CHARGE BALANCING OF RECHARGEABLE BATTERIES

Abstract: The invention relates to charge balancing of cells (3, 4) of rechargeable batteries (1, 2), such as that based on the electrochemical reaction between a negative electrode of a lithium-aluminium alloy and a positive electrode of iron sulphide. The battery (1, 2) has means to balance the charge in the cells of the battery comprising connection means (8) to enable distribution of hydrogen between all of the cells (3, 4) and means (9, 10) to remove the hydrogen from the cells (3, 4). As the battery (1, 2) is started up and hydrogen is formed from the reaction of water with the lithium, each cell (3, 4) will discharge to zero charge and thus they will all recharge from a common base point. The battery (1, 2) may further include means for maintaining the cells (3, 4) within the battery (1, 2) at a uniform state of charge by monitoring the charge on each cell (3, 4) to detect if the charge on a cell (3, 4) becomes out of balance with the other cells (3, 4) and adding hydrogen to the cells (3, 4) when the charges are detected to be out of balance to ensure that all the cells (3, 4) self-discharge.

Description: CHARGE SALMONS OF RECHARGEABLE BAUTERIES The invention relates to charge balancing of cells of rechargeable batteries principally but not exclusively having a negative electrode composed at least partly of an alkali or alkaline earth metal, for example lithium. Sheprocessbywhich cells in a rechargeablebattery are broughttoa state where each contains the same amount of electrical charge is known as charge balancing. If a number of cells in a battery are connected electrically in series it is ofcrucial importancethateachcell canbe fully charged otherwise the performance of the whole string of cells is limitedby the cell that contains the least electrical charge, ietheweakest link. If one cell becomes out ofbalancewith respecttotheothersthenthe efficiency of the battery is reduced. A battery deteriorates far more rapidly if the charges on the cells are out ofbalance than if they arebalanced, because it is likely that gross overcharging or discharging will occur during the discharge/recharge cycle and this can catastrophically damage cells inmany types of cell systems. One approach to recharging a battery is to monitor the charging of the cells. However, whenthefirstcell reaches full charge then any cell which started at a lower state of charge will never be fully charged and the battery can never reach its maximum voltage level. Alternatively, recharging can be continued to overcharge the cells to bring all of them to full charge. In this way charge balancing is achieved with lead-acid batteries, the overcharging ensuring that every cell is fully charged. However, this is not possible with some types of battery as the cells wouldbe damaged and thebattery ruined. For example, inthelithium- aluminiuir iron sulphide cell, overcharging leads to the formation of iron halide complexes at the positive electrode which eventually leads to irremedial damage. Furthermore, over-discharge of abattery brings about the anodic dissolution of the aluminium in the negative electrodes, irretrievably damaging the electrodes. Ifbattery cells are connected electrically inparallel theproblem does not arise as the cells will settle to approximately equal charge levels giving approximately equal voltages and thebatterycanoperateeffectively. Chargebalancing isnecessary to smooth the operation of thebattery by equalising thecharge ofall ofthecells. It is alsonecessary in order toavoidproblemsduringthestartupofthebatteryfromdifferentdischarges of the separate cells. Present solutions require each cell to be charged separately to achieve balanced charging and avoid over-charging or discharging of the cells. The individual cells are monitored and the charging carried out under computercontrol toensurethatallthecellshaveequalcharges. The batterymust be specially constructed and sophisticatedswitchingmeans are required to enable charging to be discontinued on those cells that become fully charged and to continue on those that are still not fully charged. A particular type of rechargeable battery is that based on the electrochemical reactionbetweenanegativeelectrodeofalithium-aluminium alloy and a positive electrode of iron sulphide. The invention is particularly concerned with this type of battery but is also applicable wherever similar reversible reactions occur. In one variation Lithium- Aluminium could be replaced by Lithium-Calcium. Alternatively lithium could be replaced by sodium, potassium, calcium or other alkali or alkaline earth metals which are sufficiently reactive. Additionally iron sulphide couldbe replacedby irondisulphideornickelsulphideorcertaintransition metal sulphides or oxides. The inventor has discoveredthat it is the presence ofhydrogen ina lithiumbatterycellthatcauses ittoself-dischargeandleadstoproblemsof out-of-balance charges. Although lithium batteries are made under the driest conditions possible it is practically impossible to ensure that all water is removedfromthebatterychemicalsandthustherewill alwaysbesome residual water. When the battery is started up thiswater reactswith the lithium to produce hydrogen. Die cells become discharged to different degrees and therefore the resultant charges will be out