A Bi-level Equalizer for Battery Cell Charge Management


Project ID: D2016-41



Lithium ion batteries for electric vehicles (EV) begin to degrade much faster after the capacity drops below about 80%, and manufacturers recommend that they be replaced at about this point.  However, these batteries still have adequate capacity for other applications, and it is expected that vast numbers will soon become available.  These batteries are expected to be very low cost, and this presents a commercial opportunity for various energy storage applications.


However, the characteristics of these second life batteries are very different from new batteries, and they will need better equalizer (EQU) circuits to balance the battery cell voltages.   To minimize cost, almost all EVs use passive EQUs (PEQ) that drain all cells to match the lowest cell voltage in the pack.  This is satisfactory for new cells that are well matched, but it is inadequate for second life cells which have wider variations. It also means the battery capacity is equal to that of the worst cell in the pack, and this results in a drastic reduction in second life battery capacity and lifetime.  Active EQUs (AEQ) can be used to avoid this issue since they provide a capacity close to the average of the cells, but they are not commercially feasible because of their high cost.

To solve this capacity/cost problem the University of Toledo has developed a PEQ/AEQ hybrid called the Bilevel EQU (BEQ).  The BEQ provides a battery capacity close to that of an AEQ but with only a minor cost increase above a PEQ.


Invention Description

Researchers at the University of Toledo have developed a Bilevel equalizer (BEQ) for a battery voltage management system that uniquely combines a PEQ and an AEQ to provide equalization at two different voltage levels. Using both types of equalizers in tandem leverages the low-cost PEQ to compensate for cell self-discharge, while using an AEQ at a higher voltage level to compensate for capacity fade.



  • Electric/Hybrid Vehicles (EVs), Military/Aerospace, and Energy Storage (ES) for Grid Connected Solar and Wind Energy

  • Equalizer for a battery management system


  • BEQ transfers charge to weak cells during charge and discharge, so capacity will be close to the average of the cells rather than that of the weakest cell

  • The second life capacity increase depends on degree of cell imbalance, but can be up to 40% higher than that of a PEQ

  • Increases performance at a low cost

  • Decreases loss in capacity over the battery lifetime



IP Status: Issued US Patents: 10,862,318 and 11,139,661



Publications:                              A Bilevel Equalizer for Large Lithium Ion Batteries Mubenga NS, Linkous Z, Stuart T. Batteries 2017, 3, 39. Dec 7 2017.

                                                A Bilevel Equalizer to Boost the Capacity of Second Life Li Ion Batteries. Mubenga NS, Sharma K, Stuart T. Batteries 2019, 5(3), 55.

                                                Bilevel vs. Passive Equalizers for Second Life EV Batteries N. Mubenga, B. Salami, and T. Stuart. MDPI Electricity 2021, 2(1), 63-76.


Patent Information:
For Information, Contact:
Katherine Pollard
Licensing Associate
The University of Toledo
Thomas Stuart
Thomas Stuart
Electric/Hybrid Vehicles
Energy Storage