Improved redox flow battery

Abstract

A redox flow battery system comprises at least one battery module 40 comprising (i) a first cell stack 50 comprising at least one cell sub-stack 60 connected in series, (ii) a second cell stack 50 com

Description

FIELD OF THE INVENTION The present invention relates to the methods and apparatus for use in battery systems particularly redox flow battery systems. In various aspects the invention relates to redox flow battery system(s) comprising a redox flow battery module(s), redox flow battery string(s), redox flow battery array(s), and set(s) of array(s). In one or more further aspects the invention relates to methods of operating the above. The invention finds particular application in redox flow battery systems and methods but may be used with other battery systems. BACKGROUND OF THE INVENTION Redox flow battery systems typically comprise multiple cells connected to one another in a bipolar electrochemical cell arrangement, electrically in series, to increase output voltage. These are usually placed in physical proximity next to one another in an arrangement referred to as a stack, or cell stack. Each cell stack is associated with at least one tank, typically at least one tank pair comprising two tanks for anolyte and catholyte, together with one or more pumps that circulate electrolyte from the tank(s), through the cell stack and back to the tank(s). These are referred to as a battery module. Sometimes more than one cell stack can share a tank. Occasionally, several battery modules are connected together in series to increase the output voltage. This is called a string. Achieving desired voltage levels for grid connected systems, particularly at reasonable cost, remains challenging. Individual stacks, and indeed battery modules, may not be identical due to variations arising from construction and component variation. Such variations result in imbalances of state of charge (SOC), and / or open circuit voltage, between battery modules, and / or, indeed, between strings, because all stacks and modules in the string operate at the same charge and discharge current. Various systems exist for balancing state of charge, and / or open circuit voltage, between battery modules within a string. Passive balancing typically involves discharging excess charge through a passive load, such as a resistor, or to other components resulting in efficiency losses. Active balancing typically involves shuttling charge from one battery module to another. US20130127396 TRIEBEL describes an electrical energy store and method for closed loop control of such energy store. The energy store uses bidirectional DC-DC converters with one DC-DC converter per cell-stack. Bidirectional DC-DC converters, which can be connected via the high-voltage side to a DC bus and which have a switching hysteresis with a charging and / or discharging curve, are also described. Provision of each cell-stack with such a DC-DC converter is expensive. US20100178533 WHITEHEAD describes a redox flow battery with parallel functional units in the form of battery cascades and a DC / DC converter, and associated switch or relay. WO2021025925 KLASSEN describes state of charge balancing in flow battery unit string by use of electrolytes flow regulation to adjust the resistance of cells in the flow battery units and the resistance of the flow battery units themselves to balance the state of charge in the flow battery unit string. Further, current bypass devices (CBDs) are described across battery units in strings to balance the state of charge between flow battery units within the flow battery unit strings by shunting currents through the current bypass devices. WO2020030762 UNDERWOOD describes controlling the number of cells in a series connection of stacks whereby less charged electrolytes discharge through fewer cells than more charged electrolytes and / or controlling the number of cells in a series connection of the stack whereby less charged electrolytes are charged through more cells than more charged electrolytes. Again, this balancing of state of charge between battery modules comprising cells in a series connection is relatively complex. WO2021190928 and WO2021191065 LUTH describe several methods for reducing imbalances using various combinations of switches and / or a DC-DC converters for each battery module. Connection of battery module(s) through a resister, or via a common DC bus using a further converter connected to the DC bus, is described. CN202435082 SHAOFENG describes an active equalization circuit for a lithium-ion battery pack. The circuit comprises an input / output reversible DC / DC converter and a switch array. CN202333899 JINHUA describes time sharing DC balancing of battery modules using a DC-DC converter and a relay array. US20160006052 LI describes charge capacity management in a redox flow battery string by adjusting the open circuit voltage value for each redox flow battery to correspond to a predetermined open circuit voltage value. GUAN and HUANG in “A Modular Active Balancing Circuit for Redox Flow Battery Applied in Energy Storage System” (IEEE access 2021.3112902, volume 9, 2021, page 127548 to 127558) describes integrating a circulating pump driving circuit and an SOC equalisation circuit together and using the energy consumption of the pump to achieve the SOC balancing control between different battery stacks by using the energy of the battery stacks to drive the pump (as a load). A DC-DC converter is provided. Such systems are complex requiring sophisticated control systems or additional devices per battery and / or result in efficiency losses. It will be appreciated from the above that there are various passive and active methods of balancing charge between battery modules or indeed between strings. Passive methods tend to be less complex and cheaper but less efficient for example, discharging one or more battery models or strings through a load, such as a resister, or to power equipment. Further such methods are difficult to control and refine. Active balancing methods typically use more complex (and expensive) methods, including for example, in which electrolyte flow is controlled, or the number of cells connected in a circuit is controlled, or DC-DC convertors and switch arrays are provided. Some of these methods introduce challenges in the field (on site): increasing risk of leaks through additional, often field-connected, joints; and increasing time and complexity of maintenance and / or replacement. At best these challenges increase the cost over the lifetime of the project, reducing commercial viability; at worst these can result in breakdowns in the field, possibly affecting even the grid, which is undesirable, practically and commercially. Cost effective, and practical, redox flow battery systems as a whole, therefore present an ongoing challenge to designers of flow battery systems. In addition, cost effective balancing remains problematic. Furthermore, approaching voltage levels and / or power levels that are suitably high for connection to DC-AC converters (and subsequent AC-AC converters) to achieve grid-level AC voltages, again at reasonable cost, remains an ongoing challenge. Providing grid-level voltages and / or power levels and balancing of battery modules together is even more challenging. Further, providing resilience in such systems, so that these offer improved reliability, in the event of one or more fault conditions, for example, a faulty cell stack, is desirable. Thus, the design of flow battery systems and the development of sufficiently high voltage and power, without significant additional cost, and preferably in a reliable manner, remains a challenge. Flow battery systems are complex having mechanical, chemical and electrical constraints and requirements. Large scale adoption of flow battery systems may be impeded where reliability is in question and / or the cost is too high for any of the component elements, and / or the cost of the footprint is too high. It therefore remains an ongoing challenge in bringing cost-effective, large-scale flow battery systems to the market. Having been around for decades, large scale adoption is in its early days, indicating that there is more to be done in addressing the challenges to provide suitable levelized cost of energy. Innovation in this area is desirable. The present invention seeks to address one or more of the above challenges, and / or of the prior art. SUMMARY OF THE INVENTION In one (e.g. a general) aspect of the invention, there is provided a battery module for a redox flow battery system, the battery module comprising: i) a first cell stack comprising at least one, preferably two or more, cell substacks (e.g. individual cell sub-stacks, which may be physically separate from one another, and / or may be, for example, a group of bipolar cells with terminal current collectors on each of the positive and negative ends, or even the same ends, typically also including other elements such as terminal end plates to form a housing, optionally with tie-rods between such end plates), and where two or more cell sub-stacks are provided, these are electrically connected in series; ii) a second cell stack comprising at least one, preferably two or more, cell sub-stacks (e.g. individual cell sub-stacks as described elsewhere herein), and where two or more cell sub-stacks are provided, these are electrically connected in series; iii) at least the first and second cell stacks electrically connected in parallel within the respective battery