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Balancing and Control System for Operation of Multiple Cells and Packs in Large Energy Storage Arrays

National Renewable Energy Laboratory

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Technology Marketing Summary

Large scale energy storage systems, such as those used in electric vehicles or grid scale systems combine multiple electrochemical cells into a pack, or in the case of grid-scale systems, multiple packs into a large array of packs. However, battery packs often underperform due to cells or packs within the array having disparate states of health or performance capability. Consequently, the performance of the entire system is limited by the weakest unit. If these limits are unknown to a battery controller and the battery controller seeks to respond to a change in load, the nonlinearities imposed by the constraints can cause instability within the array and can lead to poor performance. In the case of electric vehicles, success is dependent upon the capacity and life of the battery. Distance limitations resulting from battery capacity constraints as well as the high cost associated with the replacement of electric vehicle batteries have made electric vehicles an expensive alternative to fossil fuel powered vehicles and inhibited widespread market acceptance. There needs to be a better method for battery control and optimization.


Scientists at NREL and other research organizations have developed an active balancing system for large arrays of multi-cell and multi-pack storage systems. This technology places a DC/DC converter at each individual cell, module or pack that can transfer power to and from that unit to a shared bus. By connecting the unit cell, module or pack via the shared bus, the shared bus can serve as the control reference and means to balance the power and energy delivery. The design allows for seamless communication amongst the units, possibly with different chemistries or states of health, to ensure that the entire array acts seamlessly as one system. 

  • Extends lifetime of battery packs, including in a second-use environment
  • Reduces overall life cycle cost of battery packs
  • Balancing DC-DC converters can also supply an auxiliary load, eliminating need for a separate high voltage to 12V DC-DC converter in electric vehicle packs
  • Allows partial pack functionality with a failed cell or module
Applications and Industries
  • Battery Life Optimization
  • Large Scale Energy Storage Systems
  • Electric Vehicle Battery Packs
  • Grid Scale Systems
Patents and Patent Applications
ID Number
Title and Abstract
Primary Lab
Application 20160336767
An apparatus includes a battery state module that determines a battery state of each of a plurality of battery cells forming a battery unit. A battery state includes a health of the battery cell. A battery state of a battery cell differs from a battery state of other battery cells of the battery unit. Each battery cell is connected to a shared bus through a bypass converter that provides power from the battery cell to the shared bus. A charge/discharge modification module determines, based on battery state, an amount to vary a charging characteristic for each battery cell compared to a reference charging characteristic. Each charging characteristic varies as a function of a reference state. A charge/discharge module adjusts charging/discharging of a battery cell of the battery unit based on the charging characteristic of the battery cell.
Application 20160336765
An apparatus for model predictive control ("MPC") is disclosed. A method and system also perform the functions of the apparatus. The apparatus includes a measurement module that receives battery status information from one or more sensors receiving information from a battery cell, and a Kalman filter module that uses a Kalman filter and the battery status information to provide a state estimate vector. The apparatus includes a battery model module that inputs the state estimate vector and battery status information into a battery model and calculates a battery model output, the battery model representing the battery cell, and an MPC optimization module that inputs one or more battery model outputs and an error signal in a model predictive control algorithm to calculate an optimal response. The optimal response includes a modification of the error signal.
Application 20150214757
For battery control, an apparatus includes a shared bus and a plurality of isolated direct current (DC) to DC bypass converters. Each bypass converter is associated with one battery unit. Inputs of each bypass converter are in parallel electrical communication with the associated battery unit. Outputs of each bypass converter are in parallel electrical communication with the shared bus. Each bypass converter estimates a battery state for each battery unit and controls the battery state to a reference state.
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
ROI 14-33ProposedAvailable12/09/201612/09/2016

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To: Erin Beaumont<>