Battery Management Systems (BMS)

EaglePicher’s Power Pyramid™ employs EaglePicher’s proprietary Battery Management System (BMS) for control of battery functions and communications. Battery management is the process to ensure that a series cell battery string receives an optimum charge without damage to the battery string. A problem with large series battery strings is maintaining cell balance. For various reasons including variations in manufacture, thermal gradients across the cell modules, replacement of a used cell with a new one, etc, cells may differ in capacity, impedance, or state of charge. If cells are out of balance, system capacity is lost because the battery can only be charged until the highest cell reaches its maximum voltage or discharged until the lowest cell reaches its minimum voltage. Worse, because a cell, which is low for any of the above reasons, will have a higher terminal voltage during charge and will self-heat more during discharge, any imbalance within the pack tends to become worse with every cycle. The battery management process includes battery string charge control and cell fault protection. Safety is a major concern when charging Lithium Ion cells. Providing proper charge control for the battery system will mitigate safety concerns and extend battery life.
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EaglePicher’s Power Pyramid™ BMS is able to transfer charge between cells. It allows essentially all of the capacity of the battery to be utilized. By transferring energy out of lower capacity cells during charge, the State of Charge (SOC) of the entire pack can be brought to 100% without wasting energy. Likewise, by preferentially transferring energy out of higher capacity cells during discharge, the entire pack can be discharged to the target minimum SOC. The extra energy in higher-capacity cells is made available to the application.

To perform its function, the system must track not only the relative SOC of individual cells (i.e. percent of capacity), but also the absolute SOC (i.e. available energy). To do this, the system constantly tunes a five-parameter model of each cell. This model provides an estimate of individual cell capacity and allows the system to prepare charge and discharge energy transfer plans. By working to the algorithm, the system proactively pushes the battery toward absolute SOC balance instead of simply reacting to relative SOC imbalance after the pack is already unbalanced

Master Control System

The architecture of the system uses one small module attached to each cell and a central controller for the battery. Each module has a local processor, DC-to-DC converter, and A/D and D/A converters for measurement and control. The central controller includes a current sensor, which must be installed in one of the battery leads. Each controller supports a single series string of cells.

Voltage parameters are sensed via analog signal conditioning circuits. These circuits provide the necessary signal attenuation or gain and filtering. The outputs of the analog conditioning circuits are calibrated to individual cell/battery parameters. This provides the necessary levels for sensing via analog to digital converters within the battery’s management system.

Temperature and current parameters are sensed via analog signal conditioning circuits. These circuits provide the necessary signal attenuation or gain and filtering. The output of the analog conditioning circuits are calibrated to the cell/battery temperature and current parameters. This provides the necessary levels for sensing via analog to digital converters within the battery’s management system.

Protection during a fault condition (including over and under voltage/temperature, overload current and time) are provided by isolating the battery from the charging source or load. This is accomplished by using solid state disconnects.

Fault indication is accomplished by communicating fault conditions via SCADA/DMS interface. Either analog or digital outputs from the master controller can be made available. Fault detection uses information obtained during battery management system monitoring. Fault conditions are determined by coded parameter limits established by set points between communications.



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