module; and, iv) a (e.g. at least one) tank pair (e.g. a common tank pair, for example comprising an anolyte storage tank and a catholyte tank); v) the cell sub-stacks of at least the first and second cell stacks hydraulically connected in parallel with (e.g. to and from) the (e.g. common) tank pair. In a first aspect of the invention, there is provided a redox flow battery system (e.g. for delivering a predetermined (e.g. high) voltage at one or more predetermined (e.g. high) power levels) comprising: a string, the string comprising: at least two battery modules, electrically connected in series (e.g. to form a string); one or more or each (e.g. at least one, preferably two or more) battery module(s) comprising: i) a first cell stack comprising at least one cell sub-stack(s) (e.g. individual cell sub-stacks as described elsewhere herein); ii) a second cell stack comprising at least one cell sub-stack(s) (e.g. individual cell sub-stacks as described elsewhere herein); iii) at least the first and second cell stacks electrically connected in parallel within the respective battery module; and, iv) a (e.g. at least one) tank pair (e.g. a common (e.g. shared) tank pair, for example comprising an anolyte storage tank and a catholyte tank); v) the cell sub-stacks of at least the first and second cell stacks hydraulically connected in parallel with (e.g. to and from) the (e.g. common) tank pair. This facilitates a predetermined voltage level to be developed in the string by the at least two battery modules and associated cell sub-stacks connected in series, and further this facilitates configuration of one or more predetermined power levels to be developed by the at least two cell stacks in parallel within one or more or each battery module(s). In a second aspect of the invention, there is provided a redox flow battery system (e.g. for delivering a predetermined (e.g. high) voltage at one or more predetermined (e.g. high) power levels) comprising: - an array comprising at least two strings, each string comprising at least one battery module(s), (e.g. the two strings comprising one or more battery module(s)), the at least two strings electrically connected in parallel; - one or more or each (e.g. at least one, preferably two or more) battery module(s) in each string comprising: i) a first cell stack comprising at least one and preferably two or more cell sub-stacks (e.g. individual cell sub-stacks as described elsewhere herein), where two or more are provided, these are preferably electrically connected in series; ii) a second cell stack comprising at least one and preferably two or more cell sub-stacks (e.g. individual cell sub-stacks as described elsewhere herein), where two or more are provided, these are preferably electrically connected in series; iii) at least the first and second cell stacks electrically connected in parallel within the respective battery module; and, iv) a (e.g. at least one) tank pair (e.g. a common tank pair, for example comprising an anolyte storage tank and a catholyte tank); v) the cell sub-stacks of at least the first and second cell stacks hydraulically connected in parallel with (e.g.to and from) the (e.g. common) tank pair. This facilitates a predetermined voltage and power level being developed in an array particularly by strings when in series or parallel. In a third aspect of the invention, there is provided a redox flow battery system comprising: - one (e.g. only one) battery module, the battery module comprising: i) a first cell stack comprising at least one and preferably two or more cell sub-stacks (e.g. individual cell sub-stacks as described elsewhere herein), where two or more are provided, these are preferably electrically connected in series; ii) a second cell stack comprising at least one and preferably two or more cell sub-stacks (e.g. individual cell sub-stacks as described elsewhere herein), where two or more are provided, these are preferably electrically connected in series; iii) at least the first and second cell stacks electrically connected in parallel within the respective battery module; and, iv) a (e.g. at least one) tank pair (e.g. a common tank pair, for example comprising an anolyte storage tank and a catholyte tank); v) the cell sub-stacks of at least the first and second cell stacks hydraulically connected in parallel with the (e.g. common) tank pair; and further comprising: a housing unit comprising a shipping container of predetermined standardsize (e.g. in compliance with IS0668(2020) of length 10ft (3.05m), 20ft (6.06m), 30ft (9.12m), or 40ft (12.19m), of width 8ft (2.44m) and height 9ft 6in (2.89m) or 8ft 6in (2.59m)), wherein the one (e.g. only one) battery module is contained within the housing unit. Preferably, a 20ft (6.06m) container is used. In one or more embodiments, the battery system is configured to comply with the maximum gross mass allowed for the shipping container of standard size. In one or more embodiments, the battery system comprises: - electrolyte (e.g. positive and negative electrolyte) and the battery system is configured to comply with the maximum gross mass allowed for the shipping container of standard size. In one or more embodiments of the battery system: i) the first cell stack (50) may comprise two or more cell sub-stacks (60) electrically connected in series; and / or, ii) the second cell stack (50) may comprise two or more cell sub-stacks (60), electrically connected in series. In one or more embodiments, of one or more aspects of the invention, at least one cell stack within at least one battery module may be configured to be removable from the battery module (e.g. easily installed or removed so as to be replaceable e.g. in a quick fit, and / or field-fittable and / or plug-and-play, manner), preferably in a quick and / or reliable and / or error- minimising manner. This improves (reduces) installation time and / or servicing time and so installation and / or servicing costs. In a fourth aspect, there is provided a redox flow battery system comprising: - at least one battery module; - one or more or each (e.g. at least one, preferably two or more) battery module(s) comprising: i) a first cell stack comprising at least one (preferably two or more) cell substacks (e.g. individual cell sub-stacks as described elsewhere herein), where two or more are provided, these are preferably electrically connected in series; ii) a second cell stack comprising at least one (preferably two or more) cell sub-stacks (e.g. individual cell sub-stacks as described elsewhere herein), where two or more are provided, these are preferably electrically connected in series; iii) at least the first and second cell stacks electrically connected in parallel within the respective battery module; and, iv) a (e.g. at least one) tank pair (e.g. a common tank pair, for example comprising an anolyte storage tank and a catholyte tank); v) the cell sub-stacks of at least the first and second cell stacks hydraulically connected in parallel with (e.g. to and from) the (e.g. common) tank pair; further wherein, at least one cell stack within at least one battery module (e.g. at least one of the first and second and any further cell stack(s)) is configured to be removable (e.g. as an integrated unit) from the battery module (e.g. relatively easily installed and / or removed and / or replaceable for example, in a quick fit, and / or quick-connect, and / or plug-and-play manner e.g. in the field, preferably in reliable and / or error-minimising manner)). In one or more embodiments of the battery system with at least one removable cell stack: i) the first cell stack (50) may comprise two or more cell sub-stacks (60) electrically connected in series; and / or, ii) the second cell stack (50) may comprise two or more cell sub-stacks (60), electrically connected in series. Indeed, one or more or each cell stack may comprise two or more cell sub-stacks (60), electrically connected in series. In one or more embodiments, a plurality of sub-stacks, each with four fluid inlets and outlets (positive in and out, negative in and out) per sub-stack, with terminal electrical connections on each, may be provided between one pair of end plates to form one unitized cell stack. In a fifth aspect, there is provided at least one battery module for a redox flow battery system; - one or more or each (e.g. at least one, preferably two or more) battery module(s) comprising: i) a first cell stack comprising at least one, preferably two or more, cell substacks (e.g. individual cell sub-stacks as described elsewhere herein), where two or more are provided, these are preferably electrically connected in series; ii) a second cell stack comprising at least one, preferably two or more, cell sub-stacks (e.g. individual cell sub-stacks as described elsewhere herein), where two or more are provided, these are preferably electrically connected in series; iii) at least the first and second cell stacks; electrically connected in parallel within the respective battery module; and, iv) a (e.g. at least one) tank pair (e.g. a common tank pair, for example comprising an anolyte storage tank and a catholyte tank); v) the cell sub-stacks of at least the first and second cell stacks hydraulically connected in parallel with (e.g.to and from) the tank pair, further wherein, at least one cell stack within at least one battery module (e.g. at least one of the first and second and any further cell stack(s)) is configured to be removable (e.g. as an integrated unit) from the battery module (e.g. relatively easily inserted and / or removed