Monitoring and Charging System for a High-Power Battery

December 1, 2007

Naval Research Laboratory, Washington, DC

Figure 1 is a simplified block diagram showing (1) a high-power rechargeable battery comprising multiple series-connected modules, each module containing multiple series-connected rechargeable cells, and (2) a system for monitoring battery voltages and controlling the charging of the battery. To ensure safe, reliable operation of the battery, it is imperative to ensure that the potentials and temperatures of all the cells remain within prescribed limits. Accordingly, the system monitors the potential of each cell, the overall potential of each module, and the temperature of each cell. Because overcharging can damage a cell, if the potential of any cell exceeds a predetermined maximum allowable level, the system interrupts the charging process and starts another process, called “equalization,” in which the affected cell is partially discharged through a resistor until its potential reaches a prescribed safe lower level. If the temperature of any cell or module exceeds a predetermined maximum allowable value, or if a fault is detected, the system stops the charging process.

Figure 1. Modules Containing Cells in Series are connected in series to a charging subsystem, and the cells in each module are connected to monitoring and control circuitry on a battery-control board.

Associated with each module is a battery-control board. For measurement of the module potential, measurement of cell potentials, and equalization, all the cell terminals in the module are connected to voltage-measuring circuits and equalization-resistor-switching circuits on the battery-control board. The voltage-measuring circuit for each cell includes a differential amplifier, the output of which is digitized and fed to an on-board processor. The equalization resistor for each cell is switched into or out of contact with the cell terminals by a relay driven in response to a command from the on-board processor. The temperatures of the cells, of the module as a whole, and of the battery-control board are measured by sensors (e.g., thermistors or thermocouples), the outputs of which are also digitized and sent to the on-board processor.

The operation of the system as a whole and the operations of the battery-control boards are coordinated by a control computer. The processors on the battery-control boards communicate with the control computer via a network than can be of any wired or wireless type commonly used for communication among computers.

The system includes a charging subsystem that includes a fixed-output power supply for the control circuitry and an adjustable-output power supply for charging the battery (see Figure 2). Like the battery-control boards, the adjustable-output power supply communicates with the control computer via the network. The fixed-output power supply is used to energize a relay that controls the main power switch between the battery and the adjustable-output power supply. On each battery-control board, there is a relay, denoted a board relay, that opens a switch when equalization is needed, a cell temperature exceeds a preset limit, or a fault is detected. The board relay switches are connected in series with the main power relay, so that if any condition necessitating interruption or stoppage of charging is encountered in any cell in any module, the main power is turned off.

Figure 2. The Charging Subsystem and Power-Relay Connections are emphasized in this alternative diagram of the system of Figure 1.

Multiple layers of redundant fault sensing and control are designed into the system. During charging, the control computer must be in constant communication with the battery-control boards and the adjustable-output power supply, and the processor on each battery-control board samples cell potentials and temperatures and compares them with the corresponding preset limits at a repetition rate of the order of several times per second. The control computer commands the adjustable-output power supply to zero-output potential when it receives, from any battery-control board, a cell-potential or a temperature reading out of allowable range. The control computer also disables the adjustable-output power supply if it loses communication with, or receives a fault message from, any battery-control board. The control computer can be made to display a graphical user interface showing cell potentials, module potentials, and module temperatures, and battery-control-board temperatures. The graphical user interface can be configured to function as a means for a user to exert control over charging and equalization.

This work was done by Daniel P. Thivierge of the Naval Undersea Warfare Center for the Naval Research Laboratory.

NRL-0024