SECOND LIFE BATTERIES Fault tolerant reconfigurable battery system for stationary applications
Automotive batteries salvaged from scrapped vehicles can serve a useful second life within a stationary battery system, provided that the battery system has a suitable, reconfigurable topology. This article describes a paper presented at the PCIM Europe Digital Days 2020, which explains the challenges of using second life batteries, and proposes a solution.
As we move towards decarbonising the energy sector, the demand for stationary battery systems is increasing. At the same time, the growing Electric Vehicle (EV) market is predicted to generate an increasing supply of ‘used’ automotive batteries. Most of these can serve a useful second life within a stationary battery application, provided they are installed within a suitable Battery Electric Storage System (BESS).
However, conventional BESSs have a fixed topology where the weakest cell determines the overall performance. Therefore, if batteries from different cars, with different usage patterns and varying properties, are combined, the system’s lifetime, capacity and reliability are reduced.
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This challenge, and a proposed solution, are discussed in a paper titled ‘A fault tolerant reconfigurable Battery System for stationary applications utilizing 2nd Life Batteries’ written by Simon Bischof, Thomas Blank, and Marc Weber of the Karlsruhe Institute for Technology (KIT), Germany. The paper was presented at the PCIM Europe Digital Days 2020 Conference, 7-8 July 2020.
The proposed solution, called FlexBat, is a reconfigurable system that can actively control power flow, so improving utilisation of the available cells. This provides increased lifetime and capacity, and higher reliability.
To explain these advantages, the paper starts by reviewing existing battery systems and topologies, and then shows why one of these was selected as the basis for the improved, FlexBat design.
Comparison of battery system topologies
The most basic topology comprises multiple Battery Modules (BMs) stacked in series, with each BM comprising cells and electronics for cell supervision. These are connected to a DCDC boost converter, which provides a stable DC voltage of at least 625V, for connection to the 400V three-phase grid via an inverter. It is the simplest system, but has no reconfigurability.
Switched Matrix (SM) systems share this design, but with power switches that can bypass a module if a cell fails. More complex SM systems are available for high reliability, low capacity applications. Another approach uses a cascaded DCDC (CDCDC) design, which eliminates the central DCDC converter. The power in each battery module can be controlled by changing the voltages of each converter. As with the switch-matrix topology, faulty modules can be bypassed.
Modular Multilevel Converter (MMC) topology offers another alternative. It switches three strings of modules on and off to directly generate a three phase sine wave. This can be connected to the grid, using a relatively small inductor rather than a conventional inverter.
These topologies can be compared in terms of scalability, capacity utilisation and fault tolerance to identify which is best for use with second life automotive batteries. Table 1 shows the comparison.
A readily-scalable topology, using second life automotive battery cells, allows the implementation of capacities tailored to a given application. MMC scores poorly for scalability, because all modern EVs beside Tesla utilise high capacity pouch or prismatic cells. An MMC system using these would require 627 cells with a minimum capacity of 99.3kWh. All other topologies, by using a single string and a DCDC converter, can be implemented in smaller systems.
Capacity utilisation describes how much of the available capacity in the cells can be used for the application. Reconfigurable systems only have an advantage in this category if battery modules of varying quality (capacity, internal resistance) are used. This can be the case if modules from different cars with different usage profile or different cell generations are combined in one system. In a conventional system, the weakest cell determines the overall capacity. All described reconfigurable systems can discharge stronger modules faster than weaker modules and therefore extract more capacity. However, the MMC topology cannot transmit energy between different strings and the SM system can only fully disconnect modules. The CDCDC system can vary the power of all modules in small increments and therefore is superior in this category.
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Redundancy describes the ability of a battery system to continue operation even if one of the cells is faulty or has reached the end of its life. The conventional system provides no redundancy and has to be disconnected from the grid in case of a faulty cell. Both SM and CDCDC score high in this category. The MMC Converter does provides some fault tolerance, however if one string falls below the minimum required DC- link voltage, the system is no longer operational.
The MMC topology is viable for large battery systems >100 kWh. SM and CDCDC are also usable for smaller systems. Compared to the SM system the CDCDC offers a better capacity utilization. Therefore, the FlexBat system as proposed in the paper will be based on cascaded DCDC converters.
FlexBat battery system design
The proposed system combines decentralised control with a decentralised buck-boost converter, and is closer than other compared systems to a practical application. It provides an integrated fully-functional Battery Management System (BMS) for cell supervision. Modules are designed for at least 1.5kW each.
The FlexBat battery is constructed in a master-slave configuration where each master can be combined with up to 10 slaves in series. Each module consists of a battery-block containing up to 24 cells in series, a DCDC converter and a controller board for cell supervision and converter control. The controller board utilises an automotive rated AURIX Tricore 275 microcontroller with three cores.
The DCDC converter can work in boost, buck, bridge, and bypass mode. It is selected to boost or buck depending on whether the battery output voltage is lower than, or higher than the required DCDC output voltage. Bridge mode is very efficient, as no switching losses occur. The converter can also be switched to bypass the battery.
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Information about the cell voltage, current and temperature of each cell is transmitted to the master via CAN bus. Battery State of Charge (SoC) and State of Health (SoH) are derived from these values. Depending on these, the fraction of the output power for each module can be determined. Maximum charge and discharge power for the inverter is also automatically set.
The paper also discusses the system’s turn-off behaviour, which has been evaluated by simulation. The need for a soft turn-off rather than directly entering bypass mode in a fault condition, to avoid excessive currents in the converters, is highlighted.
A prototype system with three modules of 5.7kWh total capacity has been experimentally validated, with the exception of the emergency shutdown operation.
Conclusion and outlook
The paper claims that the presented FlexBat system offers a new approach for integrating second life automotive batteries into stationary systems. It increases capacity and reliability, therefore making second life systems an interesting alternative to conventional battery systems.
In the future the emergency shutdown procedure will be implemented to provide a fully functional reconfigurable battery system. Then the capacity will be increased to 10 kWh. This system will then be installed at Karlsruhe Institute for Technology north campus to perform peak shaving for a medium sized industrial plant.