State of Charge Balancing Control for Multiple Output Dynamically Adjustable Capacity System

Abstract

<div>A multiple output dynamically adjustable capacity system (MODACS) is developed to provide multiple voltage output levels while supporting varying power loads by switching multiple battery strings between serial and parallel connections. Each module of the system can service either a low voltage bus by placing its strings in parallel or a high voltage bus with its strings in series. Since MODACS contains several such modules, it can produce multiple voltages simultaneously. By switching which strings and modules service the different output rails and by varying the connection strategy over time, the system can balance the states of charge (SOC) of the strings and modules. A model predictive control (MPC) algorithm is formulated to accomplish this balancing. MODACS operates in various power modes, each of which imposes unique constraints on switching between configurations. Those constraints are mathematically formalized so that MPC can be applied to minimize predicted SOC differences over a finite time horizon. In this article, several variations that vary in how freely strings can connect and disconnect from the bus bars are presented. Methods allowing more flexibility in configuration changes can balance SOCs more quickly but take more computation to resolve. In contrast, simpler schemes reduce computation and simplify implementation, but take longer to balance the SOCs. Simulation results illustrate the expected behavior.</div>