The present invention relates to flow battery including a plurality of flow battery stacks, in particular Vanadiun Redox flow battery stacks.

A Vanadium Redox battery stack is described in our International Application No WO03069692, whose abstract is as follows and the first part only need be referred to for understanding of the present invention:

A redox flow battery 1 has a stack of individual cells, shown diagrammatically as a single cell 1, with anolyte and catholyte compartments 2,3 divided from each other by an ionically selective and conductive separator 4 and having respective electrodes 4,5. The battery has anolyte and catholyte tanks 6, 7, with respective pumps 8,9 and a pipework 10,11. In use, the pumps circulate the electrolytes An,Ca to and from the tanks 8,9, to the compartments 2,3 and back to the tanks. Electricity flows to a load L. The electrolyte lines are provided with tappings

21,22 via which fresh electrolyte F can be with added and further tappings 23,24 via which spent electrolyte S can be withdrawn, the respective tappings being for anolyte and catholyte. On recharging, typically via a coupling 25 for lines 26 to all the tappings, a remote pump 27 pumps fresh anolyte and fresh catholyte from remote storages 28 and draws spent electrolyte to other remote storages 29. This electrolyte is, typically by a conventional fuel cell recharging method, recharged and passed back to the storages 28.

In the above abstract replacement of electrolyte is referred to. It is more normal to recharge by current reversal, as is normal with rechargeable batteries.

Often such batteries having a stack of cells are connected in series. Some of the stacks, but not all, can be supplied with electrolyte from common tanks. Where for instance, four stacks are in series they can be supplied with electrolyte from two pairs of tanks. Due to the build-up of tolerances, which can lead to electrolyte overflow of +ve electrolyte into the -ve electrolyte tank and vice versa, itself aggravating the situation, the electrolytes can discharge at different rates, in other words the respective States Of Charge (SOC) of the electrolytes can be out of balance. This in turn can lead to a significant loss of capacity as the more charged electrolytes can discharge only until the less charged electrolytes are discharged and these can recharge only until the more charged electrolytes are fully charged.

Solutions to this problem have been proposed as in WO/2018032409, which proposes that separate power conversion systems, connected to groups of or individual stacks, can be used to modulate individually the power to and from the tank pairs, thereby balancing the SOC among the electrolytes. This is an expensive solution to a problem that may not be present with all battery stacks.

The object of the present invention is to provide an improved method of balancing the state of charge of a plurality of flow batteries electrolytes

A method of balancing the state of charge of a electrolytes in a battery having a plurality of flow battery stacks in series and supplied with electrolytes from at least two stores, the stacks each having a plurality of cells, the method consisting in the steps of:

• measuring and comparing the state of charge of the electrolytes of the respective stores and registering if the states of charge differ by more than a threshold and

• in the case of the state of the charge difference threshold being exceeded:

• controlling the number of cells in the series connection of the stacks whereby the less charged electrolytes discharge through fewer cells than the more charged electrolytes and/or

• controlling the number of cells in the series connection of the stacks whereby the less charged electrolytes are charged through more cells than the more charged electrolytes. The number of cells in series can be controlled by making connection to an intermediate electrode within a stack and leaving a number of the cells in the stack unconnected. However, preferably one or more entire stacks is switched in or out of series.

Control of the number of cells connected in series can in practice normally be done only by switching cells out of series; the alternative of switching in additional cells supplied by other electrolyte normally being impractical. Thus, when discharging, the less charged electrolyte is supplied to a reduced number of cells thus making less demand on it than the more charged electrolyte; and, when charging, the more charged electrolyte is supplied to less cells, which involves switching cells out from charging it, so that the less charged electrolyte receives more charge than the more charged electrolyte.

Whilst it can be envisaged that the switching in and out of cells, as a complete stack or as a proportion of the cells in the stack, could be by transistor switches, it is preferably by physical switches. Normally there will be two switches, one for connecting the normal battery terminal to a load line and the other for connecting a temporary (fewer cells) terminal to the load line.

