Comparison of battery string parallelization systems to improve Na-NiCl2 battery reliability

The Sodium-Metal Halide Battery reliability should be increased to enhance the competitiveness of this technology in the stationary battery market alongside lithium-ion and lead-acid batteries. Currently, the elementary Sodium-Metal Halide cells show a rather low reliability. Commercial battery power management systems are not able to completely use the energy stored if a battery composed of multiple parallel-connected strings presents faulty cells. In fact, the string parallelization is generally achieved by ideal diode circuits that do not allow the optimal management of the strings in faulty conditions. As an example, the reliability of the 48TL200 FZSoNick battery composed of 5 strings of 20 cells each is estimated to be lower than 30% after 10 working years. Two novel string parallelization approaches are compared to the commercial diode-based solution in this work. The first approach employs DC/DC converters and shows the best performances increasing the battery reliability near to 100% at the cost of higher battery cost and complexity. The second approach replaces the diodes with controllable switches. The switches increase the battery reliability beyond 95% with little increase of the battery complexity and cost.