so as to be replaceable e.g. in a quick fit, and / or quick-connect, and / or plug-and-play, manner e.g. in the field). In a sixth aspect, there is provided a battery module for a redox flow battery system, the battery module comprising: i) a receiving slot for a first cell stack, and a first cell stack located in the first receiving slot, the first cell stack comprising at least one, preferably two or more, cell sub-stacks (e.g. individual cell sub-stacks as described elsewhere herein), where two or more cell sub-stacks are provided, these are preferably electrically connected in series; ii) a receiving slot for a second cell stack comprising two or more cell substacks (e.g. individual cell sub-stacks as described elsewhere herein), where two or more cell sub-stacks are provided, these are preferably electrically connected in series; iii) at least the receiving slots for the first and second cell stacks configured for electrically connecting the first and second cell stacks (e.g. when both present) in parallel within the respective battery module; and, iv) a (e.g. at least one) tank pair (e.g. a common tank pair, for example comprising an anolyte storage tank and a catholyte tank); v) at least the receiving slots for the cell sub-stacks configured for hydraulically connecting the first and second cell stacks in parallel to the common tank pair. In one or more embodiments, at least one cell stack within at least one battery module (e.g. at least one of the first and any further cell stack(s) (e.g. a second cell stack)) is configured to be removable (e.g. as described herein) from the battery module. In a seventh aspect, there is provided a power block unit (e.g. a removable power block unit) for a battery module for a redox flow battery system, the power block unit comprising: i) a first cell stack comprising at least one, preferably two or more cell substacks (e.g. individual cell sub-stacks as described elsewhere herein), where two or more cell sub-stacks are provided, these are preferably electrically connected in series; ii) the first cell stack configured for connecting electrically in parallel with a second cell stack of a second (e.g. removable) power block unit, e.g. when located within a battery module; and, iv) the first cell stack configured for connecting hydraulically in series to a second cell stack of a second (e.g. removable) power block unit, e.g. when located within a battery module (e.g. to a preferably common tank pair). In one or more embodiments, the (e.g. removable) power block unit may comprise electrical connections (e.g. quick fit, and / or quick-connect, and / or plug-and-play electrical connections) for connecting within a battery module (e.g. when located in a receiving slot) and / or to a second (e.g. removable) power block unit. In one or more embodiments, the (e.g. removable) power block unit may comprise hydraulic connections (e.g. quick fit, and / or quick-connect, and / or plug-and-play pipework / fluid connections) for connecting within a battery module (e.g. when located in a receiving slot) and / or to a second (e.g. removable) power block unit. The removable power block unit may comprise a shunt manifold unit for electrolyte comprising the hydraulic connections (e.g. quick fit, and / or quick-connect, and / or plug-and-play pipework / fluid connections) for connecting within a battery module (e.g. when located in a receiving slot) and / or to a second power block unit. In one or more embodiments of the various aspects of the invention, at least one battery module may comprise a removable power block unit, and the removable power block unit may comprise: - the at least one removable cell stack (e.g. typically one, or the first and second, and / or any further removeable cell stacks); - a shunt manifold unit for electrolyte, for managing (e.g. configured to manage) electrolyte flow and shunt current losses between two or more cell sub-stacks of the at least one removable cell stack (e.g. when inserted within the respective battery module (e.g. when located in a receiving slot) and connected to the tank pair of the respective battery module). In one or more embodiments, the shunt manifold unit may comprise long (e.g. circuitous) respective (e.g. parallel) flow paths for flow of anolyte and catholyte between each cell stacks (e.g. which cell stacks are electrically connected in series). In one or more embodiments, the removable power block unit may be configured to move e.g. reciprocate within the battery module e.g. it may be slidably mounted, for example, on rails (e.g. sliding rails) within a battery module (e.g. within a or the standard shipping container when provided as a housing unit for a or the (e.g. only one) battery module) so as to be movable (e.g. in a reciprocating manner, such as slidable in and / or out), as a single (e.g. as a unitary structure) unit (e.g. typically, once any electrical and / or pipe work connections are closed (e.g. capped) and / or disconnected (e.g. and / or made safe)). In one or more embodiments, the removable power block unit may comprise a support structure (e.g. in the form of a chassis such as an integrated rigid chassis, such as an open frame, or indeed, a housing) which may be configured to be removable from, and / or insertable into, a or the battery module. In one or more embodiments, the redox flow battery system may comprise: 1 to 10, or 2 to 8, or 2 to 6, or 2 to 4, or 2 to 3, or 2, or 3 power block unit(s) (e.g. optionally removable power block unit(s)). In one or more embodiments, the battery module may comprise: 1 to 10, or 2 to 8, or 2 to 6, or 2 to 4, or 2 to 3, or 2, or 3 receiving slots (also known as expansion slots) each for receiving a respective, preferably removable (e.g. removable as an integral unit), power block unit(s). In one or more embodiments, one or more or each expansion slot may comprise: - electrical connection(s) for a power block unit, and associated cell stacks and sub-stacks therein; - electrolyte pipework connection(s) for a power block unit, and associated cell stacks, cell sub-stacks therein, for connecting same to the tank pair. In one or more embodiments, the redox flow battery system may comprise: - at least two battery module(s) electrically connected in series; - an active DC-DC balancer (e.g. operable at string level) for balancing the stage of charge (SOC) of at least two battery module(s), e.g. when electrically connected in series. In one or more embodiments, the DC-DC balancer may be configured to - shuttle energy and / or charge (e.g. in the form of current) from one or more battery module(s) to one or more other battery module(s). In one or more embodiments, the DC-DC balancer may comprise: - a DC-DC convertor (e.g. a bi-directional DC-DC convertor); and, - a relay array (e.g. a set of relays or switches for connecting one battery module to another within the string). In one or more embodiments, - one or more battery module(s) may be configured to have an adjustable rate of flow of electrolyte(s), so as to be able to adjust rate of flow of charge within the respective battery module(s) (e.g. so as to adjust its state of charge (SOC)) and / or; - in two or more battery module(s) may be configured to transfer electrolyte from one battery module(s) to another (e.g. so as to adjust their relative SOC). In one or more embodiments, two or more or all cell sub-stacks may be of identical (e.g. within manufacturing tolerances) configuration and / or identical construction (e.g. although mechanically separate, two or more or all cell stacks may be of common design and / or construction materials). In one or more embodiments, there may be provided a redox flow battery system comprising: - four battery modules electrically in series (e.g. as described herein); - each battery module comprising one, or two, or three cell stacks electrically connected in parallel; - each cell stack comprising four cell sub-stacks electrically connected in series, and hydraulically connected in parallel to a (e.g. at least one), or the, tank pair of the respective battery module; each cell sub-stack comprising a plurality of cells in series (e.g. of 20 to 300, or 20 to 250, or 20 to 200, or 20 to 100, or 20 to 75, or 50 to 100, or 50 to 75, or 50, or 60, or 70, or 75, or 80, or 90, or 100 cells in series). In one or more embodiments, there may be provided a redox flow battery system comprising: - four battery modules in series; each battery module comprising: - one or two or three or more (preferably two or more) receiving slots for receiving respective (optionally removable) power block units; - at least one power block unit, the power block unit comprising: at least one (typically only one) respective cell stack, the at least one cell stack comprising: one or two or three or four or more (preferably two or more, more preferably three, four, five or six) cell sub-stack(s) in series; and - a shunt manifold unit for managing (e.g. configured to manage) electrolyte flow and shunt current losses (e.g. between two or more cell sub-stacks(s) the cell substacks being connected, preferably in parallel, to the tank pair (the pair of electrolyte tanks for that respective battery module)). In one or more embodiments, the redox flow battery system comprises electrolyte (e.g. anolyte and / or catholyte). In one or more embodiments, one or more or each battery module comprises electrolyte (e.g. anolyte and / or catholyte). Thus, in one or more embodiments a balancing system may be provided comprising one or more current bypass devices, one or