This paper presents a method for achieving individual electrochemical cell balancing by using a cascaded full bridge multilevel converter where a single electrochemical cell is connected to each converter module. As a result, balancing at cell level is possible without additional circuitry, making this topology ideal for long service life grid storage and applications using second-life cells where the cells are inherently poorly matched. In order to estimate the relative state of charge between cells, the control flexibility of the multilevel converter is used to remove each cell from the current path without interrupting the operation of the system. This process eliminates the effect of the internal cell resistance and fast transient electrochemical phenomena and therefore the measured voltage serves as a high quality 'pseudo open circuit' voltage measurement. The proposed balancing strategy is validated using a 25 level cascaded full bridge multilevel converter prototype for the individual balancing of twelve lithium polymer cells, during consecutive charging and discharging cycles. Successful balancing to within 5 mV of open circuit voltage is observed between cells with 45% difference in nominal capacity and 55% initial state of charge variation.

This paper presents a method for evaluating gridconnected Battery Energy Storage System (BESS) designs. The steady-state power losses of the grid interface converter, the battery pack and the balancing circuit are calculated. The reliability of each complete system is calculated using a Markovbased modelling approach that takes into account the built-in redundancy of the system as well as performance degradation caused by faults. Finally, a simple economic analysis based on capital cost and efficiency is used to provide a basis for direct comparison between competing system designs. Three design options for a 1 MW, 1 MWh BESS connected at 11 kV are compared: a conventional BESS using parallel low-voltage power blocks, a BESS using a high-voltage intelligent battery pack and a BESS using a cascaded H-bridge converter. The results of the analysis indicate that additional power electronics included in the battery pack as part of the intelligent battery pack and H-bridge designs can enhance the reliability of the BESS by an order of magnitude under typical conditions, without increasing the overall cost of the system.

© 2015 The Authors.This paper presents an integrated modelling methodology which includes reduced-order models of a lithium ion battery and a power electronic converter, connected to a 35-bus distribution network model. The literature contains many examples of isolated modelling of individual energy storage mediums, power electronic interfaces and control algorithms for energy storage. However, when assessing the performance of a complete energy storage system, the interaction between components gives rise to a range of phenomena that are difficult to quantify if studied in isolation. This paper proposes an integrated electro-thermo-chemical modelling methodology that seeks to address this problem directly by integrating reduced-order models of battery cell chemistry, power electronic circuits and grid operation into a computationally efficient framework. The framework is capable of simulation speeds over 100 times faster than real-time and captures phenomena typically not observed in simpler battery and power converter models or non-integrated frameworks. All simulations are performed using real system load profiles recorded in the United Kingdom. To illustrate the advantages inherent in such a modelling approach, two specific interconnected effects are investigated: the effect of the choice of battery float state-of-charge on overall system efficiency and the rate of battery degradation (capacity/power fade). Higher state-of-charge operation offers improved efficiency due to lower polarisation losses of the battery and lower losses in the converter, however, an increase in the rate of battery degradation is observed due to the accelerated growth of the solid-electrolyte interphase layer. We demonstrate that grid control objectives can be met in several different ways, but that the choices made can result in a substantial improvement in system roundtrip efficiency, with up to a 43% reduction in losses, or reduction in battery degradation by a factor of two, depending on battery system use case

This paper presents a method for evaluating the performance of duty cycle balancing schemes and conventional energy redistribution active balancing schemes using a common linear programming framework. The framework is used to calculate the maximum usable capacity of a battery pack composed of cells of varying capacity coupled to a balancing circuit of a specified type and size. In the duty cycle balancing case the effect of cell redundancy for packs with DC and AC outputs is explored. In the energy redistribution case, the balancing performance of different circuit topologies with varying ratings is evaluated. An experimental validation of the numerical prediction for the duty cycle balancing method is conducted for a pack of twelve cells, where the duty cycle of each cell is controlled using a full-bridge circuit. In all cases the numerical and experimental results show an agreement in usable capacity to within 1%. The experimental results indicate that for a duty-cycle balanced pack constructed from cells with an extreme capacity range (where variation between cells is as large as the average cell capacity), the inclusion of two redundant cells in a 12 cell pack enables the extraction of 95% more capacity than in the case of no redundancy.