SE536419C2 - Balancing of super capacitors - Google Patents

Abstract

An improved device for balancing a temporary energy storage is described The temporary energy storage contains a plurality of series-connected supercapacitors. In accordance with the present invention, the device comprises a diode device which only conducts current in one direction. This diode device is connected in parallel with at least two supercapacitors in the plurality of supercapacitors included in the temporary energy storage, and in a direction so that the diode device becomes conductive if the polarity of at least two supercapacitors is reversed compared to the polarity of the charging voltage used to charge the temporary energy storage. Fig. 2

Description

lO SUPER CAPACITOR BALANCING Field of the inventionThe present invention relates to an arrangement for balancing a short terrn energy storage, as defined in the preamble of claim l.

The present invention also relates to a short terrn energy storage including such an arrangement, as defined in the preamble of claim 9.

The present invention also relates to a vehicle including such an arrangement, as defined in the preamble of claim l0.

The present invention also relates to a method for balancing a short terrn energy storage, as defined in the preamble of claim ll.

Related art and background of the invention Energy storage arrangements utilizing super capacitors are used in a large number ofapplications for which uninterrupted power supply is important, such as in computers, radiobase stations, and the like. Energy storages using super capacitors can provide a relativelyhigh effect but a relatively small amount of energy, which is basically the opposite of ordinarylead acid batteries. Super capacitor energy storages have a general advantage in that they demand less service than lead acid batteries.

Super capacitor energy storages have been found useful as short terrn energy storages inhybrid vehicles, such as cars, trucks, tractors, buses, cranes and fork lifts, as well as in trains,trams, and the like. Also, in production industry, super capacitor energy storages can be usedfor keeping the production running during short time power supply failures. Typically, suchpower failures are shorter than l minute, and are therefore easily handled by super capacitorenergy storages. In this document, a hybrid vehicle, such as a bus or the like, will mainly beused as a non-limiting example for explaining the invention. However, the invention is also applicable to all other implementations in which a super capacitor energy storage is usable.

It has been realized that energy storages including super capacitors are very suitable for use inhybrid vehicles. For example, a hybrid city bus perforrns a large number of decelerations andaccelerations during a norrnal day of service. Here, it is possible to charge the energy storageduring decelerations before e.g. bus stops or red lights and to then utilize this stored energyfor providing energy during a short time duration When driving and/or acceleration is perforrned after such a relatively short stop.

The voltage used in e.g. a bus is approximately 400 to 800 Volts. Generally, a super capacitor,i.e. a cell, can be charged With a voltage being 2.7 to 2.9 Volts. Thus, in such a bus, a pluralityof super capacitor cells, approximately 150-300 super capacitor cells, has to be seriesconnected in order to store the voltage. The super capacitor cells in such energy storages areoften grouped together in modules, each module including a number of cells, such that theenergy storage includes a number of such modules being series connected. Figure 1 shows aprior art energy storage 100, including three modules 110, 120, 130. A first one 110 of thesesmodules includes four super capacitors 111, 112, 113, 114, a second one 120 of these modulesincludes four super capacitors 121, 122, 123, 124, and a third one 130 of these modulesincludes four super capacitors 131, 132, 133, 134.

In an energy storage including a large number of series connected cells being grouped in anumber of modules, all of the super capacitors receive the same current. Preferably, all thecells have essentially the same capacitance and equivalent series resistance, such that thevoltage is evenly distributed between the cells. HoWever, the voltage over the super capacitorsmay differ from each other, since the characteristics of the super capacitors are not identical.Thus, the voltage over the energy storage may be unevenly spread over the number of cells being included in the energy storage, leading to voltage unbalanced modules.

In prior art solutions, these different voltages over different cells and/or modules have beenmitigated by binning of the super capacitors, i.e. by actively choosing super capacitors havingessentially the same characteristics for use in each cell and/or in a module in an energy storage. In the binning procedure, produced super capacitors are divided into binning groups, lO Wherein each binning group has a certain characteristic, e. g. a certain capacitance value C .Thereafter, When assembling the energy storage and/or the modules, super capacitors fromone certain of these binning groups are used for each one of the cells of the energy storageand/or of one module in order to achieve an even distribution of the voltage over the cells inthe energy storage and/or over the cells of the module. HoWever, this is an expensiveassembly process, since it demands expensive and time consuming component measurements and component selection.

Also, binning is typically also used Within each one of the modules, but When assembling theenergy storage, including a number of such modules, the different modules might have mutually differing characteristics, causing voltage unbalance.

