Energy storage systems for applications such as full electric vehicles, hybrid electric vehicles, and stationary energy storage in grid connected or off grid applications, frequently include an arrangement of multiple energy storage cell units. Each cell unit is limited by its functional mechanism and design to provide an output voltage within a certain range depending on its state of charge and operating conditions. Each cell unit is also limited by its functional mechanism and design to provide a certain maximum charge storage capability, depending on the operating conditions. Electrically connecting cell units in series increases the maximum achievable output voltage, therefore decreasing the magnitude of current required to supply a given power output. This increases the system efficiency as ohmic losses increase with current magnitude. Electrically connecting cell units in parallel increases the maximum achievable storage capacity for a given cell unit capacity and storage system output voltage level.
The individual cell units inevitably display some differences in terms of charge storage capacity, internal resistance, and other performance related factors. Even before entering their operating life, cell units inevitably have differences caused by manufacturing tolerances that allow for certain variations in cell units during manufacturing with even the most advanced state of the art manufacturing processes. Throughout the operating life, variations in cell unit performance degradation conditions or profiles further contribute to these differences. In applications in which used cell units are recycled for re-use, the cell units can be associated with notable performance differences, particularly if the cell units have been exposed to different usage profiles. Utilising cell units with different specifications can also contribute to cell unit differences.
In energy storage systems that include multiple energy storage units, such differences between cell units can impact how the overall energy storage system is managed and performs. In cell units that are electrically connected in parallel, lower performing cell units contribute or accept a lower current during a discharge or charge process, respectively. This leads to higher performing cell units contributing to or accept a higher current during a discharge or charge process, respectively. Such rate increases can decrease the system efficiency, increase cell unit degradation, and potentially present safety risks. It is therefore often necessary to constrain the entire system to a lower power input or output level. In cell units that are electrically connected in series, lower charge capacity cell units can contribute or accept less electric charge during a discharge or charge process, respectively. Due to the series arrangement, higher charge capacity cell units are limited to contribute only an equal amount of charge as the lowest charge capacity cell unit. This means that the cell unit with the lowest charge capacity limits the charge storage capacity of the full energy storage system.
Conventional battery management systems typically use switched resistors to dissipate surplus energy from higher charged cell units, or switched capacitors or switched inductors to transfer energy from higher charged cell units to lower charged cell units. The primary role of these systems is to equalise the state of charge differences of cell units connected in series at a particular point in the charge discharge cycles, for example at the end of charging. Equalising the state of charge at one specific point in the cycle ensures that the lowest capacity cell unit in a series arrangement is able to be fully used. It does not, however, allow higher capacity cell units to contribute more energy to the output.
For example, assume two fully charged battery cell units connected in a series arrangement have capacities for 1 Ah and 10 Ah, respectively. If this system discharges at a rate of 1 A then, assuming no equalisation during the discharge, the entire system has a discharge time of one hour during which it will provide 2 Ah consisting of 1 Ah from the lower capacity cell unit and 1 Ah from the higher capacity cell unit.
In order to overcome the limitations posed by the lowest capacity cell unit in an energy storage system comprising multiple cell units connected in series, a more advanced approach is required. Switched capacitor or switched inductor balancing systems can be operated to transfer energy on a continuous basis, for example transferring energy from higher charge capacity cell units to lower charge capacity cell units throughout part or all of the discharge process. However, the electrical pathways and components used to equalise the cell units are typically rated to energy throughputs that are only a fraction of the rating of the full energy storage system. As such, the systems typically can only account for a fraction of the difference between the cell units.
For example, assume two fully charged battery cell units connected in a series arrangement have capacities for 1 Ah and 10 Ah, respectively. If this system discharges at a rate of 1 A and additionally transfer energy from the lower charged cell unit to the higher charged cell unit at a rate of 0.1 A, then after one hour, the system has provided a capacity of 2 Ah. At this point, due to the energy transfer, the lower charge capacity cell unit still holds 0.1 Ah and the higher charge capacity cell unit still holds 8.9 Ah, allowing discharging to be continued for approximately 0.1 hours longer and resulting in a full energy storage system capacity that is around 0.2 Ah larger than without any equalisation system. The additional discharge time and energy that can be maintained from higher charge capacity cell units increases with the energy rating of the equalisation system, which can increase the cost and space requirements among other factors. This leads such battery management approaches to predominantly be useful for energy storage systems with relatively small differences only, such as energy storage systems based on not previously used cell units with the same specifications. Furthermore, using switched capacitors or switched inductors requires energy to be transferred via intermediary storage devices such as capacitors or inductors, respectively, which can be associated with losses that negatively impact the full energy storage system efficiency.
A further method to address the limitations posed by differences between cell units that are connected in series is to use voltage converters. Typically, each cell unit is connected to one voltage converter, and the voltage converters are connected in parallel leading to a coupling on the direct current side. This can then be either directly or via a further voltage converter connected to an inverter. Another option is to connect each cell unit to one voltage converter, and connect each voltage converter to an inverter and connect the inverters in parallel so that the energy from the cell units is connected on the alternating current side. A further option is to use voltage converters with the output connected in series. Disadvantages of using voltage converters include the considerable component cost of converters, some prospective limitations in controllability of cell charging and discharging depending on controller type and layout, and the limited efficiency of voltage converters, partly due to energy losses in storage elements used for voltage conversion such as inductors and/or capacitors.
Switches can also be used to connect or bypass the cell units. By bypassing lower-performing cell units, additional charge and discharge capacity can be unlocked from the other cell units. Some disadvantages of current systems using this approach are that for each cell unit connected in series, an additional switch is placed in any given current path contributing an associated on resistance and energy loss.
It is an aim of the invention to provide a battery system which overcomes or ameliorates one or more of the disadvantages or problems described above, or which at least provides the consumer with a useful choice.
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