more relays and one or more DC-DC balancers typically bi-directional DC-DC balancers, typically one per module. In some embodiments, one or more cell stacks may comprise a plurality of cell sub-stacks e.g.one or more, or 2-12 or 2-10, or 2-8, or 2-6, or 2-4, or 3, or 4 cell sub-stacks. The cell sub-stacks may comprise a plurality of individual redox flow cells electrically connected in series there together e.g. one or more of 10-400, or 10-300, or 10-200, or 10-100, or 20-80, or 20-60, >or <40, >or <50, >or <60, >or <80, >or <100, >or <200. In some embodiments, one or more or each cell stacks may each comprise one or more cell sub-stacks. In one or more embodiments, the redox flow battery system comprises electrolyte (e.g. anolyte and / or catholyte) for example within the tank pair and cell stacks and associated electrolyte connections. In one or more further aspects, the invention relates to method(s) of manufacturing, repairing and / or operating the system(s), battery module(s), and power block unit(s), shunt manifold unit(s) and other apparatus described herein. Any feature or element in any one or more embodiments of any one or more aspects of the invention described herein may be used in any one or more embodiments of any other aspect(s) of the invention, as would be understood by those skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example only, with reference to example embodiments in the following Figures, in which like numerals refer to like reference features. Figure 1 shows a schematic plan view of a flow battery system comprising a set of arrays ARI to ARN. Figure 2 shows a schematic view of a string, Sij, here the j-th string for use in the i-th array. Figure 3 shows a schematic plan view of a battery module, Bijk, here the k-th battery module of string j of array i. Figure 4 shows a schematic plan view of array i illustrating a series of strings 1 to M, each string having 1 to n battery modules (Bijk, k=l to n), each string of battery modules, Bijk, being provided with a string level DC balancer. Figure 5 shows a schematic plan view of a string comprising four battery modules and a string interface and control module also known as a string control unit (SCU). Figure 6 shows a schematic plan view of three power blocks, two of which are optional, each comprising a cell stack ‘T’ of ‘q’ cell sub-stacks Cq, for use within a battery module. Figure 7 illustrates a schematic view of an array comprising twelve strings, each string comprising four battery modules. Figure 8 illustrates a bi-directional DC-DC converter and associated relay array for use with a string of series connected battery modules. Figure 9 illustrates a schematic view of an array comprising n strings connected by a common high-voltage DC (HVDC) bus DETAILED DESCRIPTION OF THE INVENTION The invention will now be described in more detail, without limitation, with reference to the accompanying Figures. In this document, the use of terms ‘first’, ‘second’, ‘third elements’ or ‘primary’, ‘second’ or ‘tertiary elements’ etc. (e.g. a ‘second element’) does not, where only one is mentioned, require the presence of a ‘first’ such element, unless the context requires otherwise. So, for example a ‘second element’ ‘secondary element’ does not require the presence of a ‘first element’ or ‘primary element’, although such a ‘first element’ or ‘primary element’ may be present. Further it will be understood that ‘first’ and ‘second’ or ‘primary’ and ‘secondary’ elements, where mentioned, may refer to alternatives, typically of an equivalent or similar nature. For example, in this document first and second cell stacks refer to similar or identical elements, here, typically in the form of equivalent or similar cell stacks, used in different locations within the topology of a battery system. In this document, module balancing means balancing between modules, in other words, balancing (e.g. of state of charge (SOC) and / or open circuit voltage (OCV) of respective battery modules) between battery modules e.g. within a single string of battery modules. In this document, string balancing means balancing between strings, in other words, balancing, (e.g. of state of charge and / or open current voltage) between strings e.g. within an array of strings. In this document, pre-charging means charging a string from a depleted state (e.g. a new or newly serviced string). Potentially, this may be a new site where all strings are discharged e.g. prior to commissioning. In this document, service discharging means discharging so that some service operations e.g. which require the stack of cells in a particular battery module to be fully discharged, can be carried out. The term contactor is used for switching components designed to open and close with high current flow e.g. higher current flow, than say a relay or switch. In other words, a contactor is a switching component that can be used for applications at string level, where higher make or break currents are required. The term relay or switch will be used for switching components that open and close at lower currents, such as in connection with cell stacks and / or battery modules. The terms relay and switch are used interchangeably herein. A cell sub-stack may comprise a group of cells connected in series, which share electrolyte fluid inlets and electrolyte fluid outlets (four in total: positive in and out, and negative in and out). A cell sub-stack, e.g. an individual, cell sub-stack may be physically separate from a neighbouring cell sub-stack. A cell sub-stack may comprise a group of bipolar cells with terminal current collectors on each of the positive and negative ends, or even the same ends, typically also including other elements such as terminal end plates to form a housing (e.g. optionally with tierods between such end plates). Each cell sub-stacks may comprise series connected cells within a pair of end plates. In one or more embodiments, multiple cell sub-stacks with terminal connections on each, may be connected in series between one pair of ‘common’ end plates e.g. to form one unitized cell stack, each sub-stack having its own respective positive and negative inlet(s) to and from one or more tanks e.g. a tank pair (typically to the same common tank pair). Figure 1 illustrates a redox flow battery system comprising a number of arrays 20. Arrays 20 may be located on a particular site or may be distributed across several sites in one or more groups of arrays 20. Each array 20 is connectable to the grid. Here, N arrays 20 are shown labelled ARi where i = 1-N. An i-th array ARi here comprises a number of strings 30 of battery modules. Each string 30 is labelled Sij, where i refers to the i-th array and j refers to the j-th string, where j = 1-M. Each string comprises one or more, preferably two or more battery modules 40 connected in series typically together, with an associated power conversion system 90 (PCS e.g. comprising an AC / DC converter and an array level AC / AC transformer) for connecting the 1 to j-th string into the array (and so to the grid). A string 30 of battery modules 40 can be seen in more detail in Figure 2. Here, four battery modules 40, labelled Bijk, are shown. Typically, k battery modules 40, where k=l to n, preferably 2 to n, are provided. Thus, in several preferred embodiments at least two battery modules 40 in series are provided in at least one string 30. Referring now to Figure 3, an individual battery module 40 here the k-th battery module of the j-th string of the i-th array is shown. Within one or more, and preferably two or more, and typically each battery module, one or more and preferably two or more, cell stacks 50 are provided. Indeed, one or more each cell stack 50 may be removable, in other words configured so as to be (relatively easily) removed and / or inserted into a suitable receiving location e.g. a receiving slot within the battery module. Here, battery module 40, Bijk, has the capacity for three cell stacks 50, one of which, stack Tl, is in place. Stacks T2 and T3 are indicated in dotted lines in Figure 3 to indicate the receiving slots where these may be located and that one or both of second and third stacks T2 and T3 may be omitted on initial commissioning or during repair. Stack Tl may likewise be configured so as to be (e.g. easily) removable, following initial installation and commissioning. When inserted into battery module 40, cell stack(s) T1 (T2 and T3, where provided) are connected into and form part of the battery module 40. As will be described in more detail later, in some embodiments, cell stacks Tl, T2 and T3 are configured so as to be removably located in (and appropriately connected within) battery module 40. Each battery module 40 typically comprises, firstly, a tank pair (see 46 in Figure 6) comprising a pair of tanks for electrolyte (anolyte and catholyte), and, secondly, appropriate tank connections (see 47 in Figure 6) in the form of electrolyte hydraulic connections for connecting to and from a predetermined number of cell stacks 50 to the tank pair 46. Similarly, battery module 40 also comprises appropriate electrical connections (see 48 in Figure 6) for connecting up to a predetermined number of cell stacks 50 (Tl, T2, and T3) in parallel. As shown in Figure 3, each cell stack 50, and, in particular, where two or more cell stacks 50 are provided (e.g. three cell stacks Tl, T2 and T3), is connected in parallel with one or more or all neighbouring cell stacks 50 (e.g. between the positive and negative sides of the battery module 42 and 