Further, the characteristics for the cells Will vary over time. Thus, even if the characteristicsWould have been essentially identical at the time for assembly of the energy storage and/or themodules, the characteristics for the different super capacitors change over time. The differentsuper capacitors may experience differing temperature conditions, due to e.g. differingWorking temperatures, Which may depend on e.g. sunlight exposure, environmenttemperature, motor temperature, asymmetric cooling systems, or the like. This causes the characteristics for each cell, and also for each module, to change over time. lO Also, appliance of a too high voltage over a cell results in a lowered capacitance for that cell,where the capacitance is lowered differently for each cell. When the capacitance is lowered,the Voltage over that cell will be even higher, since the voltage over a capacitor is inversely proportional to the capacitance of the capacitor; where U is the voltage, Q is the charge on the conductors, and C is the capacitance.

Also, the charge is expressed as Q = J-ídt, where i is current. Therefore, all series connected cells have the same charge Q since they all experience the same current i running through Q them. The voltage equation U = ä can then, for these series connected cells, be simplified to the equation U ~ From th1s s1mpl1f1ed equation, 1t can easily be real1zed that a lower value for the capacitance C results in a higher value for the voltage U. Thus, the super capacitor isaged faster and faster due to the overcharging of the cell, resulting in more and more voltage unbalance between cells and/or modules.

Thus, there is a need for an improvement in voltage distribution between the cells in a multiple cell energy storage.

In prior art document EPl64l099, a Zener diode is connected across each super capacitor tolimit the maximum voltage over the super capacitor to a Zener voltage of the diode, e.g. 2.5Volt, during charging. However, the power dissipation for this solution is substantial forhigher levels of currents, which results in extensive generation of heat. Thus, by this prior artsolution, heating is an unwanted by-product, which has to be taken care of by cooling arrangements, such as refrigerating devices, cooling flanges or the like.

In prior art document US6278604, an arrangement for limiting the voltage over each cell of amultiple-cell energy storage by continuously bypassing current through a leakage resistance, and thereby distribute voltage between the cells, is presented. It is here stated that it is lO damaging for the cells to be operated close to a rated capacitor voltage during time periodsbeing longer than a certain time. Such high voltages are, according to this prior art solution,mitigated by connecting an equivalent amount of additional leakage resistance in parallel Witheach cell, i.e. in parallel With the leakage resistance of the cell. The leakage resistances of thecells are here dependent on the additional parallel resistances, Whereby the additional leakageresistances can be used for adjusting the cell leakage resistances, such that all cells haveessentially the same resulting leakage resistance after addition of the additional leakage resistors.

The steady state voltage over each cell is deterrnined by the product of the leakage current andthe leakage resistance. The leakage current is here the same for all cells, since it is providedby an arrangement keeping the energy storage at a constant voltage, i.e. at steady state. Thus,the steady state voltage is not dependent on the capacitance of the cells, and cells having thesame resulting leakage resistance also have the same voltage over them. The additionalresistance should here be an integral part of the multiple-cell energy storage. By this parallelcoupled equivalent additional leakage resistance, the total leakage resistance is loWered, and therefore the voltage over the cell is also loWered.

It is also stated in US6278604 that a discrete diode can be connected in parallel With the cellto mitigate these high voltages over the cell. HoWever, to use a discrete diode instead of aresistor in this solution Would introduce a voltage dependent resistance being similar incharacter as a Zener diode. Therefore, this Would lead to substantial power dissipation and heat generation, Which has to be mitigated by cooling arrangements.

The prior art solution being presented in US6278604 has a number of disadvantages. Onedisadvantage is that it is only practically usable for static implementations, in Which static(non-varying) voltage levels are used. For dynamic implementations, utilizing varying voltagelevels, such as implementations in vehicles and the like, this prior art solution Would performpoorly, since it can not balance voltage differences resulting from differing capacities occurrin in a d namic im lementation.Y Also, the use of leakage resistors according to this prior art solution also has negative effects.

The additional leakage resistance causes considerable standby losses, Which have to be lO mitigated by additional Cooling arrangements. Further, a failure of one single resistor in onecell would increase the leakage resistance of that cell and could cause cascade failures in the system, which results in a non-robust system.

Further, additional leakage resistance components have to meet both electrical andmechanical requirements, e. g. electric conductive rubber has to be used instead of just anyrubber having the mechanical requirements. This leads to compromises in performance, sinceessentially no components are optimized for both electrical and mechanical purposes. This compromise results in performance degradation for dynamic implementations.

Also, the prior art solution being presented in US6278604 can not handle differences incapacitance very well. Here, a 10% difference in capacitance between two cells results in avoltage difference of approximately 10% between the cells, since the additional leakageresistance is too small to be able to bypass 10% of the current, and thus to prevent uneven voltage distribution of the energy storage.

