Model-Based Investigation of Dual Energy Storage Selection for Advanced Start-Stop Vehicles

Dual energy storage solutions have seen significant interest as a means to combine high energy content and high power capability in a single energy storage architecture.  Such architectures, which can consist of batteries, capacitors, fuel cells, and other devices, have been used automotive and other applications, allowing the requirements of long-term energy support, high charging power, and high discharge power to be supplied by different devices.  Examples include 12V li-ion/lead-acid architectures for advanced start-stop, fuel-cell/li-ion high-voltage hybrids, Li-ion/capacitor combinations, and others.

A challenge in combining multiple energy storage devices is ensuring that they are matched in terms of operating characteristics, such as voltage, resistance, and other characteristics.  While power electronics (such as DC/DC convertors) are often used, another approach is to simply passively couple the devices.  Doing so requires that the energy and power of the devices should be compatible in terms of capacity, voltage and resistance to achieve the optimal system performance.  Typically, that means ensuring a high degree of voltage overlap between the two devices over their operational ranges, usually with a focus on open-circuit voltage. 

In this work, we consider an alternate approach for highly-dynamic power profiles, such as those found in automotive hybrid or grid-based power regulation.  Here, we utilize mismatch between both the open-circuit voltages and resistances of the two devices to steer current into a higher-voltage, lower-resistance device during a high-current pulse, and then toward a low-voltage, high-energy device during periods of low current. We use electrochemical and equivalent-circuit models to study the current split, voltage, and SOC response during a variety of charge and discharge pulses and some designed driving profiles in order to understand the trade-offs between these different selections.