44). Typically, there are p cell stacks where p = 1 to P. In preferred embodiments p=2 or more, or 1 to 12, or 2 to 12, or 2 to 10, or 2 to 8, or 2 to 6, or 2 to 4, or 2 to 3, or 2, or 3, or 4 cell stacks 50, these being provided electrically in parallel providing configurable (e.g. varying) power rating(s) for the battery module 40 depending upon how many cell stacks 50 are installed. In any case, in some embodiments, the battery module 40 is further configurable to hold up to P cell stacks, in some embodiments, preferably each in an individually removable (e.g. replaceable) manner, for removal and / or replacement and / or repair in the field. This configuration within the battery module 40 in some sense, resembles the provision of expansion slots in IT equipment, whereby up to P expansion slots are provided for receiving up to P cell stacks 50. Thus, in one or more embodiments a battery module may be provided with up to P cell stacks with all configurable slots filled, each filled with a cell stack 50, or with a reduced number of cell stacks 50, that may be increased later when upgrading a system. Referring to Figure 3, as an example, the system may be provided with up to three cell stacks 50 (Tl, T2, T3), one in each of the, here three, configurable receiving slots. Within each stack 50, up to Q cell sub-stacks 60 (Cq, where q = 1-Q) may be provided electrically connected in series. Here, four cell sub-stacks 60 (Cl, C2, C3, C4) are provided electrically connected in series together within a respective cell stack 50. Thus, when two or more configurable slots are filled with respective cell stacks 50, say two stacks, Tl and T2, or Tl and T3 or three stacks, say Tl, T2 and T3, between eight and twelve cell sub-stacks 60 can be provided, cell sub-stacks 60, are connected, firstly, in series within a stack 50 and, secondly, in parallel between respective cell stacks 50. It is preferred that each battery module is provided with its own tank pair and cell sub-stacks 60 within each cell stack of a common (shared) battery module 40 are hydraulically connected in parallel to the tank pair. As cell sub-stacks 60 are also connected in series electrically (within their respective cell stack 50), an electrical connection path is formed through the electrolyte going into and out of each sub-stack in the stack, and this is managed via a shunt manifold, preferably via a shunt manifold unit, as will be described later. Referring now to Figure 4, a redox flow battery system 10 is shown comprising an array 20. Array 20 comprises a series of M strings, electrically connected in parallel. For simplicity, Figure 4 will be described below with reference to two strings 30. In some embodiments however, one, two, or up to twelve or more strings 30 may be provided in an array 20 connected in parallel. The detail of stacks 50 (e.g. Tl, T2, T3) and sub-stacks 60 (e.g. Cl, C2, C3, C4) within each battery module is shown in one battery module 40 only, for clarity. Nevertheless, it will be understood that one or more or all battery modules 40 within a string 20 may resemble those of Figure 3. It will be appreciated that the provision of configurable arrangements of cell stacks 50 in parallel within each battery module 40, means that, within a given string, and indeed within an array 20, a variety of configurable power levels can be achieved. Further, this provides some resilience and / or redundancy for delivery of energy from array 20, should a particular sub-stack 60 within a stack 50, or a stack 50 itself, enter a fault condition, because the nonfaulted stack 50 and / or battery module and / or string 30 will charge or discharge rated power, or just nominally below rated power by array 20 at least initially, enhancing resilience. A string level DC-DC balancer 70 (also referred to as merely a DC balancer for brevity) for a string 30 of battery modules 40 is shown. The DC-DC balancer 70 for the string 30 will be described in more detail below. Typically, this operates actively on a string level, shuttling charge (e.g. facilitating current flow) between battery modules within a string. It will, of course, be understood that where all cell sub-stacks 60 within each stack 50 within a single battery module 40 share a tank pair, these will be at the same state of charge (SOC). Referring now to Figure 5, a redox battery system 10 comprising a string 30 of four battery modules 40 (Bl, B2, B3, B4) is shown. Battery modules B1-B4 are connected in series between HVDC bus via the string interface and control module 80, and, in particular, via contactors KI and K2. Here, each battery module 40 is located within a standardised shipping container (preferably 20ft, 6. Im in length) which is not shown. Various interconnections between the battery modules Bl, B2, B3 and B4 are shown and a string interface and control module 80. Here, one string interface and control module 80 (also referred to as a string control unit (SCU)) is provided for the string of four battery modules B1-B4 in series. String interface and control module 80 also comprises two string contactors KI and K2 which connect the string to a HVDC bus and into the rest of the array (not shown). String control unit 80 comprises a string controller 82, which manages contactors KI and K2, a door interlock, DC balancer 70, and AC auxiliary power distribution via an AC distribution module 92. DC balancer 70 may comprise its own control module e.g. implemented in the software, (not shown). Strings 30 within an array are connected through two contactors KI and K2 of each string to further strings in the array 20. This allows each string 30 to be connected to and disconnected from the array 20 to support and improve (optionally optimize) operation including balancing between strings 20, and for isolating strings for service. The contactors KI and / or K2 may be controlled by a string controller of the string itself or in response to a higher-level controller. Battery modules B1-B4 are also each connected to DC balancer 70. DC balancer 70 shuttles charge between the string as a whole, and a selected battery module. There are several ways to do this. Example methods are described in WO2021190928, WO021191065, US20130127396, CN2020435082, and CN202333899. US20130127396 describes an energy store comprising a multiplicity of cell stacks, each cell stack being equipped with a DC-DC converter (see para 9), and “systems with different energy stores and different states of charge for a plurality of electrolyte fluid circuits are also possible because the DC-DC converters de-couple the individual energy stores” (see para 10). Indeed, one example of a DC-DC converter that may be used is shown in Figure 8. Twelve battery modules 40 (B1-B12) are shown, each battery module provided with a series of switches connectable to a DC-DC controller 71 together providing a DC-DC balancer 70. Here the DC-DC convertor may time share battery module balancing by using an array of switches 72. By controlling the operation of various switches within switch array 72 connected to the positive and negative sides of the battery modules B1-B12, charge can be balanced between battery modules B1-B12. This is similar to the arrangements shown at CN202333899 and CN202435082. As shown in Figure 8, the series of switches 72 can be operated to connect one battery module 40 to another within string 30 (and / or to the remainder of the string) to transfer current and achieve balanced SOC..A balancing device (e.g. DC balancer 70) is also operated at the same time to provide the driving voltage for the desired current flow between modules. The DC balancer 70 moves current from modules that are more charged, e.g. than others or than an average bus voltage, and puts it into battery modules that are less charged. Alternative methods of balancing between battery modules are known (such as controlling the number of connected cells in a stack, controlling the flow of electrolyte, connecting to a load, etc.) and may be used as alternatives, or indeed in addition to using the DC balancer 70. Indeed, in some embodiments the DC balancer module 70 may be provided as a separate module which uses one or more of these alternative methods. Alternatively, or in addition, battery modules 40 within a string 30 may be operated in a manner that also supports balancing within the string (intrastring) or between strings 30 within an array 20 (inter-string). This includes adjusting electrolyte flow within battery modules (see WO2021025925), or parameters that can influence string balancing methods. Such adjustments may be done by a string controller, such as string interface and control module 80 or at any control level above the level of the string 30. A further alternative to assist in balancing battery modules within a string is by controlling the number of cells within the string (see WO2020030762). Indeed, WO2021025925 also describes current balance devices (CBD) as being used to balance the state of charge between flow battery units within the flow battery unit strings by appropriately shunting current through the current bypass devices. (CBD) Referring now to Figure 6, a battery module 40 comprises 1 to 3 cell stack(s) 50, each comprising 1 to q, in one example, four cell sub-stacks 60. Battery module 40 is shown here with a first power block unit 100A comprising a respective cell stack T1 (e.g. of q sub-stacks 60 (Cl, C2... .Cq) along with a shunt manifold unit 106 (in its broadest sense, pipe work preferably, configured as an integrated unit for each of the Cq sub-stacks) for