One other disadvantage is that a large number of additional leakage resistances, one additionalleakage resistance per cell, have to be controlled to have an equivalent amount of leakageresistance, which leads to an expensive manufacturing process, since cell measurements andcomponent selection has to be performed for each one of this large number of additional leakage resistances.

Thus, the prior art solutions either add to manufacturing costs, fail to offer an even voltagespread over time due to the fact that super capacitor characteristics change over time, or generates unwanted heating that has to be taken care of Aim and most important features of the inventionIt is an object of the present invention to provide a short term energy storage balancingarrangement and a short term energy storage balancing method that solves the above stated problem.

The present invention aims to provide a robust short term energy storage balancing being more efficient than the energy storage balancing procedures known in the background art. lO The object is achieved by the above mentioned arrangement for balancing a short term energy storage according to the characterizing portion of claim l.

The object is also achieved by the above mentioned short terrn energy storage including such an arrangement according to the characterizing part of claim 9.

The object is also achieved by the above mentioned vehicle according to the characterizing portion of claim l0.

The object is also achieved by the above mentioned method for balancing a short terrn energy storage according to the characterizing portion of claim ll.

The short terrn energy storage balancing according to the present invention is characterized inthat the arrangement includes a diode means, which is conductive in only one direction, andwhich is connected in parallel with at least two super capacitors of the short terrn energystorage, i.e. the diode means is connected in parallel with a group of cells. The diode means isconnected in a direction such that it will only be conducting if a polarity over the at least twosuper capacitors is reversed in relation to a charging voltage having been used for chargingsaid short terrn energy storage. By this arrangement, the diode means starts conducting if apolarity over the at least two super capacitors is reversed, such that the reversed polarity is counteracted.

The present invention has a number of advantages. It makes manufacturing of the short terrnenergy storages less expensive, since a relatively low number of low cost conventional diodes,only one diode means per at most every second cell, are used, and since no adjustment isneeded for the diode means of the invention. This is a great improvement over the prior artsolutions, in which one diode or resistor per cell is needed and where the characteristics forthat diode or resistor has to be carefully chosen to achieve the wanted effect. Also, the diodecan be arranged extemally of the cells, and do not have to be added within the cells themselves. lO Also, the present invention takes care of the situation Where the super capacitors experience areversed polarity, While the prior art solutions are directed to situations Where charging of theenergy storage is perforrned. The fact that the present invention focuses on the polarityinversion problem results in that corrosion of the super capacitors is mitigated, and in that noprocessing unit is needed for controlling the balancing process, Which makes the solution ofthe present invention very easy to implement in a Way such that it adds very little complexity to the system.

Thus, the present invention provides a short term energy storage being more easilyimplemented, faster and more robust than the prior art energy storages. The present inventionperforms Well in dynamic systems, such as systems for different kinds of vehicles and the like.

The present invention offers a good balancing, Which results in operational voltages being close to a voltage being optimal and efficient for the energy storage.

Also, since the present invention utilizes a diode solution instead of using a solution utilizingleakage resistors, it presents a solution being robust Without having the above stateddisadvantages of leakage resistance solutions. Thus, the risk of cascade failures is loWeredconsiderably. Also, the standby losses are loWered considerably and there is no need forcooling arrangements in this regard. Further, the present invention is Well suited for coping With capacitance differences.

According to an embodiment of the invention, the balancing arrangement is arranged forperforming the balancing during discharge of the short term energy storage. Since thebalancing is performed during discharge, the implementation can be made With very lowcomplexity. This is due to the fact that discharge, and thus balancing, can be activated inpreset intervals, such as e.g. every night, Which can be achieved With very little addedcomplexity. In prior solutions, a possible unbalance has to be monitored the Whole time,Which requires use of a processing unit for controlling the balancing process. Also, thebalancing according to the present invention can be performed very quickly, since very little excessive heating has to be taken care of, Which results in improved up-time availability. lO According to an embodiment of the present invention, the super capacitors of the short termenergy storage are grouped together into modules, either logically or physically, and at leastone the diode means of the balancing arrangement is connected in parallel with all the supercapacitors being included in one module. Thus, in a short terrn energy storage utilizing thisembodiment of the present invention, each module can be provided with a diode means beingconnected in parallel with all the super capacitors included in the module. Thus, the diode can here be applied extemally, outside the housing of the module.

This embodiment has advantages in that a simple and robust balancing of the modules, andthereby of the short terrn energy storage, is achieved. Also, very low negative voltages overonly some of the modules are achieved during discharge by this embodiment, since the diodemeans bypasses the possible negative voltages around the modules such that the voltage is notreversed over the modules to a great extent, which also mitigates corrosion of the supercapacitors of the modules. An extemally arranged diode is also easily exchangeable. Also,this embodiment is very useful and efficient for situations where the modules have mutually different properties, where each property may change over time.