of balance when recharged. The object of the invention is to provide a rechargeable alkali or alkaline earthmetalbatteryandamethodofoperationtoenablethechargeon the cells in the battery to be balanced. It is a further object of the inventiontoprovideabatteryandmethodbywhich eachcell inthebatteryis similarlychargedwithouttheneedtochargethemseparatelyandbywhichthey can be maintained at a uniform state of charge. The invention provides in one aspect a rechargeable alkali or alkaline earth metal battery having means to balance the charge in the cells of the battery comprising: a) connection means between all of the cells so that they are in a coπmon atmosphere, whereby hydrogen is distributed between all of the cells; and b) means by which hydrogen is removed from the cells. Conveniently each cell may be open to a coπmon manifold to connect themall toa common atmosphere. Alternativelytheconnectionmeansbetween the cells may be by pipes between each of the cells. The aim of exposing all the cells to a common atmosphere is to ensure that the concentration of hydrogen in the atmosphere is the same for each cell. Hence, as the battery is started up and hydrogen is formed froπ the reaction of water with the lithium, each cell will discharge to zero charge and thus they will all recharge from a common base point. The cells all discharge to zero as when one cell is fully discharged the hydrogen that caused it to discharge is released and made available to discharge further cells. In one arrangement hydrogen is provided in the cσs ion atmosphere. Even if no hydrogen is present, other imperfectionsmay eventually cause the cells to become out of balance and thus require rebalancing. If the cells have been made as perfectly as possible there may be no hydrogen present, or insufficient to fully discharge one cell so that the hydrogen iεnot released to discharge further cells. If this iε the case, the addedhydrogen ensures that all the cells discharge and thus that recharging commences from zero charge on all the cells. Hydrogenmay be added even to the systemwhich can produce sufficient hydrogen itself in order to speed up the self-discharge process. In one arrangement the cells are arranged in series as an open stack within a stainless steel enclosure. The cells are thus directly open to a common atmosphere. When recharging of a battery iε commenced thehydrogen mustbe removed fromtheatmosphereexperiencedby themor theywill discharge again. The hydrogen can diffuse out through the stainless steel casing as the casing is hot from the operation of the cells before they discharged, and hot stainless steel iε permeable tohydrogengas. However, if the casing iε stainless steel walls will not heat up and so hydrogen cannot escape. In this case it is advantageous to include a small "hot box" connected to the cells and part of the casing around the cellsbut outside thevacuumarea so that as the cells heat up during operation the "hot box" also heats up, allowing hydrogen to diffuse out. Alternativelyametal hydridemaybeprovidedtoabsorb thehydrogen andhold it readyfor re-use if thecells shouldbecomeoutofbalanceagain. Heating the metal hydride will release the hydrogen back into the cells to dischargethemallifnecessary. Otherwisethehydrogenwillbeabsorbedby themetal hydrideandsowill notaffecttheatmospheretowhichthecellsare exposed. In an alternative arrangement, particularly suitable for casings around a vacuumor made of amaterial that is not permeable to hydrogen, the hydrogenmaybe pumped off. Preferablythecells areallowed tocoolbelow the freezing point of the electrolyte before applyingpumping to remove the gas. This avoids the risk ofdamage tothecell thatcanbecausedbyefflux of hydrogen from the molten electrolyte. With a pumping system, it is thenadvantageous totakebatteryleads and other electrical connections through the pumping pipework. Ωius the cells in a battery can be completely enclosed and sealed from the ambient atmospherebut electricalconnectionscanstillbemadetootherbatteriesor external connections. If a number of batteries are being used, these are advantageously connected so that they see a commonatmosphere. In thisway it iεposεible to charge a number ofbatteries in series such that every cell iεbalanced. Operation of cells or batteries connected in parallel can also be improvedbyconnectingthemtoacαπronatmosphere. Whencellsorbatteries areconnected inparallelthechargebalancesbetweenthemapproximatelyand theywork effectively. However, iftheyexperiencethesameatmospherethe charge balances more evenly between them and their performance can be optimised. The battery may further include means for maintaining the cells within the battery at a uniform state of charge comprising: a) means to monitor the charge on each cell and to detect if the charge on a cell becomes out of balance with the other cells; b) meansbywhichhydrogen is addedtothecellswhenthechargesare detected to be out of balance to ensure that all the cells self- discharge; and c) means to recharge the cells when they have self-discharged. The invention further provides in a secondaspect amethod of charge balancing the cells in a rechargeable alkali oralkaline earthmetal battery comprising the steps of: 1) connecting all thecells so thattheyare inacoππonatmosphere; 2) adding hydrogen gas to the cells such that they