connecting to a tank pair 46 via battery module tank connections 47A. Similarly, power block unit 100A comprises electrical connections 108 for q sub-stack 60 (Cl, C2.. .Cq) for battery module electrical connections 48. Power block unit 100A comprising cell stack Tl, shunt manifold unit 106 and, power block electrical connections 108, is typically housed within an integrated (e.g. rigid) frame, chassis or super structure or housing (not shown) e.g. in a manner so the integrated chassis or frame, and so the power block unit itself, may be (relatively) easily manipulated in the field to be inserted into battery module 40, or indeed removed from battery module 40. This is facilitated, if particular configurations and components and, in particular, tank connections 47 (47A, 47B, 47C) and electrical connections 48 (48A, 48B and 48C) e.g. quick fit and / or easy connections, are provided within the battery module 40. In several preferred embodiments, battery module 40 comprises a container, preferably a standard sized (e.g. ISO) shipping container, within which a tank pair 46, tank connections 47, and electrical connections 48 for up to P power block units 100 (100A, 100B, 100C), here three power block units (see Figure 6), are provided. Thus, the standardised shipping container is preconfigured during manufacture to receive one or more, more preferably two or more, more preferably three or more, power block units 100. Further, power block units 100 are configured during manufacture to be receivable within a housing e.g., in the form of a standard sized shipping container, of battery module 40. This is shown schematically in Figure 6 in which shunt manifold unit 106 and connections 108 of power block unit 100 (10A, 100B, 100C) are preconfigured within power block units 100 (10A, 100B, 100C) for connection to pre-configured tank 47 and electrical 48 connections within battery module 40 and in particular, within the housing for battery module 40. These pre-configured locations within the battery module housing having pre-configured tank and electrical connections 47, and 48, (e.g. quick-fit or easy connect) connectors may be thought of as ‘expansion slots’ for receiving additional power block units 100 where only one is provided, or in which power block units may be easily replaceable, e.g. in the field. One or more or all power blocks are removable from a preprepared location for a power block unit, typically also referred to as an expansion slot, within a battery module. Referring now to Figure 7, a redox flow battery system 10 comprising 12 strings 30 and an AC-DC power conversion system (PCS) connecting the string to the grid or further arrays is shown. Here, each string 30 (S1-S12) comprises four configurable battery modules each of which may comprise up to P Power block units 100 (100A, 100B, 100C) to provide configurable power levels e.g. 25kW, 50kW, lOOkW. In some embodiments, power block units 100 provide 10-200kW, or 10-150kW, orl0-100 kW, 20-80kW, or 25kW, or 50kW, or 75kW, or 80kW, or lOOkW or 150kW, or 200kW each. Whilst the amount of power provided by each power block unit 100 is decided during manufacturing, the provision of configurable slots (also referred to as expansion slots each for receiving a respective slot - in power block unit) within e.g. a battery module 40 facilitates the addition, and / or replacement, of power block unit(s) in the field should additional power be required for example. Further, should a power block unit 100 degrade (e.g. if one or more cell stacks degrade or cease to function), a replacement power block unit can be provided relatively easily. Indeed, where parallel connected power block units are provided, the battery module can operate at reduced power, if a power block unit ceases to function until it can be serviced. This arrangement particularly increases resilience which is of significant use for grid connector systems. Together the twelve strings S1-S12 provide up to 3.6 (nominally 3.5MW) if each battery module within each string is provided with three power block units of 25kW. Referring now to Figure 9, a redox flow battery system 10 comprising an array 20 of n strings 30 each string comprising four battery modules is shown. Each string 30 is provided with a string control unit (SCU) 80 (80-1, 80-2 etc.). There is provided a DC balancer controller 70, a string controller 82 and a high voltage DC bus controller, as well as AC auxiliary power distribution controller 92. In addition, in some embodiments, an array controller module 120 may be provided with an individual string control unit 80. Referring to the figures, the shunt manifold unit 106 provides a parallel electrolyte feed to the four cell sub-stacks 60 arranged electrically in series. In some embodiments, each power block unit comprises four 50 cell sub-stacks and a shunt manifold assembly (also known as the shunt manifold unit 106). One to three power block units 100 may combine with a tank pair, pumps, pipes, and remaining flow battery equipment, to form a DC battery module 40. A power block unit has three interfaces, 1) positive and negative electrical connections, 2) electrolytes, positive in and out, negative in and out, and 3) electrical for charging and discharging and stack voltage measurements. The two or more, or preferably four or more battery modules grouped in strings (and their internal cell sub- stacks) govern the systems DC voltage output range. The shunt manifold unit 106 connects between the main electrolyte tank pair 46 and each cell sub-stack 60 acting as a resistive path to minimise self-discharge current that is shunted through the electrolyte circuit. While the cell sub-stacks 60 (Cl, C2, C3, C4) are electrically connected in series, their electrolyte connection creates a parallel electrical path through the electrolyte inlets and outlets... The shunt manifold unit is effectively a length of tube that increases the parallel resistance between cell sub-stacks. In other words, the shunt manifold units act as a resistive path to minimise self-discharge current that is shunted through the electrolyte circuit. In one or more embodiments, a DC-DC power converter allows power transfer between any battery modules within a string of (e.g. four) battery modules connected in series. It does so by transferring current from the string or battery module to / from a different battery module in the string. In one or more embodiments, the invention provides a configurable utility scale energy storage system (ESS) 10 composed of multiple (e.g. 300kWh) DC battery modules 40 with a rated power of 75kW when three power block units are installed (or 25kW when one power block unit is installed, or 50kW when two power block units are installed). A single DC battery module has a typical voltage range of 200VDC to 350VDC. Generally, the smallest configurable system will have four DC battery modules 40 in a string 30 in series to provide a DC output voltage of 800VDC to 1300VDC making the system compatible with commercially available power conversion systems. A string 30 of four DC battery modules 40 has a capacity of 1.2MWh. The ESS can be scaled larger by adding more strings 30 in parallel creating an array, e.g. 12 strings in parallel, may have an energy capacity of 14MWh with a nominal power rating of 3.5MW. In one or more embodiments, strings 30 of DC battery modules may be connected in parallel to form a single DC combined battery bank. Banks are combined and then interconnected with a power conversion system (PCS) to create an array 20. While operating, it is expected that strings 30 and battery modules 40 within strings 30 will become imbalanced. Imbalanced refers to the SOC of modules or strings, and this can impact the overall energy storage capacity of an array 20. There are several sources of imbalance, many coming from tolerances that occur in manufacturing of a flow battery, and / or its components, in addition to how it is operated and / or maintained. For example, identical cell stacks will experience different usage which can result in imbalances. There are many known options for balancing, commonly sorted into two classes, passive and active. Passive typically refers to a solution that uses some sort of resistive load or shunt to discharge a battery when its SOC is higher than those in a string or array. Active solutions use some sort of active device to shuttle energy between battery modules in a string or array. Thus, passive balancing may result in losses. In one or more embodiments, each string will preferably have a single DC balancer 70. An array level balancer (inter-string) may be provided by an electrical arrangement in which strings are connected to a common bus rather than via an active device. String level balancing (intra-string) may occur as described herein and / or in various examples of the art, several examples of which are also referred to herein. In practice, typically, the DC power interface for battery modules 40 is floating and isolated from the battery module chassis, earth and the battery module’s container housing. The battery module DC power interface is ungrounded so that battery modules 40 can be connected in series to form an ungrounded string. Indeed, in some embodiments the module may not have a contactor, meaning the stack voltage is always presented at the battery module’s DC terminals. In an active mode, the battery module is fully operable and provides DC power availability for charging and discharge. In this mode, all auxiliary systems are fully engaged with their module