Manufacturing costs can also be cut by this embodiment of the present invention, since lowcost standard modules can be coupled together, while differences between the characteristicsfor these standard modules are compensated for by the arrangement according to theembodiment. Such a simple manufacturing is impossible in the prior art solutions, in whichthe resistors or the diodes taking care of the balancing have to be integrated parts of the energy storages.

According to an embodiment of the present invention, at least one of the reversed diodemeans is connected in parallel with a subset of the super capacitors of a module. For thisembodiment, it is advantageous that only the super capacitors within each subset of a module have to have identical or compensated properties.

Also, to perform the balancing on groups of cells being at least two cells, e. g. balancing on amodule level, according to the present invention, mitigates reversed polarity of the group ofcells, e. g. of the modules, since the polarity is here not influenced to a great extent by the forward voltage drop of the diode connected to the group of cells. Generally, diodes have a lO lO positive forward Voltage drop in the range of 0.1 V. Thus, if one backward diode would bespent for each and every cell, as has been suggested in prior art, the protection againstnegative voltage over the cells would not be sufficient, also not when 0.1 or 0.2 Volt Schottkydiodes are used. According to the present invention, the forward voltage drop of a diode isdistributed over at least two cells and thereby only a fraction of the forward voltage drop ofthe diode connected to group of cells results for each cell. The remaining negative voltagesseen by a single cell can be counteracted by other measures, e. g. by the use of resistors as proposed in prior art if necessary.

Detailed exemplary embodiments and advantages the balancing of the short terrn energystorage according to the invention will now be described with reference to the appended drawings illustrating some preferred embodiments.

Brief description of the drawings Fig. l schematically shows a prior art short terrn energy storage.

Fig. 2 schematically shows a short terrn energy storage according to an embodiment of theinvention.

Fig. 3 schematically shows a short terrn energy storage according to an embodiment of theinvention.

Fig. 4 schematically shows a short terrn energy storage according to an embodiment of theinvention.

Fig. 5 schematically shows a short terrn energy storage according to an embodiment of the invention.

Detailed description of preferred embodiments By the present invention, an arrangement for balancing a short terrn energy storage includinga number of series connected super capacitors is provided. The balancing arrangementincludes at least one reverse connected diode means, i.e. the diode means is reversed inrelation to the normal working voltage over the capacitor, where the normal voltage is thevoltage over the capacitor when being charged and when storing energy. The diode means is astandard diode means, being conductive in only one direction, and is thus not e.g. a Zener diode, which under certain conditions may conduct in two directions. lO ll In use, the at least one diode means of the balancing arrangement is connected in parallel withat least two of the super capacitors being included in the short terrn energy storage,respectively, and in a direction such that the diode means will be conducting if a polarity overthe at least two super capacitors is reversed. Here, and in this document, a polarity over asuper capacitor is said to be reversed if the voltage over the super capacitor is negative inrelation to the voltage being applied to the super capacitors when being charged. Thus, thepolarity is reversed in relation to a charging voltage having been used for charging the shortterrn energy storage. In other words, the polarity of the super capacitor is reversed in relationto the voltage over the super capacitor during norrnal operation, i.e. when storing energy, which is later to be used by the system.

The present invention utilizes a diode solution instead of using a solution utilizing leakageresistance having been used in some prior art solutions. Thereby, the present invention offersincreased robustness, since this considerably lowers the risk for cascade failures resulting from one faulty component. Also, the present system has very low standby losses.

The short terrn energy storage can, according to an embodiment, include one or moremodules, each including a number of super capacitors being connected in series within that module.

Further, according to the method of the present invention, balancing of the short terrn energystorage is performed by the use of the balancing arrangement of the invention. It is hereutilized that the at least one diode means of the balancing arrangement is conductive in onlyone direction and has been connected in parallel with at least two super capacitors of the shortterrn energy storage. Thereby, a diode means starts conducting if a polarity for the at least twosuper capacitors, over which the diode means is connected, is reversed in relation to thevoltage over the at least two super capacitors when storing energy, which results in that thereversed polarity is counteracted and balancing is performed. Basically, the bypassing of thedischarge current being performed by the reversed diode means has as a result that the risk ofreversed polarity of the modules is mitigated, as will explained below in connection of figure 2. lO 12 The solution of the present invention lowers the manufacturing costs in relation to prior artsolutions, in which components, such as Zener diodes, are coupled over each one of the supercapacitors of a module. Thus, the number of diode means of the present invention is muchlower than the number of the components being used in the prior art solutions. Also, each oneof the used prior art components, e.g. Zener diodes or resistors, have to be adjusted due tochanging temperatures over time, while no adjustments are needed for the solution accordingto the present invention. Also, due to low losses, the reversed diode means can be very easilydesigned for the nominal current of the energy storage, which also reduces the manufacturing costs.

