a certain maximum time after the earlier charging has elapsed. 
   The charging current and the voltage difference between different monoblocks of the accumulator is measured in order to determine the time to end charging. The time derivates of these quantities follow certain pattern characteristics, when the accumulator cells reach their full charge. It is advantageous to select a situation, in which the time derivates of the charging current and the voltage difference have essentially zero values as the end criterium or in which a certain maximum time has elapsed after the time derivate of the potential difference of the monoblocks reached its positive maximum value. 
   The criteria according to the present invention, the fulfillment of which control the starting and ending of charging, are at least partially tied to such reference values, which are measured from the accumulator itself instead of e.g. the starting of charging always only after a certain constant (regular) time after the earlier charging. A good adaptivity is achieved by this i.e. the method and system according to the invention adapt especially well to the individual characteristics of each accumulator to be charged. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following the present invention is described in more detail by referring to beneficial embodiments presented as examples and referring to the enclosed figures, in which 
       FIG. 1  shows a known simple standby electric supply, 
       FIG. 2  shows a known more advanced standby electric supply, 
       FIG. 3  shows general outlines of a VRLA accumulator, 
       FIG. 4  shows one beneficial embodiment of the system according to the present invention, 
       FIG. 5  shows the behavior of the accumulator voltage in different situations, 
       FIG. 6  shows a method for starting the charging according to a beneficial embodiment of the invention, 
       FIG. 7  shows a method for starting the charging according to another beneficial embodiment of the invention, 
       FIG. 8  shows the behavior of the charging current and the voltage difference between the blocks at the end of the charging, 
       FIG. 9  shows a method according to a beneficial embodiment of the invention for stopping the charging, 
       FIG. 10  shows the operation of a system according to a beneficial embodiment of the invention as a state diagram and 
       FIG. 11  shows a system according to another beneficial embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   In connection with the above description of the state of the art  FIGS. 1 and 2  are referred to, so in the following description of the invention and of its beneficial embodiments reference is made mainly to  FIGS. 3–11 . The figures use the same reference numbers for the parts corresponding each other. 
     FIG. 3  shows a VRLA accumulator i.e. a string  300 , which consists of monoblocks  301 ,  302 ,  303  and  304  connected in series. Each monoblock comprises of the same number of cells, separate cells are not shown in  FIG. 3 . The number of monoblocks in string depends on of how many cells each monoblock is constructed. Usually the number of blocks is 4, 6, 8 and 24. The voltage over one monoblock can be marked with u Bi , in which i=1, 2, 3 or 4. The current flowing through the string can be marked with i B  and its sign is in the figure selected so that the charging current i.e. the current to the positive pole of the string is marked positive. 
     FIG. 4  shows an arrangement, in which the VRLA accumulator  300  presented in  FIG. 3  is connected to a switching and charging device i.e. the IBCM module  401 . The two conductor line  400  to the left is an input and output line, through which the arrangement presented in  FIG. 4  can be connected to the feed line between the rectifiers and the load (not shown in the figure). The IBCM module  401  consists of a switching and stabilizing block  402  and a control block  403 . There is a two-way connection between them so that the control block  403  receives information about the state of the switching and stabilizing block  402  and is able to control its operation. The switching and stabilizing block  402  has been connected to the positive and negative poles of the VRLA accumulator  300 . There is a number of measurement connections from the VRLA accumulator  300  to the control block  403  so that, the control block is able to measure the voltage u Bi  of each block separately and, in addition, the charging current i B  and the temperature T B  of the VRLA accumulator. 