maintaining terminal voltage through active pumping management. This is the main operating mode with a battery module where charging and discharging takes place. The battery module actively communicates operating limits and status information to allow higher level e.g. string, or array, controllers to manage overall system operations. In one or more embodiments, cell stacks 50 are made from connecting four sub-stacks 60 electrically in series, A battery module 40 may be made of either one, two or three such stacks 50 connecting electrically in parallel. The sub-stacks 60 are mounted individually to a support structure (e.g. a chassis of a power block unit) with series interconnecting wiring physically constrained and protected from any bonded metal. All components of the sub-stack 60 remain fully isolated from the chassis of the power block unit. Sub-stack fluid interfaces are made through the pipework of a shunt manifold unit to the bulk electrolyte feed and return lines to the tank pair 46. The voltage of battery modules 40, isolated from the chassis / earth, may be elevated to several hundred volts above the string negative (zero volts reference) due to the series architecture. In some embodiments, the cell stacks 50 within the battery module 40 are combined in parallel to deliver the module rated power, or indeed a fraction of this, as required by the particular desired application. The power delivery of the cell stacks 50 may be managed in a passive sense, in that there may be no active power converters or devices that control limits or otherwise actively manage an individual cell stack’s power contribution. Thus, in one or more embodiments there may be provided a system and method of achieving a predetermined voltage level and / or of achieving a predetermined power level. The voltage level achieved per battery module may be greater and equal than 200 volts, 200-500 volts, or 200-400 volts, or 200-350 volts, or 200-320 volts. In a particularly preferred embodiment, 48 modules may be provided in 48 standard sized container housings divided into 12 strings each of 4 battery modules at one of three power levels. Where one power block unit per battery module is provided, this gives a 48 x 25kW = 1.2MWDC level. Where two power block units per battery module, this provides 48 x 50kW = 2.4MWDC. Where three power block units are provided per battery module this provides 48 x 75kW = 3.6MWDC level. It will be understood that where one or more power block unit(s) are provided per battery module, should a fault arise in a cell stack within one power block unit taking this offline, the overall power level may fall but the voltage will remain constant. This will improve reliability of the system from a grid perspective facilitating time for planning of fault correction. In one or more embodiments battery modules are of common design and manufacture, in other words of identical design and manufacturers insofar as is possible within manufacturing tolerances. Similarly, cell stacks, and preferably also cell sub-stacks, are of common design, in other words identical to each other insofar as is possible within manufacturing tolerances. This facilitates redundancy and so reliability. This also facilitates efficient manufacturing. In one or more embodiments, within one or more or each battery module, fluid and / or electrical connections exist for up to a predetermined number of power block units, typically three to twenty, preferably 3 to 10. If only one or two power block unit slots or locations for power block units are used, then any remaining fluid connections are capped and / or the remaining electrical connections are isolated in any unused locations / slots. These arrangements facilitate augmentation and / or replacement of power block units in the field. Thus, in one or more embodiments the invention provides configurable redox flow battery system(s) comprise one or more configurable redox flow battery module(s). In one or more embodiments, the invention provides a configurable redox flow battery system for delivering one or more predetermined voltages at a plurality of configurable power levels. In one or more embodiments, imbalances between strings may be addressed by disconnecting a string and bringing this back online once the SOC is matched. In one or more embodiments DC balancers for battery modules supports balancing between battery modules and can be done regardless, in other words during normal operation. In one or more embodiments, DC balancing may be achieved by controlling the number of cells within a battery module, and / or controlling the charge and / or discharge of a battery module via the voltage as explained in the art and / or redistributing electrolyte between modules and / or regulating the flow rate of electrolyte between modules. Other methods may be known from the art and could be used with suitable adaptation. The present inventors have recognized that implementing one or more various components of a redox flow battery system as commonly designed and / or removable (e.g. (easily) installable and / or replaceable) can make a useful contribution to ultimate overall cost of energy e.g. by reducing construction and / or ongoing maintenance costs and providing for augmentation and repair in the field. The present inventors have recognized that by a suitable design of the topology of components, and / or the component themselves, within a battery system to provide suitable (e.g. high enough to be grid level) voltage and power levels and / or configurable voltage(s), and / or power level(s), a more practical and / or reliable battery system can be provided, this facilitates connection into an array of battery systems and / or string balancing and / or the grid. Further, this can reduce ongoing maintenance costs and / or indeed risk of breakdown. Indeed, introducing simplification and / or configurability into the components of a battery system may have multiple benefits. Not only are the number of different components required reduced, but components that need to be maintained or serviced, or replaced, can be dealt with more easily. Further, initial assembly is simplified, and configurations can be varied in the field. Furthermore, resilience in the field is increased. In one or more embodiments, there is provided redox flow battery system which reduces, and may avoid, need for DC / DC converters to achieve voltages of-1500 VDC as required for low cost, high efficiency commercially available PCSs for utility scale energy storage. In one or more embodiments, there is provided redox flow battery system which can assist in reducing overall shunt losses by breaking an e.g. 1500 VDC string voltage across “n” battery modules (n=4), allowing the -375 VDC of each battery module to be managed using shunt manifold technology. In one or more embodiments, there is provided redox flow battery system may assist in reducing the maximum potential fault current caused by a potential electrolyte leak to ground in a module due to the limited maximum nominal voltage of e.g. -375 VDC per module. In one or more further aspects the invention relates to methods of operating the above. In some embodiments there is provided a method of operating a redox flow battery system as described herein. In some embodiments there is provided a redox flow battery system comprising: a string (30), the string comprising: at least one or two battery modules (40), electrically connected in series; one or more or each battery module(s) (40) comprising: i) a first cell stack (50) comprising one or two or more cell sub-stacks (60) electrically connected in series; ii) a second cell stack (50) comprising one or two or more cell sub-stacks (60), electrically connected in series; iii) at least the first and second cell stacks (50) electrically connected in parallel within the respective battery module (40); and, iv) a tank pair (46); v) the cell sub-stacks (60) of at least the first and second 5 cell stacks (50) hydraulically connected in parallel to the tank pair (46). The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the 10 scope of the invention. LIST OF REFERENCE NUMERALS 10 - Battery system 20 - Array of strings (ARi, i-th array, i = 1 to N) 30 - string of battery modules (Sij, j-th battery module (in array i), j = 1 to M 40 - battery module (Bijk, k-th battery module (in string j of array i), k = 1 to n) 42 - positive side of battery module 44 - negative side of battery module 46 - tank pair, (e.g. pair of tanks for electrolyte (anolyte and catholyte)) 47, 47A, 47B, 47C - tank connections for power block units 48, 48A, 48B, 48C - electrical connections for power block units 50 - stack of cells (Tp, p-th stack, p = 1 to P) 60 - sub-stack(s) (of cells) (Cq, qth sub-stack, q = 1 to Q) comprising set of cells e.g. 10-200 cells, or 50-100 cells, or 50 cells 70 - DC-DC Balancer (for string of battery modules) 72 - Relay Array for battery modules Tl, T2, T3 - cell stacks 1 to 3, electrically in parallel Cl, C2, C3, C4 - cell sub-stacks 1 to 4, electrically in series 80, 80-1 - String Control Unit (SCU), 80-2 modified SCU hosting String Controller Module 82 - String Controller 90 - PCS - Power Conversion System for connecting string or array to grid (e.g. comprises an AC / DC converter and a transformer) 92 - AC auxiliary power distribution unit 100, 100A, 100B, 100C - Power Block Unit, each comprising a respective stack (Tl, T2, T3) and pipework and electrical connections for a respective stack (Tl. T2, T3) 106 - shunt manifold unit 120 - Array controller