Preferably they will be ganged together, whereby they cannot both be closed at the same time. The corollary is that at an intermediate position in the process of switching, they are likely to be both be open at the same time. To avoid this causing an interruption of power supply to or indeed charging from the load line, each switch is preferably provided with a respective unidirectional conduction element, conveniently a diode, arranged to allow charging/discharging current flow from the temporary terminal and to the normal terminal, when both switches are open.

According to another aspect of the invention there is provided a flow battery comprising:

• a series array of at least two stacks of cells having at least two respective electrolyte stores; • means for measuring and comparing the state of charge of the electrolytes in the respective stores;

• switch means for switching a bypass shunt into the series array in place of at least some by-passed ones of the cells or stack(s) of cells, this by-pass switch means being normally open;

• switch means for switching out the by-passed ones of the cells or stack(s) of cells from the series array, this switch-out switch means being normally closed; and

• a controller for actuating the switches in event of the measuring and

comparing means detecting a difference above a threshold in state of charge in the respective stores, whereby during charging and/or discharging the state of difference can be reduced.

Preferably the measuring means is a respective reference cell associated with each electrolyte store. Alternatively the reference cell could be associated with one or more of the stacks connected to the electrolyte stores.

Again the state of charge can b measured in the manner described in W09003666.

Preferably, the switch means for switching out is connected to the by-passed cells at one end of their series connection together.

Preferably, the bypass switch means is connected in a shunt line across the said by-passed ones of the cells or stack(s) of cells. The switch means could comprise transistor switches. However, we prefer to use electromagnetic relays.

The switching out switches could be duplicated at either side of the cells/stack(s) to be switched out, but are preferably provided at one side only, this being effective to remove them from being active in the battery. In order to avoid an interruption of supply or indeed charging when the switches are made, each preferably has a diode in parallel with it, whereby opening of a first switch is followed by immediate conduction through the diode of another switch prior to closure of this switch.

Preferably, the bypass diodes, that is the diodes in parallel with the switches in the bypass shunts, are oriented to conduct current during discharge, to facilitate continuous discharge when the cells/stack(s) being switched out are no longer in the array; and the diodes in parallel with the switching out switches are oriented to conduct current during charging, to facilitate continuous charging as switched out cells/stack(s) are switched back into the array.

In one embodiment, the battery includes:

• an additional shunt line is provided for switching out all of a group of stacks of cells supplied by a particular one of the electrolyte stores, the additional being provided at an end of the group remote from an end to which the switch-out switch means is connected;

• an additional bypass switch means being provided in the additional shunt line; and

• an additional switch-out switch means is provided in series with the said switch-out means, remote from the group of stacks,

the arrangement being that:

• the said shunt line is connected between the said switch-out switch means and the additional switch-out switch means and

• the additional switch means is connected to the remote side of the

additional switch-out switch means;

• switch means for switching out the by-passed ones of the cells or stack(s) of cells from the series array, this switch-out switch means being normally closed; and

• a controller for actuating the switches in event of the measuring and

comparing means detecting a difference above a threshold in state of charge in the respective stores, whereby during charging and/or discharging the state of difference can be reduced.

Preferably, the battery is a vanadium redox flow battery.

To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which:

Figure 1 is Figure 1 of prior art W00069692;

Figure 2 is a block diagram of a flow battery in accordance with the invention;

Figure 3 is a more detailed diagram of switching-in and bypassing switches for the battery of Figure 1, with six switch state diagrams (a) - (f);

Figure 4 is a block diagram of another battery in accordance with the invention.

Referring first to Figures 2 to 4 of the drawings, a flow battery 1 consists of a series of four Vanadium Redox stacks 2i .. 2 ₄ , normally electrically connected in series. The stacks are paired with electrolyte tanks 3. By paired is intended that each tank supplies in parallel two stacks via pipework 4, with electrolyte pumps 5. As shown, each tank is divided to have +ve & -ve compartments and respective pumps. This is conventional and will not be described further.

Each tank has a state of charge meter 7, conveniently in the form of a reference cell supplied with the tank’s electrolytes.

