Modular Energy Storage System for Hybrid Drive Systems Based on Lithium-Ion Cells

Hybrid drives offer significant potential for reducing fuel consumption and emissions in off-highway applications thanks to the recovery and storage of kinetic or potential energy. Systems supplier MTU is developing a modular hybrid system comprising of standardized components. One of the key subcomponents in this system is the energy storage system (ESS), which stores the produced energy for use when it is actually needed.

In the LiANA+ project funded by the German Federal Ministry of Economics and Technology (Bundesministerium für Wirtschaft und Technologie) in accordance with a decision by the German Federal Parliament, project partners MTU Friedrichshafen GmbH, Akasol GmbH and Sensor-Technik Wiedemann GmbH (STW) joined forces with the Chair for Mechatronics at the University of Rostock and the Center for Solar Energy and Hydrogen Research, Baden-Württemberg (Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg) to develop a modular high-performance energy storage system based on lithium-ion cells for use in a wide range of applications and power classes.

Akasol tested nine different cell types based on criteria such as power density, safety, price, cycle stability, quality and availability and then selected two of these cell types for prototype development. In terms of detailing and time prioritization, attention focused on the larger 46 Ah cell, since the simulations indicated the prospect of a promising solution.

Cell characterization and modeling

The dynamics, power and efficiency of an electrical accumulator depend on the state of charge (SOC), state of health (SOH) and temperature of the battery. To characterize the power of the accumulator, electrical and thermal measurements were performed on the cells at ZSW. For the simulation of the battery pack and the model-based determination of state of charge and state of health, the electrical behavior of the cell is reproduced and parameterized by means of a terminal voltage model with open-circuit voltage V0, ohmic resistance RΩ and two RC elements.

Modularization of the energy storage system

Hybrid Modularized Energy Storage

To cater for various off-road applications in the future, a modular approach has been adopted for the battery system.
The existing solution at Akasol, has had its design optimized to allow better cooling properties and, therefore, higher continuous power with a longer life. The module includes twelve lithium-ion cells and ensures their thermal and electrical connection. It also contains the measuring system for monitoring the state of the cells.

Several modules are grouped together in a battery housing, as shown in Figure 1 (On right, Figure 1: Energy storage system for LiANA+). Various parallel and serial connections allow these battery banks to be arranged into energy accumulators with different power and energy values. The LiANA+ project created banks of 15 serially connected modules creating the nominal voltage of 666 V. Three of these battery banks are connected in parallel to produce a total energy content of about 92 kWh and a peak output of 552 kW.

Functional safety

Lithium-ion harbors an intrinsic hazard potential that needs to be managed by a combination of design and electronic measures. The electronic measures are a key component of the battery management system (BMS). Their implementation is subject to the applicable rules and regulations for functionally safe systems, which are defined in the IEC 61508 standard and application-specific standards.

The risk assessment produced in the LiANA+ project stipulated the safety objectives for the electronic safety equipment, determined the safety functions and their requirement levels up to SIL2, and defined the shutdown of a battery bank as a safe state. The BMS realizes these functions, and the manufacturer of this system, Sensor-Technik Wiedemann GmbH, is able to provide verification in accordance with the applicable standards. By way of example, the ISO 13849 standard applies to the safety of machinery, while the EN 50128 and EN 50129 standards apply specifically to rail technology.

Components of the battery management system

Each of the modules installed in the battery includes a cell supervision circuit (CSC, Figure 2, below, from a storage module). Each CSC further includes a 16-bit microcontroller plus a redundant highly accurate measuring device for individual cell voltages and temperatures.

Each battery is assigned two contactors which allow two-pole switching of the respective battery. These are supplemented by a highly accurate, shunt based current measurement and insulation monitoring as well as a precharging unit that allows controlled charging of circuit capacitance.

In the case of parallel operation, one BMS (Figure 3) can be assigned a master function. This BMS assumes the coordination role and makes the energy storage system appear to be an individual battery with correspondingly higher capacity.

There is a 32-bit microcontroller with floating-point processor. Its basic software is already decoupled from the functionally safe components and performs the more complex algorithms determining the state of the ESS.

Die Batterie hat ein Gewicht von 500 KilogrammValidation of the entire system

Testing and validation of the functions of the energy storage system in interaction with the other hybrid drive components was performed on a hybrid test bench adapted for this purpose. The tests confirmed the functions of the system and were followed by an endurance run in order to gain long-term experience with lithium-ion energy storage systems of this size.

(Figure 3: Battery management system (BMS) with high-voltage connection for a battery bank, to the right)

Potential fuel saving for a diesel-hybrid rail vehicle

To evaluate the fuel-saving potential of the selected rail vehicle compared to conventional railcars, an optimum operating strategy was calculated based on a control-oriented simulation model at the University of Rostock. The main components were modeled in terms of the longitudinal dynamics of a two-part local railcar converted into a hybrid vehicle.

In accordance with the basic idea of the Bellman optimality principle – each end of an optimum sequence of decisions is optimum per se – the sequence of decisions is built up successively from an end state in the direction opposite to the simulation direction. Here the battery state of charge is selected as the dynamic state variable and the load distribution between electric motor and diesel engine as the control variable. The result for the speed profile is a fuel saving of 18.1% compared to the conventional diesel vehicle. The corresponding progression of the energy accumulator state of charge meets the requirement for a balanced state of charge and also displays a maximum charge stroke of 4%, superimposed by numerous microcycles <1%. Useful life simulations give reason to expect that about 125,000 of these load cycles are possible. Superimposed with the calendric aging of the cells, an operating time of over 10 years can be expected.