Claims

1. A redox flow battery system comprising:a string (30), the string comprising:at least two battery modules (40), electrically connected in series;one or more or each battery module(s) (40) comprising:i) a first cell stack (50) comprising at least one cell sub-stack(s) (60);ii) a second cell stack (50) comprising at least one cell sub-stack(s) (60);iii) at least the first and second cell stacks (50) electrically connected inparallel within the respective battery module (40); and,iv) a tank pair (46);v) the cell sub-stacks (60) of at least the first and second cell stacks (50) hydraulically connected in parallel with the tank pair (46).

2. A redox flow battery system comprising:- an array (20) comprising at least two strings (30), each string (30) comprising at least one battery module(s) (40), the at least two strings electrically connected in parallel;- one or more or each battery module(s) in each string comprising:i) a first cell stack (50) comprising at least one cell sub-stack(s) (60);ii) a second cell stack (50) comprising at least one cell sub-stack(s) (60);iii) at least the first and second cell stacks (50) electrically connected inparallel within the respective battery module (40); and,iv) a tank pair (46);v) the cell sub-stacks (60) of at least the first and second cell stacks (50) hydraulically connected in parallel with the tank pair (46).

3. A redox flow battery system comprising- one battery module (40), the battery module comprising:i) a first cell stack (50) comprising at least one cell sub-stack(s) (60);ii) a second cell stack (50) comprising at least one cell sub-stack(s) (60), electrically connected in series;iii) at least the first and second cell stacks (50) electrically connected in parallel within the respective battery module (40); and,iv) a tank pair (46);v) the cell sub-stacks (60) of at least the first and second cell stacks (50) hydraulically connected in parallel with the tank pair (46).a housing unit comprising a standard-sized shipping container wherein the one battery module is contained within the housing unit.

4. A redox flow battery system according to any of claims 1 to 3 in which:i) the first cell stack (50) comprises two or more cell sub-stacks (60)electrically connected in series; and / or,ii) the second cell stack (50) comprises two or more cell sub-stacks (60), electrically connected in series.

5. A redox flow battery system comprising:- at least one battery module (40);- one or more or each battery module(s) (40) comprising:i) a first cell stack (50) comprising at least one cell sub-stack(s) (60);ii) a second cell stack (50) comprising at least one cell sub-stack(s) (60);iii) at least the first and second cell stacks (50) electrically connected inparallel within the respective battery module (40); and,iv) a tank pair (46);v) the cell sub-stacks (60) of at least the first and second cell stacks (50) hydraulically connected in parallel with the tank pair (46);further wherein, at least one cell stack (50) within at least one battery module (40) is configured to be removable from the battery module (40).

6. A redox flow battery system according to any of claims 1 to 5 in which:i) the first cell stack (50) comprises two or more cell sub-stacks (60) electrically connected in series; and / or,ii) the second cell stack (50) comprises two or more cell sub-stacks (60), electrically connected in series.

7. A redox flow battery system according to any of claims 1 to 4 in which at least one cell stack (50) within at least one battery module (40) is configured to be removable from the battery module (40).

8. A redox flow battery system according to claim 5 to 7 in which at least one battery module (40) comprises a removable power block unit (100), the removable power block unit (100) comprising:- the at least one removable cell stack (50);- a shunt manifold unit (106) for electrolyte, for managing electrolyte flow between two or more cell sub-stacks (60) of the at least one removable cell stack (50).

9. A redox flow battery system according to claim 8 in which the shunt manifold unit (106) comprises long respective flow path(s) for flow of anolyte and catholyte to and from each cell sub-stack(s) (60).

10. A redox flow battery system according to claim 8 or 9 in which the removable power block unit (100) is mounted on rails within a battery module (40) so as to be slidable in and / or out as a single unit.

11. A redox flow battery system according to any of claims 8 to 10 in which the removable power block unit (100) comprises a frame, or chassis, which is configured to be removable from, and / or insertable into, the battery module (40).

12. A redox flow battery system according to any of claims 8 to 11 comprising:1 to 10, or 2 to 8, or 2 to 6, or 2 to 4, or 2 to 3, or 2, or 3 power block units (100).

13. A redox flow battery system according to any of claims 8 to 12 comprising:1 to 10, or 2 to 8, or 2 to 6, or 2 to 4, or 2 to 3, or 2, or 3 slots or locations, one or more or each for receiving a respective removable power block unit (100).

14. A redox flow battery system according to claim 13 in which one or more or each slot or location comprises:- electrical connection(s) for a power block unit (100) and associated cell stack(s) (50) and sub-stack(s) (60) therein;- electrolyte pipework connection(s) for a power block unit (100) and associated cell stack(s) (50) cell sub-stack(s) (60) therein for connecting same to and from the tank pair (46).

15. A redox flow battery system according to any of claims 1 to 14 comprising - at least two battery module(s) (40) electrically connected in series;- an active DC-DC balancer (70) for balancing the stage of charge (SOC) of at least two battery module(s).

16. A redox flow battery system according to claim 15 in which the DC-DC balancer (70) is configured to shuttle energy and / or charge from one or more battery module(s) (40) to one or more other battery module(s) (40).

17. A redox flow battery system according to claim 15 or 16 in which the DC-DC balancer (70) comprises:- a bi-directional DC-DC convertor;- a relay array.

18. A redox flow battery system according to any of claims 1 to 17 in which - one or more battery module(s) (40) are configured to adjust rate of flow of electrolyte(s) to adjust the rate of flow of charge within the respective battery module(s) and so adjust its state of charge (SOC) and / or;- two or more battery module(s) (40) are configured to transfer electrolyte from one battery module(s) to another.

19. A redox flow battery system according to any preceding claim in which two or more or all cell sub-stack(s) (60) are of identical configuration and / or construction.

20. A redox flow battery system according to any preceding claim comprising: - four battery modules (40) electrically in series;- each battery module (40) comprising one, two or three cell stacks (50) electrically in parallel;- each cell stack (50) comprising four cell sub-stacks (60) electrically connected in series and hydraulically connected in parallel to the tank pair of the respective battery module;each cell sub-stack (60) comprising a plurality of cells in series.

21. A redox flow battery system according to any preceding claim comprising: - four battery modules (40) in series;each battery module (40) comprising:- one or two or three or more receiving slots for receiving respective removable power block units (100);- at least one removable power block unit (100), the removable power block unit comprising:- at least one cell stack (40), the at least one cell stack (40) comprising one or two or three or four or more cell sub-stack(s) (40) in series; and- a shunt manifold unit (106) for managing electrolyte flow.

22. A redox flow battery system according to any preceding claim comprising electrolyte (e.g. anolyte and catholyte).s

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