Thus, by the present invention, voltage differences between a number of super capacitors ormodules of a short term energy storage can be easily and efficiently equalized. This isachieved by balancing the super capacitors of the short term energy storage during discharge as described above.

Since the arrangement for balancing the short term energy storage is arranged for achievingbalance during discharge, a very fast equalizing of voltages over the super capacitors ormodules can be performed, in relation to systems performing equalization during charging ofthe modules. This is due to the fact that the effect causing heating loss results from multiplying the discharge current with the built-in potential of the diode means, which is relatively low for the diode means used in the present invention, e.g. approximately 0.3 Volts.

Because of the relatively low built-in potential, a low level of heating is also created. Sincenot so much heating is generated, the discharge and balancing can be performed relativelyquickly, as cooling procedures can be omitted. As a comparison, the Zener voltage of a Zenerdiode being used in a prior art solution is much higher than the built-in potential of a standarddiode, e. g. approximately 2.5 Volts, which results in a much higher generation of heat.Therefore, balancing circuits utilizing Zener diodes during charging have to make pauses inthe charging in order to not generate too much heating energy. Typically, Zener diodearrangements have to rest at certain voltages for several tens of minutes or even hours to not become overheated.

Also, the present invention offers a low complexity solution since no processor is needed for activating the process. The balancing process is performed automatically, driven by the basic 13 normal behavior of the diode means, Which are activated by the voltages being present in thecells and/or modules during discharge. Thus, once the discharge is started, the balancing andequalizing procedures being perforrned by the invention do not have to be controlled. Thedischarge can be timed in a very low complex manner, e.g. during night pauses of the vehicle,such that no processing power is needed for monitoring the unbalance situation and for activating the discharge.

According to an embodiment of the present invention, the reversed diode means is in useconnected in parallel With all of the super capacitors in a module. Thus, the diode means ishere in use connected over the Who le module, so as to balance all series connected supercapacitors being included in the module as a Whole. Figure 2 schematically shoWs a non-limiting example of a short term energy storage 200, according to this embodiment of theinvention, including three series connected modules 210, 220, 230, and Where the polarity of avoltage being used for charging the energy storage is indicated by plus and minus signs. Thus,the polarity of the super capacitors When storing energy is also indicated by these plus and minus signs.

A first one 210 of theses modules includes four series connected super capacitors 211, 212,213, 214. A second one 220 of these modules includes four series connected super capacitors221, 222, 223, 224. A third one 230 of these modules includes four series connected supercapacitors 231, 232, 233, 234. As is understood by a skilled person, such a short term energystorage 200 can have essentially any number of modules, Where each of these modulesinclude essentially any number of super capacitors. Generally, the number of modules and/orsuper capacitors to be included in the energy storage is decided based on for Whichimplementation the energy storage is to be used. Also, according to the present invention, atleast one of the modules 210, 220, 230 is provided With an arrangement 215, 225, 235 for balancing the module.

The balancing arrangement 215, 225, 235 includes at least one diode means, Which isconductive in only one direction, i.e. the diode means is not e.g. a Zener diode. The diodemeans are connected in a reverse direction in parallel With all of the super capacitors of eachmodule, respectively. Thus, each of the diode means is connected in parallel With all the series connected super capacitors of a module such that the diode means Will be conducting if a 14 polarity of the module, i.e. over all of the super capacitors of the module, is reversed inrelation to the polarity of the super capacitors when storing energy. Hereby, a simple androbust approach for balancing the super capacitors is achieved, and thereby equalizing thevoltages over the modules 210, 220, 230 can be utilized, since the unbalanced modules can bebalanced during discharge. Thus, equalized modules 2l0, 220, 230 are achieved bydischarging them completely once unbalance is detected, whereby an overall voltage of zeroVolts is achieved for the short term energy storage and for all modules 2l0, 220, 230. Thebalancing arrangement 2l5, 225, 235 according present invention makes it possible toperform this discharge such that equalization is easily achieved, as is explained in the following.

Generally, when discharging the modules 2l0, 220, 230, the voltage is lowered equally foreach one of the modules. Thus, if unbalance is present between the modules, the differentmodules will not reach a module voltage of zero Volts at the same time during the discharge,and the voltage over one cell of the module will also have a large negative value. As anexample in relation to the prior art energy storage shown in figure l, suffering from leakage ofcharge due to e.g. unequal leakage currents, if a first voltage over the first module ll0 isequal to 70 Volts, a second voltage over the second module l20 is equal to l00 Volts, and athird voltage over the third module l30 is equal to l00 Volts in an unequalized condition, anenergy storage overall voltage of zero Volts can result after discharge. Typically, after thisdischarge, the first voltage is -20 Volts, the second voltage is l0 Volts, and the third voltage isl0 Volts, which result in an overall voltage of zero Volts. However, as is clear from thisexample, the voltages over the different modules have not been really equalized, and also the voltages over one of the modules have a large negative value.

