This work presents the design and development of an integrated High Temperature Polymer Electrolyte Membrane (HT-PEM) fuel cell system which is used for the charging of Li-Ion battery stacks. The necessary hydrogen is provided either by a renewable hydrogen production station or from an LPG reformer system which are both located at CERTH. In this work the automated monitoring system is analyzed providing online vital information such as battery state of charge, state of health, temperature and capacity. The design is based on a novel integrated management methodology that incorporates monitoring, evaluation and control features, which are configurable, expandable and flexible according to the specific end-user requirements.
Furthermore the Battery Monitoring System (BMS) is designed and developed along with a system for the protection of each individual cell of the battery pack. More specifically, the system is developed for a battery stack that consists of 15 Li-ion battery cells of 3.2V/120Ah each, which are connected in series to produce a 48V. . In order to protect the battery and the individual cells a state of the art BMS has been designed, implemented and tested. The main requirements for this BMS are the monitoring of the state and the protection from overcharging of each individual cell and at the same time the reduction of the overall cost as much as possible. The main idea of the BMS is that while monitoring the voltage of each cell during charging, in case that a cell goes beyond a predefined limit value (set for example by the manufacturer), the charging procedure to be interrupted. This is implemented through a resistance that is connected in parallel with each specific cell such as all the charging current to go through the resistance and not the cell. This process is repeated if necessary until all cells are fully and equally charged.
When a resistance is placed in parallel to a cell its state of charge and voltage should remain constant i.e. all the charging current goes through the resistance. However, due to transient phenomena it was observed that the voltage is not remaining constant but is oscillating between two values. One of them is the value where the resistance is placed in parallel to the cell and the other one is a lower value where the resistance is removed when the cell voltage drops. It was observed that the frequency of this oscillation is different for each cell and that it is greatly depended on its state of charge and health. Moreover, it was found after several charging cycles and in case that a cell is overcharged, that its voltage continues to increase even above the limit where the resistance is placed. In this case a second safety limit is used by the BMS and when this limit is exceeded the charging process is interrupted. After all using that BMS, it is possible to balance the state of charge of all the cells and therefore to protect the state of the battery.
The flexible and modular design of the system can be applied to a wide range of traction and stationary battery applications since the involved platform is generic and can be adapted upon demand. The smart soft sensor features combined with the supervisory monitoring capabilities enable the optimum utilization of the battery pack. Finally the BMS provides informative decisions to the user that monitors the stationary system or operates the forklift and enables proactive maintenance operation to ensure that the system will provide the maximum of its capabilities.