The battery is connected to an inverter/charger 6 to receive charge from a source of electricity 8 and supply it to a load 9, as required.

In accordance with the invention, end ones of the stacks, i.e. stacks 2 _I ,2 ₄ , each has across it a shunt or by-pass switch 111, 1 l ₄ in a shunt or by-pass linel2i,l2 ₄ . Normally the shunt switches are open. The by-pass lines extend from the common points 14i,14 ₃ of the stacks 2 _I ,2 ₂ & stacks 2 ₃ ,2 ₄ and to the +ve & -ve supply lines l5 _+ve , 15 _ve to the inverter/charger 6. End parts of the +ve & -ve supply lines l5 _+ve , l5 _ve have switching-out switches 16i,16 ₄ , between the respective +ve terminal of stack 2i & the point of connection of the shunt line 12i with the +ve supply line l5 _+ve and the respective -ve terminal of stack 2 ₄ & the point of connection of the shunt line l2 ₄ with the -ve supply line l5 _Ve .

A controller 17 for the battery is connected to the switches 11 i,l l ₄ & 16 _I ,16 ₄ and the inverter charger 6 for all of their control. It is also connected to the state of charge meters 7. Should the controller detect a more-than 5% difference in the state of charge of the electrolytes for the respective stacks 2i,2 _¾

& 2 ₃ ,2 ₄ , it operates to switch out one of the stacks 2i & 2 ₄ according to which is more charged and whether the battery is being charged or is supplying electricity.

Whilst the switches could be switched simultaneously, as ganged together, this can result in a momentary short circuiting of one of the stacks or a momentary open circuit, which is liable to damage the inverter/charger or some apparatus or device connected to it. Accordingly the switches are provided with diodes Dl 1, D16 to conduct as soon as opening of the other switch for the stack is opened and before the closure of the previously open switch.

The arrangement is shown in more detail in Figure 3.

It will be appreciated that increasing the state of charge on the electrolytes of stacks 2 _I ,2 ₂ is equivalent to decreasing the state of charge on the electrolytes of stacks 2 ₃ ,2 ₄ .

Further the arrangement of the switches (and their diodes) at the end ones of the stacks enables the state of charge of the electrolytes for both of the stacks connected to the respective tanks to be modified. The controller monitors the state of charge of the electrolytes of the stacks. Where the state of charge of the electrolytes from different tanks exceeds a threshold, typically 5%, the controller alters the mode of charging or discharging via the inverter charger, (i.) to charge preferentially more heavily discharged electrolyte or (ii.) to discharge preferentially more heavily charged electrolyte. In practice when the threshold is reached, the mode change can be to either (i.) or (ii.). Where there are more stacks and in particularly more tanks, the electrolytes responsible for the threshold being exceeded are the logical ones to be dealt with in accordance with (i.) or (ii.). However it remains possible to operate with (i.) on one tank or (ii.) on the others for instance. It will normally be preferable to operate during charging, whereby the maximum rate of discharge remains available.

The detailed sequence of steps below is restricted to switching out and back in of one end stack during both discharging and charging.

The steps of bypassing stack 2i during discharge are as follows for a detection of the electrolytes of stacks 2 _I ,2 ₂ being less charged by the 5% threshold than those of the stacks 2 ₃ ,2 ₄ (which is the same as the electrolytes of the stacks 2 ₃ ,2 ₄ being more charged):

1. Firstly as shown in Figure 3(a), prior to bypassing the stack, the switch 111 is open and the diode D 111 is reverse biased; no current flows in the by-pass linel2i. The switch-out switch 16i is closed and discharge includes from the stack 2i;

2. The switch-out switch 16i is opened and with the diode Dl6i being reverse biased, current ceases to flow through and from the stack 2i. Simultaneously, the diode D 111 becomes forward biased with removal the stack’s voltage across it. It immediately starts conducting and without the switch 111 being opened current flows in the linel2i;

3. Now, the switch 111 is closed and current passes through it and not the diode D 111. The result is that twice as much energy is drawn from the electrolytes of stacks 2 ₃ ,2 ₄ , not separately shown in Figures 3(a) - (b), as from that of stack 2 ₂ , whereby its less charged electrolyte drops in state of charge more slowly until it is aligned with that of stacks 2 ₃ ,2 ₄ ;

4. The switch 111 is now opened and the state of Figure 3(b) is reverted to with current passing through diode Dl 11;

5. Now the switch-out switch 16i is closed and the situation of Figure 3(a) is reverted to, with the difference that the electrolytes of the respective pairs of stacks are equally charged/discharged. They will remain so until another accumulation of tolerances and events causes the difference in state of charge threshold to be exceeded again and the same adjustment to be made again.