According to the embodiment of the present invention shown in figure 2, including a diodemeans being connected in parallel with each of the modules, and being connected in adirection such that the diode means will be conducting if a polarity over the module isreversed in relation to the voltage having been used for charging the short term energystorage, the voltage is instead lowered on all modules such that a much more even level ofvoltages are achieved for the modules, i.e. equalization of the voltages between the modules isachieved. This is because of the fact that reversed voltages, i.e. negative voltages for the modules, will be mitigated by the reverse coupled diode means. Thus, the bypassing of the discharge current being performed by the reversed diode means has as a result that thereversed polarity of the modules, which have reached the module voltage level of zero Voltsduring discharge, is mitigated. Thereby, the discharge of the voltage, and thereby thebalancing of the modules, can be continued until the all-over voltage reaches zero and all of the modules are discharged.

For the example given above, the present invention would achieve equal charges also withleakage of charge in the cells. The first voltage would, after such a discharge, typically be -0.3Volts, the second voltage would typically be 0.15 Volts, and the third voltage would typicallybe 0. l5Volts, which also result in an overall voltage of zero Volts. The levels of the first,second, and third voltages are here exemplified as -0.3, 0.15, and 0.15, respectively, which allare results of voltage levels in the diode characteristic of the diode means. A skilled personrealizes that different diode means would result in different values after discharge according to the embodiment of the invention.

Generally, according to the embodiment of the invention, much lower negative voltages oversome of the modules are achieved through balancing of the super capacitors during discharge.The diode means, which is reversed in relation to the normal voltage over the super capacitor,automatically takes care of the negative voltage over the module, such that the voltage is notreversed over the module to a great extent. It has been shown that a voltage of up to -0.7Volts, i.e. a negative voltage of 0.7 (for a Si diode), for a cell is not very harrnful for the supercapacitor, while higher negative voltages are very harrnful for the cells due to corrosion of thesuper capacitors. Thus, the present invention mitigates reversed voltage over the cells, which also mitigates corrosion of the super capacitors.

For this embodiment of the invention, where the reversed diode means is connected in parallelover the whole module, the conducting state voltage of the diode means is very low in relationto the module voltage over the module, which may contain a large number of cells. Also, theenergy storage can be designed and assembled on a module level, which is cost effective.Generally, modules including super capacitors can be bought at a relatively low cost. Thesemodules have norrnally been separately assembled such that the super capacitors within eachmodule have similar characteristics. Often, each module includes cooling arrangements, and the like, and often works fine separately. But when a number of such modules are assembled 16 in series with each other to form an energy storage, different modules possibly havingdiffering module Characteristics, resulting from the fact that the different modules often havebeen assembled from super capacitors having differing characteristics, respectively, cause unequalized voltage distribution in the energy storage.

By this embodiment of the invention, the different modules having differing characteristics,respectively, can easily be assembled together in a series connection, since the differences arecompensated by the reversed diode means being connected in parallel with each one of themodules. This lowers the costs for manufacturing since relatively non-expensive off-the-shelfstandard modules can be coupled together as long as the capacitance stays within certain limits.

Further, according to an embodiment of the present invention, each of the at least onereversed diode means is in use connected in parallel with a subset of the super capacitor in amodule. An example of such an embodiment is schematically illustrated in figure 3. Here, theenergy storage 300, includes three series connected modules 310, 320, 330, each beingprovided with an arrangement 315, 325, 335 for balancing the modules, which includes adiode means and is connected over two of the super capacitors 312, 313, 322, 323, 332, 333,respectively, in each of the modules. As is understood by a skilled person, the short termenergy storage 300 can have essentially any number of modules, each including essentiallyany number of super capacitors. The polarity of a voltage being used for charging the energystorage, and thus also the polarity of the super capacitors when storing energy, is indicated by plus and minus signs in the figure.

According to this embodiment of the invention, the diode means is connected over essentiallyany number between two and the total number of the series connected super capacitors withinthe module, such that a reversed polarity for that subset of super capacitors is avoided. Also, according to this embodiment of the invention, one or more of the modules 310, 320, 330 can include two or more such diode means.