   The measurement arrangement shown in  FIG. 4  for measuring the quantities u Bi , i B  and T B  is naturally only an example. It is not essential for the present invention how the values of the quantities in question are measured and defined, as far as they can be used by the control block  403 .  FIG. 4  shows for clarity only one VRLA accumulator  300 , although from the application point of view of the application of the present invention it does not matter how many accumulators have been connected to a certain IBCM module. Each accumulator connected to the IBCM module can be handled as an independent unit in the way shown in  FIG. 4  concerning the accumulator  300 . 
   The switching and stabilizing unit  402  has been dimensioned so that it can produce a certain charging voltage U C  and a certain charging current I C . The maximum possible values of these quantities must be selected so that they are as high as possible, but however smaller than the detrimental level to the accumulators. When the maximum value of the charging current is determined it must be taken into consideration that no excessive requirements are laid on the components of the equipment due to too high charging current value. The higher the maximum possible value of the charging current, the sooner the accumulators can be fully charged, but the more expensive components must be used for accomplishing the switching and stabilizing block  402 . The optimal maximum value for the charging current can be selected by defining a utility function for charging time and by solving a two-dimensional optimizing problem, the dimensions of which are the manufacturing costs and the utility function mentioned above that expresses the charging time. 
   It has been assumed in  FIG. 4  that the switching and stabilizing block complies with the so called constant current and constant voltage principle. This means that when the charging starts a certain maximum charging current value limits it. The charging voltage rises to its maximum value during constant current charging. When the maximum value of the charging voltage has been reached the charging current gets lower quickly, because the charging is now limited by the maximum value of the charging voltage. 
     FIG. 5  shows in principle the operation of the embodiment according to  FIG. 4 . 
   The vertical axis shows the voltage of the accumulator and the horizontal axis shows the time in some arbitrary units. The maximum charging voltage U C  and a certain minimum voltage U MIN  have been marked on the voltage axis. In the standby state the operation of the embodiment follows a cycle, in which the linear fall of the voltage from U C  to U MIN  caused by the internal self discharge of the accumulator, and the following fast voltage rise back to U C  caused by switching on the charging, are repeated. In the discharging state between the moments  501  and  502  the accumulator is connected through the switching and stabilizing block to the load, in which case the voltage of the accumulator falls during discharge of the accumulator. Returning to the standby state means that the cyclically alternating charging and self discharging cycles continue. Most essential for the present invention in  FIG. 5  is how the criteria for starting and ending the charging are selected during the standby state so that the charging control functions optimally also after a normal discharge. 
   From the theory describing the chemical functioning of lead accumulators is known the so called Nernst&#39;s equation, according to which there exists a nearly linear relationship between the open cell voltage u ocv  of the cell and the specific gravity SG, which can be represented at 25° C. (298° K) temperature using the equation
 
