Using Cell Balancing to Maximize the Capacity of Multi-cell Li-Ion Battery Packs

Introduction

Common multiple cell configurations for Li-Ion cells in battery packs consist of three or four cells in series, with one or more cells in parallel. This combination gives both the voltage and power necessary for Portable Computer, medical, test and industrial applications. While these configurations are common today, they may not be as efficient as they could be. The reason for this is that any capacity mismatch between cells in a series connection of cells results in a reduction of overall pack capacity.

There are two kinds of mismatch in the pack, State-of-Charge (SOC) and capacity/energy (C/E) mismatch. Each problem limits the pack capacity (mAh) to the capacity of the weakest cell. It is important to recognize that the cell mismatch results more from limitations in process control and inspection than from variations inherent in the Lithium Ion chemistry. As such, these types of cell to cell variation more likely occur in Li-Ion prismatic cells, due to more extreme mechanical stresses, and in Li-Ion Polymer, due to the newer processes involved.

The use of cell balancing can improve the performance of series connected Li-Ion Cells by addressing both State-of-Charge and Capacity/Energy issues. SOC mismatch can be remedied by balancing the cell during an initial conditioning period and subsequently only during the charge phase. C/E mismatch can be remedied by balancing during both charge and discharge periods. Even though the defect level for a given cell manufacturer may be very low, the extra confidence that a pack will not be returned, because of an early end of life, may be well worth the extra effort. It provides another level of quality assurance.

To provide some insight into the issues, this article discusses cell balancing, offers some guidelines for implementing cell balancing, and provides some actual results achieved through cell balancing techniques.

Definition of Cell Balancing

For portable systems requiring 6V or more of operating voltage, battery packs utilize battery cells connected in series. A series connection results in a pack voltage equal to the sum of the cell voltages. For portable computers (PCs), the battery pack typically has 3 or 4 cells in series with nominal voltages of 10.8V or 14. 4V. In the majority of these applications, the system requires more energy than is provided by a single series string of battery cells. Since the largest cell typically available (i.e. 18650) has a capacity 2000mAh, a PC requiring 50–60 Whr. of energy (5000–6000 mAh) requires three cells connected in parallel to each of the series cells.

Cell balancing is defined as the application of differential currents to individual cells (or combinations of cells) in a series string. Normally, of course, cells in a series string receive identical currents. A battery pack requires additional components and circuitry to achieve cell balancing.

Cell balancing is only considered when multiple cells in a battery pack are connected in series and usually when there are three or more series cells. Battery pack cells are balanced when all the cells in the battery pack meet two conditions.

1. If all cells have the same capacity, then they are balanced when they have the same relative State of Charge (SOC.) SOC is usually expressed in terms percent of rated capacity. In this case, the Open Circuit Voltage (OCV) is a good measure of the SOC. If, in an out of balance pack, all cells can be differentially charged to full capacity (balanced) then they will subsequently cycle normally without any additional adjust ments. This is mostly a one shot fix. The customer usually has instructions with a new pack to provide an overnight conditioning on the first cycle. Overnight conditioning typically consists of one complete discharge, followed by one complete charge cycle. Conditioning the pack overnight reduces the demands on the cell balance circuitry by minimizing the load and maximizing the charge time.
2. If the cells have different capacities, they are also considered balanced when the SOC is the same. But, since SOC is a relative meas ure, the absolute amount of capacity for each cell is different. To keep the cells with different capacities at the same SOC, cell balancing must provide differential amounts of current to cells in the series string during both charge and discharge on every cycle. Since charge and discharge cycles times can be shorter than the initial charge time, this process demands higher currents. Therefore, it is a much more demanding issue.

When the cells in the battery pack are not balanced, the battery pack has less available capacity. The capacity of the weakest cell in the series string determines the overall pack capacity. In an unbalanced battery pack, during charging, one or more cells will reach the maximum charge level before the rest of the cells in the series string. During discharge the cells that are not fully charged will be depleted before the other cells in the string, causing early undervoltage shutdown of the pack.

Manufactured cell capacities are usually matched within 3%. If less than optimal Li-ion cells are introduced in to a series string pack or cells have been on the shelf for a long period prior to pack assembly, a 150mV difference at full charge is possible. This could result in a 13-18% reduction in battery pack capacity.