According to an embodiment of the invention, each module of the short time energy storageshould include a diode being coupled in parallel with at least two of its super capacitors, unless it can be proven that the capacitance and the leakage resistance for that module are 17 both higher for the module Without the diode than the capacitance and leakage resistance ofeach one of the other modules in the short time energy storage, Which all have a such a diodeincluded in the modules. Also, if more than one module are arranged to not have a diodecoupled in parallel With at least tWo super capacitors, then all modules lacking such a diodeshould have similar capacitance and leakage resistance. HoWever, at least one of the modulesin the short time energy storage has at least one diode being coupled in parallel With at least one of its super capacitors.

According to an embodiment of the present invention, the reversed diode means is in useconnected in parallel With all of the super capacitors in a pair of modules, Where the modulesthemselves are also coupled in parallel pair. Figure 4 schematically shows a non-limitingexample of a short term energy storage 400, according to this embodiment of the invention,including three pairs of modules 410, 410”; 420, 420”; and 430, 430”. Within each of thesepairs, the modules are parallel coupled. Thus, in the first pair, module 410 is parallel coupleWith module 410”; in the second pair, module 420 is parallel couple With module 420”; and inthe third pair, module 430 is parallel couple With module 430”. The pairs as a Whole are thenseries coupled together, as is shown in figure 4. Thus, the first pair of modules 410, 410” isseries coupled to the second pair of modules 420, 420” and to the third pair of modules 430,430”.

In figure 4, polarity of a voltage being used for charging the energy storage is indicated byplus and minus signs. Thus, the polarity of the super capacitors When storing energy is also indicated by these plus and minus signs.

Each one of the modules 410, 410”, 420, 420”, 430, 430” includes four series connected supercapacitors. Thus, module 410 includes super capacitors 411, 412, 413, 414; module 410”includes super capacitors 411”, 412”, 413”, 414”; module 420 includes super capacitors 421,422, 423, 424; module 420” includes super capacitors 421 ”, 422”, 423”, 424”; module 430includes super capacitors 431, 432, 433, 434; and module 430” includes super capacitors 431 ”,432”, 433”, 434”. According to the present invention, at least one of the first pair of modules410, 410”, the second pair ofmodules 420, 420”, and the third pair ofmodules 430, 430” isprovided With an arrangement 415, 425, 435 for balancing the modules of the pair. 18 The balancing arrangement 215, 225, 235 includes at least one diode means. The diode meansare connected in a reverse direction in parallel with all of the super capacitors of each pair ofmodule, respectively, and the diode means will be conducting if a polarity of the module is reversed in relation to the polarity of the super capacitors when storing energy.

However, as is understood by a skilled person, such a short term energy storage 400 can haveessentially any number of modules, where each of these modules include essentially any number of super capacitors.

According to an embodiment of the present invention, the modules are coupled in parallelpairs. Figure 5 schematically shows a non-limiting example of a short term energy storage500, according to this embodiment of the invention, including three pairs of modules 5 l0,5l0”; 520, 520°; and 530, 530”. Within each pair, the modules are parallel coupled. Thus, inthe first pair, module 5 l0 is parallel couple with module 5 l0°; in the second pair, module 520is parallel couple with module 520°; and in the third pair, module 530 is parallel couple withmodule 530”. The pairs as a whole are then series coupled together, as is shown in figure 5.Thus, the first pair of modules 5 l0, 5 l0° is series coupled to the second pair of modules 520, 520” and to the third pair of modules 530, 530”.

Each one of the at least one reversed diode means is in use connected in parallel with a subsetof the super capacitor within a module. An example of such an embodiment is schematicallyillustrated in figure 5, in which each one ofthe modules 5 l0, 5 l0°; 520, 520°; and 530, 530°.isprovided with an arrangement 5 l5, 5 l5°, 525, 525°, 535, 535” for balancing the modules.

The arrangement includes a diode means being connected over two of the super capacitors ineach of the modules. As is understood by a skilled person, such the short term energy storage500 can have essentially any number of modules, each including essentially any number ofsuper capacitors. The polarity of a voltage being used for charging the energy storage, andthus also the polarity of the super capacitors when storing energy, is indicated by plus and minus signs.

According to this embodiment of the invention, the diode means is connected over essentiallyany number between two and the total number of the series connected super capacitors within the module, such that a reversed polarity for that subset of super capacitors is avoided. Also, 19 according to this embodiment of the invention, one or more of the modules 5 10, 5 l0”; 520, 520°; and 530, 530” can include two or more diode means According to an embodiment of the present invention, the at least one diode means is located outside a casing of the module, as is schematically shown for one embodiment in figure 2, butcan be utilized for other embodiments as well. This implementation has the advantage that thediode means are easily accessible for exchange or service. Also, this embodiment can be usedfor balancing modules without having to open their casings, which can be useful when a quick and easy installation of the balancing arrangement onto bought modules is to be achieved.