 u   ocv =0.84 +SG.   (1)
 
   If the change of the open cell voltage is represented by Δu ocv  and the change of the specific gravity by ΔSG, so it is possible, according to the equation (1), to write
 
Δu ocv =ΔSG  (2)
 
   On the other hand it is known that there is essentially a linear relation by a proportional coefficient k between the capacity C of the cell and the specific gravity SG, therefore it can be written
 
ΔC=kΔSG  (3)
 
and on the basis of equations (2) and (3)
 
Δ u   ocv   =ΔC/k.   (4)
 
   The cells in the same monoblock can be regarded as functioning in the same way, in which case the total open cell voltage change Δu Bi,ocv  of a certain i:th block is derived simply by multiplying the result concerning one cell by the number n of cells i.e.
 
Δ u   Bi,ocv =( n/k )Δ C .  (5)
 
   The change in the open cell voltage of the accumulator in relation to the percentage change of its capacity is constant relating to the particular accumulator, the value of which can be estimated theoretically. The manufacturers of the accumulator deliver usually the exact value, which is based on measurements. If this constant is marked with L, its definition can be written as
 
 L=Δu   ocv /[100·(Δ C/C )],  (6)
 
from which it can be derived a percentile change of the capacity corresponding to the change Δu ocv  of the open cell voltage.
 
Δ u   ocv   L·[ 100·(Δ C/C )],  (7)
 
   In addition, the temperature characteristics of the open cell voltage of the lead accumulator cell is known. The temperature characteristics follows the equation
 
Δ u   ocv   /ΔT= 0.23 mV/K  (8)
 
and can be directly generalized to the temperature behavior of the open cell voltage of a whole monoblock by multiplying the constant value according to the equation (7) by the number n of the cells in the monoblock.
 