self-discharge; 3) removing hydrogen from the cells; and 4) recharging the cells. Thehydrogenmaybe removedbeforerechargingcommencesorduringthe recharging operation. This method can be used to discharge the cellsbefore storage of the battery. Hydrogen is added to the cellstocausethemall to self-discharge to zero so that they canbe stored in a state of no charge andwhenthebattery is required again the hydrogen is removed and the cells recharged. The method may include the further step of monitoring the charge on each cell todetect if a cell becomes out ofbalancewith respect to the other cells, prior to adding hydrogen to the cellswhenthey are detected tobe out of balance such that each cell self-discharges. Conveniently the state of charge of the cells is monitored by measuring the voltage on each cell and hence the means for monitoring the charge may be a voltmeter which indicates the voltage on each cell. The invention is useful even for "perfect" cells which have been manufactured with no water present in the chemicals, and hence no self- discharging problems due to internally produced hydrogen. No cell can be absolutelyperfect and other imperfections cancausethecells tobecome out of balance after the battery has been cycled many times. The battery arrangement and method of operation of the invention enable the cells to be brought to a zero state of charge, by the addition of hydrogen, and then recharged froma coπmonbasepoint toensurethat the chargesonall thecells are balanced once again. In all the above aspects of the invention the means for recharging electrical cells maybe any of thosewell known in the art. Hydrogenmaybe added to the cells froman outsidesourceormaybestoredwithinthebattery system, for example in ametal hydride, and regenerated as required. The hydrogenmaybe removedfromthecellsbydiffusionthroughthehotcellwall, byabsorptionintoametalhydrideorotherhydrogenabsorbingchemicalormay be pumped off. The invention will now be described by way of example only with reference to the drawings of which: Figure1 shows, incross-section, asimplified, schematicdiagram of two lithium-aluminiumiron sulphide batteries connected in series. Figure2 shows, incross-section, asimplified, schematicdiagram of an evacuated lithium-aluminiimr iron sulphide battery showing a "hot box" to enable hydrogen to diffuse out of the battery casing. Figure1 shows, incross-section, asimplified, schematicdiagramof two lit um-aluminium/iron sulphide batteries 1 and 2 connected in series. Each battery 1, 2 is shown containing stacks of three cells 3a,b,c, and 4a,b,c, open to the atmosphere within the respective batteries 1,2. The cells are connected in series with the negative terminal of cell 3c being connected by a connecting lead 5 to the positive terminal of cell 4a. An input 6 is connected to the positive terminal of cell 3a and an output 7 is connected to thenegativeterminal ofcell 4c. The input6 andoutput7 may beconnectedtofurtherbatteriesorapowersupply inputfor rechargingoran output for which the batteries 1, 2 provide the power source (none shown). A connecting pipe 8 is fitted between the two batteries 1 and 2 to allow distribution of gases between the batteries so that each cell sees a common atmosphere. If further batteries are connected in series (not shown) r connectingpipes arefittedbetweenonebatteryandthenexttoallow all of them to see the same atmosphere. Theoverall electrochemical reactionof thecellscanberepresented by the following equation: 2Li(Al) + FeS ≤? Li2S + Fe + Al Electrolyte is neither consumed nor generated in the reaction. If rechargingofthecells3, 4 iscommenced anywaterpresentreacts with the lithium inthecells toproduce hydrogengas. The hydrogenacts as a "shuttle" for transporting lithium from the negative to the positive electrode. The reaction scheme can be written as follows: 2Li + H2 → 2LiH so ι u tion → 2LΪE + FeS → IJ.2S + Fe + H Theliberatedhydrogen reacts still furtherwithmorelithiumandtheprocess eventually completely discharges the cell. The final products in both electrodes are the same as fromtheelectrochemicaldischargereactionandso can be reversed by a normal recharge process. The hydrogen is distributed between the batteries 1 and 2 via thepipe 8. The hydrogencauses the cells 3, 4 to discharge completely so that their resultant charge is zero. Once thecellshavedischargedtheybeginto rechargeagain. Thewalls, 9 and10, of the batteries 1, 2 aremade of stainless steel and as theybecome hot from the operation of the batteries the hydrogen can diffuse out, enabling the cells to be fully recharged for effective operation of the batteries. Figure 2 shows an arrangement by which hydrogen candiffuse out of a battery 20 having an evacuated casing 21 such that the battery walls 22 are prevented from heating up and thus never become permeable to hydrogen. As in batteries 1 and 2 shown in Figure 1, the battery 20 contains a stack of cells 23a,b,c which are connected together in series. An electrical input 24 and output 25 are connected to further batteries, power supplies etc as required (none shown) . Apipe 26 connects thebattery20 to any other batteries in the system (not shown) or is blocked off if no other batteries are connected. The cells 23a,b,c are in a common atmosphere within the container 29 and the pipe 26 enables all connected batteries to