When the difference in state of charge threshold is detected during charging, a similar sequence of steps is gone through, except that the diode Dl6i conducts during the middle step.

The steps of bypassing stack 2i during charging are as follows for a detection of the electrolytes of stacks 2i,2 ₂ being more charged by the 5% threshold than those of the stacks 2 ₃ ,2 ₄ (which is the same as the electrolytes of the stacks 2 ₃ ,2 ₄ being less charged):

11. Firstly as shown in Figure 3(a), prior to bypassing the stack, as in step (1) above the switch 111 is open and the diode D 111 is reverse biased; no current flows in the by-pass linel2i. The switch-out switch 16i is closed and discharge includes from the stack 2i;

12. The switch-out switch 16i is opened, but in contrast to step (2) above and with the current flowing in the opposite direction, the diode Dl6i is forwards biased, and current continues to flow through the stack 2i. The diode D 111 remains reverse biased and no current flows in the shunt linel2i;

13. Now, the switch 111 is closed and current passes through it in the shunt line. The diode Dl 11 becomes reverse biased and charging current ceases to flow in it and the stack 2i. The result is that twice as much energy passes into the electrolytes of stacks 2 ₃ ,2 ₄ , not separately shown in Figures 3(a) - (b), as that into the stack 2 ₂ , whereby its more charged electrolyte rises in state of charge more slowly until it is aligned with that of stacks

23, 2 ₄ ;

14. The switch 111 is now opened and the state of Figure 3(b) is reverted to with current passing through diode Dl6i and charging of stack 2i resumes;

15. Now the switch-out switch 16i is closed, charging of all the stacks

continues and the situation of Figure 3(a) is reverted to, with the difference that the electrolytes of the respective pairs of stacks are equally

charged/discharged. They will remain so until the difference in state of charge threshold exceeded again and the same adjustment to be made again.

The switches, which could be transistor switches, are electro- mechanical relays.

Turning now to Figure 4, a battery with many more stacks is shown together with the possibility of preferential charging/discharging of a variety of stacks. The basic circuitry for effecting this remains the same.

The battery has 48 stacks, but they are permanently connected in triplets, of stacks 102. These grouped in groups of twelve stacks, reference 1021, 1022, 1023, 1024. All the stacks in each group are supplied from common electrolyte stores 1031, 1032, 1033, 1034 respectively.

Within each group there are a triplet 104 of three stacks arranged in parallel between one end busbar 105 and a“quarter point” busbar 106. Another end busbar 107 is provided with a set 108 of nine stacks arrayed between it and the quarter point one as three parallel strings of three stacks in series. The voltage across the triplet, i.e. between the busbars 105,106, is of course one third of the voltage across the set of nine, i.e. between the busbars 106,107. Representative ones of the cells in representative ones of the groups has a reference cell 1091,1092,1093,1094. These are all connected to a controller 110 for the battery. Each group of stacks has an inner switch and diode circuit 111 for switching in or out its triplet of cells and an outer switch and diode circuit 112 for switching in or out the entire group of 12 cells. These will not be described in the same detail as the above described embodiment. Their diodes operate in an analogous manner to maintain continuity of current flow. The inner circuit switches 1121, 1122 can be alternately opened and closed to bypass or reconnect the triplet of cells as required. The outer circuit switches 1221,1222 likewise can switch in or out the entire group of cells. The former switching is likely to be for state of charge adjustment. The latter switching is likely to be infrequent and for eventualities such as electrolyte leakage or replacement.