   It is assumed that the open cell voltage of a certain i:th monoblock is known at a time t 1 . If a minimum value u Bi,ocv,min  is to be defined to which the open cell voltage is allowed to fall so that the capacity of the monoblock is not reduced more than a certain percentile part (ΔC/C)·100, a formula can be written for this minimum value on the basis of the equations (1)–(8) presented above 
                 u     Bi   ,   ocv   ,   min       ≈         u     Bi   ,   ocv       ⁡     (   t1   )       +       n   ·   0.23     ⁢       mV   K     ·     (       T   B     -       T   B     ⁡     (   t1   )         )         -     n   ·   L   ·       Δ   ⁢           ⁢   C     C     ·   100         ,           (   9   )             
 
in which
 
   
     
       
             
             
           
         
             
                 
             
           
           
             
               u Bi,ocv (t1) = 
               open cell voltage of the i: th monoblock at time t1, 
             
             
               n = 
               number of cells in monoblocks, 
             
             
               T B  = 
               temperature of the monoblock at the moment of 
             
             
                 
               observation 
             
             
               T B (t1) = 
               the temperature of the block at time t1 
             
             
               L = 
               constant, which describes the influence of the 
             
             
                 
               percentage change of the capacity on the cell voltage 
             
             
                 
               and 
             
             
               (ΔC/C) · 100 = 
               is the maximum allowed percentile loss of capacity 
             
             
                 
               during the observation period. 
             
             
                 
             
           
        
       
     
   
   The formula (9) according to the beneficial embodiment of the present invention is used for defining the minimum voltage U MIN  presented in  FIG. 5 . Because the accumulator consists of several monoblocks due to the manufacturing tolerances and individual properties of which their open cell voltages differ slightly, the method shown in  FIG. 6  is used most beneficially. This method is described in more detail in the following. 
   The state  601  is the initial state, in which the open cell voltage of each monoblock is measured in a state in which the accumulator is essentially fully charged. Thus the above mentioned time t 1  is in question, so the measurement results are marked with U Bi,ocv (t 1 ), in which the index i gets as many values as the accumulator in question has blocks. The cell voltage of the lead accumulator stabilizes to its actual open cell voltage only after certain time (approximately 1–2 days) after the previous charging has ended, so it is most beneficial to select the moment t 1  so that it has passed at least 24 hours after the previous charging has ended. The most suitable period, which separates the moment t 1  from the ending of the charging can be sought by experimenting. 
   In addition, in state  601  the lowest measurement result is selected. It is assumed that the lowest open cell voltage was measured from the j:th block, in which case the lowest value selected in the  601  can be marked with u Bj,ocv (t 1 ). After that only the j:th block in question is monitored. 
   In the state  602  the formula (9) is used to calculate a minimum value to which the open cell voltage is allowed to fall so that the capacity does not become smaller than a certain predefined percentile part. The specifically selected smallest value u Bj,ocv (t 1 ) is substituted in formula (9) for calculating the minimum value so the calculated minimum value can be marked with u Bj,ocv,min . The state  602  is a part of a cycle, which monitors how the open cell voltage of its j:th block decreases, the j:th block being the block the open ell voltage of which was found to be the lowest in state  601 . The states  603  and  604  form the other parts of the cycle. The cycle is repeated till the open cell voltage of the j:th block reaches the minimum value calculated in state  602  or till a certain maximum time t cmax  has elapsed from previous charging. If either of these criteria is fulfilled it leads to the state  605  in which the charging of the accumulator is started. 
   The simple embodiment presented above is based on monitoring the decreasing of the open cell voltage only in one block. Also other kinds of embodiments of the present invention can be presented.  FIG. 7  shows an embodiment in the initial state  701  of which the open cell voltage of each block is measured at the moment when the accumulator is essentially fully charged taking into consideration the settling time of the open cell voltage described above. The measurement results are marked again with u Bi,ocv (t 1 ), in which the index i gets as many values as the accumulator in question has blocks. 
   In the embodiment shown in  FIG. 7  none of the measurement results is given precedence to, instead in state  702  an individual minimum value is calculated for each block using the formula (9), in which case the open cell voltage is allowed to decrease so that the capacity is not getting smaller than a certain defined percentile part. For calculating the individual minimum value for each block the measured open cell voltage value u Bi,ocv (t 1 ) is substituted in the formula (9). 
   The state  702  is again a part of a cycle, in which this time is observed how the open cell voltage of each block decreases. The states  703  and  704  form the other parts of the cycle. The cycle is repeated till the open cell voltage of one block reaches the individual minimum value, which has been calculated for each block separately or till a certain maximum time t cmax  has elapsed from the previous charging. If either of these criteria is fulfilled it leads to state  705  in which the charging of the accumulator will be started. 
   In addition to the embodiments described above, embodiments according to the present invention can be presented, in which for calculating minimum values and for monitoring the individual open cell voltages of the different blocks some interblock calculations are applied. For example, all the voltages to be observed can be taken as mean and median values of interblock voltages. In this case, however, a part of the benefits of the method according to the present invention is lost, because information of individual blocks is lost. 
   In the following it is studied when charging the accumulator is beneficial to stop i.e. how the arrangement according to the present invention functions near the maximum voltage U C  of the charging shown in  FIG. 5 .  FIG. 8  shows an experimental measurement, in which the curve  801  shows the value of the charging current i B  in relation to the maximum value of the charging current and the curve  802  shows the value of the maximum difference Δu max  between the block voltages u Bi , which can be mathematically defined using the formula
 