experience the same atmosphere. Hydrogen thus distributes between the cells 23a,b,c, and the cells in further batteries, and causes them all to self-discharge to zero. The cells are then recharged. However, unlike the batteries 1,2 of Figure 1, the cell walls of the battery do not heat up during operation of the battery because of the vacuumwithin the casing 21. Hence, a "hot box" 27 is enclosed within the container 29. This box 27 iε made of stainless steel and as the cells heat up during operation of the batterythebox27 isheatedupaswell, becomingpermeabletohydrogen. The hydrogen can then diffuse out through the stainlesε steel wall of the box 27 and pass out of the battery 20 through pipe 28. The invention haε the advantage that it enableε lithium cellε, and hence batterieε, to be charged in series whereas previously each cell had to be charged separately. The invention also avoids the need for the sophisticatedmonitoring systems that were necessarywithprior art methods and means of charging lithium batteries. Sharing a common atmosphere ensures that the cells are in balance almost immediately from the moment of activation and remain so for many cycles. If at anytime thecellsbecomeunbalanced during operation theycan be simply and easily rebalanced by the deliberate addition of hydrogen to discharge themtotally so they canthenbe recharge fromzerostateofcharge. The hydrogen can be added froman external source or can be storedwithin the battery ready for use, for example in a metal hydride, the hydrogen being generated or reabsorbed by control of the temperature of the metal hydride. The invention iε not limited to Lithium Aluminium/Iron sulphide batteries but can be used wherever similar reversible reactions to those diεcussedbetween lithium andhydrogen and lithiumhydride and iron εulphide occur.

Claims: CLAIMS 1. A rechargeable battery with a negative electrode composed at least partly of an alkali or alkaline earth metal characterised in that there iε included means to balance the charge in the cells of the battery comprising: a) connection means between all of the cells so that they are in a common atmosphere, whereby hydrogen is distributed between all of the cells; and b) means by which hydrogen iε removed from the cellε. 2. A rechargeable battery according to claim 1 characterised in that each cell is open to a common manifold to connect all the cells to a comrron atmosphere. 3. A rechargeable battery according to claim 1 characteriεed in that the connection means between the cellε comprisespipesbetween each of the cellε. 4. A rechargeable battery according to any one of the preceding claims characteriεed in that hydrogen is provided in the common atmosphere. 5. A rechargeable battery according to any one of the preceding claims characteriεed in that the cellε are arranged in series as an open stackwithin a stainless steel enclosure. 6. A rechargeable battery according to any one of the preceding claims characteriεed in that a metal hydride is provided to absorb the hydrogen. 7. A rechargeable battery according to any one of claims 1 to 5 characterised in that the hydrogen is pumped off. 8. A rechargeable battery according to claim 7 characterised in that the cells are allowed to cool below the freezing point of the electrolyte before applying pumping to remove the gas. 9. A rechargeable battery according to claim7 or claim8 characteriεed in that the battery leadε and other electrical connectionε are routed through the pumping pipework. 10. A rechargeable battery according to any one of the preceding clai ε characterised in that thebattery further includesmeans formaintaining the cells within the battery at a uniform state of charge comprising: a) means to monitor the charge on each cell and to detect if the charge on a cell becomes out of balance with the other cells; b) meansbywhichhydrogeniεadded tothecellswhen thechargesare detected to be out of balance to ensure that all the cells self- diεcharge; and c) means to recharge the cellε when they have self-discharged. 11. Amethod of charge balancing the cells in a battery having a negative electrode composed at least partly of an alkali or alkaline earth metal comprising the steps of: 1) connectingall the cells so that theyareinaconrmon atmosphere; 2) adding hydrogen gas to the cells such that they self-discharge; 3) removing hydrogen from the cells; and 4) recharging the cells. 12. Amethod of charge balancing the cells in a battery according toclaim 11 characterised in that thehydrogen iε removed before recharging co menceε or during the recharging operation. 13. Amethod of charge balancing the cells in abattery according to claim 11 or claim12 characteriεed in that the method includes the further stepof monitoring the charge on each cell todetect if a cell becomes out ofbalance with respect to the other cells, prior to adding hydrogen to the cells when they are detected to be out of balance such that each cell self-dischargeε. 12. Amethod of charge balancing the cellε in a batteryaccording toclaim 13 characteriεed in that the state of charge of the cellε iε monitored by meaεuring the voltage on each cell.

Inventors: HAZZARD GRAHAM, PEARCE LEONARD JOHN, HAZZARD, GRAHAM, PEARCE, LEONARD, JOHN
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1988
Main classification: H01M10
IPC codes: H01M10