According to an embodiment of the present invention, the at least one diode means is locatedwithin a casing of the module, as is schematically shown for one embodiment in figure 3, butcan be utilized for other embodiments as well. This has the advantage of protecting the diodemeans from e. g. water and impacts, when the short term energy storage is used in e.g. a rough environment.

As is clear to a skilled person, in a short term energy storage including a plurality of modules,the diode means can be located outside the casing of some modules and inside the casings ofsome other modules. Also, both the embodiment in which the diode means is coupled inparallel with all of the super capacitors of the module and the embodiment in which the diodemeans is coupled in parallel with a subset of the super capacitors of the module can be provided with diode means being either within or outside the casing of the module.

Further, the reversed diode means can be essentially any diode means being conductive inonly one direction, such as a Schottky diode, a silicium/silicon (Si) diode, or the like. Thediffering advantageous characteristics for each of these diode types can be utilized by theinvention. For example, a low built-in potential of the Schottky can be utilized to make the equalizing procedure even faster, since even less power is dissipated as heat.

According to an embodiment of the invention, the diode means includes at least two diodes.These diodes are then parallel connected and/or series connected with each other. This has anadvantage in improved properties for the diode means, resulting in increased reliability for the balancing arrangement. lO The balancing arrangement and method for balancing according to the inVention may bemodified by those skilled in the art, as compared to the exemplary embodiments described above.

As is obVious for a skilled person, a number of other implementations, modifications,Variations and/or additions can be made to the above described exemplary embodiments. It isto be understood that the inVention includes all such other implementations, modifications, Variations and/or additions Which fall Within the scope of the claims.

Claims (14)

lO 21 Claims

1. l. Arrangement for balancing a short terrn energy storage including a plurality of supercapacitors being connected in series, Wherein said arrangement includes at least one diodemeans being conductive in only one direction, characterized in that each one of said at least one diode means is connected in parallel Withat least tWo super capacitors of said plurality of super capacitors, and in a direction such thatsaid diode means Will be conducting if a polarity over said at least two super capacitors isreversed in relation to a polarity of a charging Voltage having been used for charging said short term energy storage.

2. Arrangement as claimed in claim l, Wherein said arrangement is arranged for balancing said short term energy storage during discharge of said short term energy storage.

3. Arrangement as claimed in anyone of claims 1-2, Wherein said diode means includesat least two diodes being connected together according to at least one connection in the groupof: - a series connection; and - a parallel connection.

4. Arrangement as claimed in anyone of claims l-3, Wherein said diode means includesat least one diode in the group of:- a schottky diode; and - a silicium (Si) diode.

5. Arrangement as claimed in anyone of claims l-4, Wherein said plurality of supercapacitors are grouped into at least one module, and at least one of said at least one diodemeans is connected in parallel With all of the super capacitors being included in one of said at least one module.

6. Arrangement as claimed in anyone of claims l-4, Wherein said plurality of super capacitors are grouped into at least one module, and at least one of said at least one diode lO 22 means is connected in parallel With a subset of the super capacitors being included in one of said at least one module.

7. Arrangement as claimed in anyone of claims 5-6, Wherein said at least one diode means is located outside a casing of said one module.

8. Arrangement as claimed in any one of claims 5-6, Wherein said at least one diode means is located Within a casing of said one module.

9. Short term energy storage including a plurality of super capacitors being connectedin series,characterized in that said short term energy storage includes an arrangement as claimed in any one of claims 1-8.

10. Vehicle, characterized in that said vehicle includes:- a short term energy storage including a plurality of super capacitors being connected inseries; and - an arrangement as claimed in any one of claims 1-8.

11. Method for balancing a short term energy storage including a plurality of supercapacitors being connected in series, characterized in that a at least one diode means, being conductive in only one direction andbeing connected in parallel With at least tWo super capacitors of said plurality of supercapacitors, starts conducting if a polarity over said at least tWo super capacitors is reversed inrelation to a polarity of a charging voltage having been used for charging said short term energy storage, such that the reversed polarity is counteracted.

12. Method as claimed in claim 11, Wherein said plurality of super capacitors aregrouped into at least one module, and at least one of said at least one diode means isconnected in parallel With all of the super capacitors being included in one of said at least one module, such that reverse voltage is counteracted in said one module as a Whole. 23

13. Method as clain1ed in clain1 11, Wherein said number of super capacitors are groupedinto at least one module, and at least one of said at least one diode n1eans is connected inparallel With a subset of the super capacitors being included in one of said at least one module, such that a reversed polarity of said subset of super capacitors is avoided.

14. Method as clain1ed in anyone of clainis 11-13, Wherein the balancing is perforrned during discharge of said short terrn energy storage.