Δ u   max =max i1,i2   [u   Bi1   −u   Bi2 ]  (10)
 
   In  FIG. 8  the horizontal axis represents time and the vertical axis represents both the relative value of the charging current and the value of maximum difference of the block voltages; the units are irrelevant. At the moment  803  the above mentioned maximum value U C  of the charging voltage is reached, the charging current starts to get smaller quickly: its time derivate (di B /dt) is high and has a negative value. At the same time the value of the maximum difference Δu max  of the block voltages U Bi  rises strongly, because the voltage of the first full charged block rises and the voltage of other blocks gets smaller correspondingly: the time derivate (dΔu max /dt) is also high and it has a positive value. When the other cells become fully charged, each in its turn, the absolute value of the time derivate (di B /dt) of the charging current gets smaller. The time derivate (dΔu max /dt) of the maximum difference of the block voltages changes first negative and can after that oscillate a few times to both directions around the zero, but starts at the end to approach steadily zero. At the moment  804  both derivates (di b /dt) and (dΔu max /dt) are essentially zero i.e. smaller by their absolute value than a certain small threshold value. 
   According to a beneficial embodiment of the present invention the charging is ended as shown in the flow diagram of  FIG. 9 . In the state  901  it is noticed that the time derivate (di B /dt) has a high and negative value and the time derivate (dΔu max /dt) has a high and positive value. The threshold values for considering the values of the time derivates high can be found out experimentally. The states  902  and  903  form a cycle the purpose of which is to observe the time derivate (dΔu max /dt) of maximum difference of the block voltages in cosecutive periods of time and to store the information when it reached its highest value. When this information has been stored i.e. higher values are no more observed, state  904  is entered, in which it is observed how the absolute values of both time derivates (di B /dt) and (dΔu max /dt) decrease toward zero. States  904  and  905  make a cycle, which is repeated till both time derivates (di B /dt) and (dΔu max /dt) are essentially zero or till a certain maximum time tmax has elapsed from the moment when the time derivate (dΔu max /dt) of maximum difference of the block voltages reached its maximum value. Positive result detection in any of the states  904  and  905  leads to state  906  in which the charging of the accumulator is stopped. 
   It is possible to make changes and additions to the method presented in  FIG. 9  without departing from the principle of the invention. For instance it is possible to add a restriction to stop the charging, according to which the charging is also stopped if a certain maximum time has elapsed after it has been started, even if neither of the criteria according to states  904  and  905  is met. In addition, to stop the charging can be made dependable of the measured temperature of the accumulator so that exceeding a certain predetermined threshold temperature causes the stop to the charging. 
     FIG. 10  is a state diagram, which describes the operation of the standby electric supply according to the invention. There are three states defined in it, which are the standby state without charging  1001 , the standby state with charging  1002  and the state  1003 , in which the electrical energy is discharged from the accumulator to the load. According to a beneficial embodiment of the invention the transfer from state  1001  to state  1002  happens when one of the criteria is fulfilled, which have been described above in connection with  FIGS. 6 and 7 . Correspondingly the transfer from state  1002  to state  1001  happens when one of the criteria is fulfilled, which have been described above in connection with  FIGS. 8 and 9 . The transfer to state  1003  happens as such in a known way when the rectifier or rectifiers which can as such be according to the state of the art cannot for reason or other to supply the load with the electrical energy it requires. Correspondingly, when the power supply disturbance of the rectifiers ends, the standby electric supply returns in as such a known way to state  1002 , from which the return to the state  1001  happens when the charging according to the criteria mentioned above is ended. 
     FIG. 11  shows one way for extending the invention for a system which has several VRLA accumulators connected parallel to each other. Two accumulators  300 ′ and  300 ″ are shown in the figure, but in the invention the number of accumulators connected to the system is not restricted in any way. In the system shown in  FIG. 11  each accumulator is handled separately in the measurements i.e. the measurement of the charging current, the measurement of the temperature and the measurements of the block voltages are made separately for each accumulator. The accumulators are, however, connected parallel to each other for charging, so in the system either all accumulators are being charged or no accumulator is being charged. For the embodiments described above this means that if the criterium according to  FIG. 6  for starting the charging is used, the block of the accumulator is searched, the open cell voltage of which is lowest at the moment t 1  and charging all accumulators is started when the open cell voltage of the block in question reaches the minimum value defined for it. In an embodiment according to  FIG. 7  the open cell voltages of all blocks in the whole accumulator system are monitored separately and each of them are compared with the minimum value individual for each block. Correspondingly if the embodiment according to  FIG. 9  is applied for ending the charging the charging is stopped when in all the accumulators the time derivates of the charging current and of the voltage difference of the blocks are essentially zero or when a maximum time has elapsed from the moment when high values of the derivates were detected or when a temperature exceeding a certain threshold is measured in one of the accumulators. 
   In a system consisting of several accumulators, the accumulators can also be connected using individual switching means to the IBCM module, in which case each accumulator can be charged separately if required. In this case the structure of the IBCM module becomes very complicated. The embodiment in  FIG. 11  can be simplified so that the charging current is not measured separately from each accumulator but the charging current of the whole accumulator system. 
   In the above only those systems have been handled, in which the measurement and follow-up of voltage and current values which describe the state of the accumulator system is done locally essentially in the same unit, which unit also, if required, connects the accumulator to the charging supply and disconnects it from the supply. The invention can also be applied so that the state of the accumulator system can be monitored and the switching commands can be given in addition to or instead of the local unit via a remote control system. In this case it is not necessary to have other equipment in connection with the accumulator system but the measurement elements, switches and telemetric equipment by which the measurement results are transmitted and the switching commands are received e.g. via Internet or telephone network. 
   The features of the invention described above can be applied in many different ways together or separately. It is e.g. possible to use the method described above according to the invention only for starting the charging of the accumulator and to stop the charging after a certain constant charging time or when the maximum charging voltage has been reached. On the other hand charging the accumulator can be started according to another criterium and use the above described method according to the invention only for stopping the charging of the accumulator. The most beneficial result can be attained, however, so that the invention is applied both to the starting of the charging and for stopping the charging.