State of Charge Balancing Control for Multiple Output Dynamically
Adjustable Capacity System 14-13-02-0014
This also appears in
SAE International Journal of Electrified Vehicles-V133-14EJ
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.