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  • Writer's pictureSemco Infratech

A Bilevel Equalizer for Lithium-Ion Batteries

Electric-powered vehicles such as drones (UAVs), Electric cars, electric scooters, buses, Trucks, etc. are now in widespread use, and recent reports indicate their development is going to accelerate.


Virtually all of these types of EVs now use lithium-ion batteries (LIB), but LIBs require electronic equalizer circuits (EQU) to balance the cell voltages. All present versions have cost and/or performance problems. However, a new type of SEMCO’s hybrid EQU called the Bilevel Equalizer (BEQ) has been proposed that avoids these problems.


Electric-powered aerospace and military vehicles such as drones (UAVs) are also undergoing intense development, and these use lithium-ion batteries (LIB) almost exclusively. However, all large LIBs require equalizer circuits (EQU) to balance the voltages of the series connected cells (perhaps 200 or more), and all EQUs currently in use have certain cost and/or performance problems.


However, previous references have described a new type of hybrid EQU called the Bilevel Equalizer (BEQ) that mitigates these problems. This present study provides further insight into the BEQ design and proposes possible criteria that can be used for designing both the active and passive parts of the system.


The vast majority of large LIBs presently use passive equalizers (PEQ), which simply use a transistor to connect a resistor in parallel with each cell until it discharges to the same level as the lowest cell voltage in the pack. A typical circuit is shown in Fig. 1.


Basic PEQ circuit

Fig. 1. Basic PEQ Circuit.


PEQs are popular because they are simple and cheap, but heating and energy loss are obvious disadvantages. PEQs also are of no use during discharge since they cannot transfer charge to lower voltage, and thus the Ah discharge capacity of the battery is equal to that of the worst cell in a pack of perhaps 200-300 cells.


This problem is usually not important when the cells are new and well-balanced, but as they age, large variations develop, and the loss in discharge capacity due to even 1 or 2 weak cells can become serious.


This reduces the useful life of the battery, which of course increases the lifetime cost. PEQ heating problems also must be considered. This severely limits the size of the equalization currents, typically to less than 200-300 mA, and this limits the ability of the PEQ to equalize the pack when large imbalances are present.


There are several types of active equalizers (AEQ) that transfer charge between cells and thus avoid the problems with PEQs, but they are rarely used due to their complexity and much higher cost. All of these prove to be expensive even for modest AEQ currents, and the cost becomes prohibitive for the higher AEQ currents that are required for large cell imbalances and load currents.


The limitations of PEQs are widely recognized, but since presently available AEQs bring new cost and complexity problems, designers of battery management systems (BMS) have avoided them. Another problem is system inertia. Once a company has an operational BMS with a PEQ, they are reluctant to change, especially if the advantages of an AEQ do not become important until after a few years of service. Thus, these problems persist, and if left uncorrected they will degrade the lifetime performance of these large LIB applications.


BILEVEL EQUALIZER


This quandary has motivated the design of a new EQU that provides performance close to an AEQ but with only a modest cost increase above a PEQ. This circuit is a hybrid AEQ/PEQ called the Bilevel Equalizer (BEQ) because it provides equalization at two different voltage levels. In this system, the battery is organized into sections of series connected cells. The AEQ portion balances the section voltages, and there is a PEQ for each section which balances the section cells. This is especially advantageous for large applications such as those for electric aerospace vehicles because the BEQ can be implemented by adding an AEQ to an existing PEQ system with only minor changes to the original hardware. Fig. 2 (a) shows the AEQ circuit that constitutes the active part of the BEQ.

Schematic Diagram for BILEVEL EQUALIZER

(a) Schematic

current in L1 diagram

(b) Current in L1

In this system B1 – B3 represent sections of a series connected cells. The number of cells/sections is usually 4 to 14, and for sections of 12 -14 cells the efficiency is typically in the range of 85 to 90%. Components Q1, Q2, and L1 constitute one AEQ unit, so this circuit has 2 units. To transfer charge from B1 to B2, Q1 is turned on for 0 < t < t1, and i1 flows into L1. At t1, Q1 turns off and i1 flows from L1 into B2 via the body diode of Q2. The period t2 – t1 is less than t1 because of a slight gap in the FET gate drive signal and parasitic losses.


Since the B’s can consist of any number of cells, a 196-cell battery might be organized into 14 sections of 14 cells each. This would only require 13 AEQ units (number of sections – 1), whereas an AEQ with a bidirectional DC-DC converter for each cell would require 196 AEQ units. Therefore, if both types are operated at the same value of equalization current, the cost of the AEQ in the BEQ will be much lower than using an AEQ for each cell.


Another important cost advantage is the absence of the transformers that are present in virtually all other AEQs. AEQs with a DC-DC converter for each cell are presently limited to EQU currents less than 1 Adc, and they are still quite expensive even at these low current levels. Currents in this range also are inadequate for larger batteries that might require EQU currents in the range of 5 Adc or more. Because of its relative simplicity and the low number of AEQ units, the circuit in Fig. 1 can easily be designed to economically provide equalization currents in these higher current ranges.


The block diagram of a BEQ where the cells are divided into 5 sections is shown in Fig. 3. This might represent a 60-cell LIB with 12 cells/section and a maximum voltage of about 240 Vdc. This system uses a PEQ for each section to provide equalization at the cell level for the cells in that section. AEQ units identical to those in Fig. 2 (a) are used to equalize the section voltages. The AEQ boxes shown in blue in Fig. 3 are the only new hardware items needed to convert a PEQ to a BEQ.

Fig. 3. BEQ for a Battery with 5 Sections of Cells

Although the conversion of a PEQ to a BEQ does not require any significant hardware changes, it does require new software since the equalization strategy is different, e.g., the PEQs now drain the cells to the lowest cell voltage in each 12-cell section instead of the entire pack.


Conclusion


In spite of their power losses and lack of equalization during discharge, PEQs remain the most common type of EQU due to their lower cost. AEQs provide much better performance, but they are rarely used because of high cost and complexity. This present study, along, shows that SEMCO’s BEQ hybrid provides an attractive solution since its performance for large imbalances is much better than a PEQ, and its much lower component count and absence of transformers indicate a much lower cost than an AEQ of equivalent size.


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