With the limitations of the HE cell, the battery designer must carefully manage the current demand in the system to avoid an over-current situation, which affects battery life time. Additionally, when the HE cell voltage is low and close to the system shutoff voltage, large current pulses in conjunction with the voltage drop due to battery resistance can cause the system to shut down prematurely. To help prevent these over-current situations, the HE cell capacity must be increased. The addition of a HP cell, however, will effectively increase the total systemís pulse current capability without increasing the battery capacity. This allows the full system performance to be realized.
On the one hand, the two-strings of cells, i.e. two separate batteries, can be connected by a DC-DC link only. In this configuration, both strings require an own charge-equalization circuit for the cells in series connection.
On the other hand, both types of cells can be directly connected in parallel on a cell-to-cell level. In the second configuration, the amount of current supplied by the HP cell and the HE cell will be inversely proportional to the electrical series resistance (ESR) of the HE cell and the ESR of the HP cell. While this connection is feasible during situations with low current demand, the HE cell must be disconnected from the HP cell during full-power recuperation and acceleration. Therefore an electronic circuit should be placed between the HE cell and the HP cell (illustrated by simple switches). For the balancing of the charge between cells in series connection, only one complex equalization circuit is required. But in this configuration, the electronic circuit between both strings of cells must be designed for high power capability, which leads to high costs for the electronic components.
In a third option, both concepts are merged together and additionally a new designed DC-DC converter will be placed between the two strings. In this configuration, the electronic circuit between the two strings of cells is designed for the purpose of charge-equalization between the strings only. The DC-DC converter will include the feature, that the energy/power flow can be controlled in a very flexible way between the inverter and the two strings of cells. The new operating modes and the fact, that a DC-link between the battery and the inverter exists, will improve the overall efficiency of the electric powertrain by up to 7% because the E-machine can be operated at an optimum voltage level (see [Eckhardt 2010Gleichspannungswandler hoher Leistungsdichte im Antriebsstrang von Kraftfahrzeugen", PhD Thesis, November 2010, ISBN: 978-3-8322-9574-5]).
The SuperLIB project includes the required development, prototyping and testing of the specific charge equalization circuits and of the new DC-DC converter for coupling HE and HP cells to provide additional power to the electric components. The switches between the HE and HP cells represent the low power path, similar to switches in the passive cell equalization circuits. The DC-DC converter will be responsible for the energy flow between the two strings. This new topology permits a smart management of the storage system. Thus, the vehicle mileage and performance will be improved. Furthermore, the connection with the new type of DC-DC converter will have the capability of a redundant system and the reliability will be significantly improved. Even the breakdown of a single cell in one of the strings, the dual-cell battery system will be able to provide a limp-home function with the remaining working cells. In addition, the safety of the system is one of the important aspects. Thus an assessment of the functional safety will be performed explicitly, which will consider boundary conditions of the electronic architecture and of the smart control system.
The SuperLIB project focuses on smart control system solutions for traction batteries. The general concept consists of a combination of measures for
- a dual-cell battery concept comprising high-power and high-energy cells,
- including an electronic architecture for an efficient energy and current distribution,
- managed by a smart control strategy, and
- monitored by cell integrated temperature sensors