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

APPLICATION NOTE

AN167 Rev 0.00 Page 1 of 9

Jun 7, 2005

AN167

Rev 0.00

Jun 7, 2005

Introduction

Common multiple cell configurations for LiIon 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 LiIon prismatic cells, due to

more extreme mechanical stresses, and in LiIon Polymer,

due to the newer processes involved.

The use of cell balancing can improve the performance of

series connected LiIon 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 adjustments. 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 measure, 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.

Using Cell Balancing to Maximize the Capacity of

Multi-cell Li-Ion Battery Packs

AN167 Rev 0.00 Page 2 of 9

Jun 7, 2005

SOC Balancing

SOC balancing is used when all cells in the pack have the

same capacity. As such, cells are considered balanced when:

Looking at an individual cell, the state of charge is defined as:

The capacity of a cell is defined as:

To determining the capacity of a cell (C), the cell is fully

discharged to its minimum operating voltage, then charged,

while measuring the current and the time, until it reaches an

open circuit voltage (OCV) of 4.20V. With perfectly performing

cells, the SOC in this condition is 100%. A unique open circuit

voltage (OCV) is also associated with the 50% SOC and

usually known as V

MID

. A Typical V

MID

is 3.67V.

The definition of balanced cells is strictly true, even if the cells

have different capacities. In order to charge cells with different

capacities to the same SOC requires that some cells be

charged or discharged more or less than others in absolute

terms. This requires the use of a differential current (cell

balancing technique) and is termed Capacity/ Energy

Maximization.

Capacity/Energy Maximization

The term Capacity/Energy Maximization refers to the setting of

all series cells in a pack to the same SOC even though they

have different capacities. Managing the SOC at all times,

maximizes the energy output of the pack.

To maximize energy output, the cells must all be fully charged.

That is, the SOC of all cells is 100%. If the cells have different

capacities, some cells will charge and discharge more than

others. Consider the case where, in a 3 cell series stack, C

= C

. The only way to balance the pack is to apply a

differential charging current (cell balancing) to the cell with

higher capacity (C

). The same has to be done as the pack

discharges. If not done, then the total pack capacity diminishes

because the pack discharge shuts off when the cell with the

least capacity reaches its cutoff voltage. More important, in this

case, is that the weakest cell reaches cutoff while capacity

remains in the other cells. Over time, this degrades the

weakest cells faster than the others, thus accelerating capacity

loss over many charge/discharge cycles.

By matching the voltage of each cell in the string on the way

down, more current is drawn from the higher capacity cells.

During discharge this process requires that there is some

additional voltage loss while balancing, but in the end, when all

cells arrive at zero SOC, there is an increase in the total

energy extracted from the pack over time.

Cell Manufacturing State of the Art with

respect to Balancing

Li-ion cylindrical cells today generally have very good quality

control, with cell capacity matched to ±3 percent. The input

capacity is essentially exact and being within a few

milliampereseconds. So the absolute amount of charge (mAh)

is initially exact and the SOC is within a few percent.

Liion Self-discharge and the Need for SOC Mat ching

LiIon Chemistry does not create cell imbalance and has no

mechanism for reversible self discharge. On the other hand,

there is a process by which the cell chemistry stabilizes and

creates what is known as irreversible loss. Most of this loss is

experienced inside the factory. Since these cells are rejected in

the factory, gross errors are not observed by the user.

However, a small amount of irreversible loss can be

experienced in high temperature discharge and over long

periods of time at room temperature. The maximum amount

tends to be less than 10 percent and all cells stored or used

together will experience this effect at the same rate. So cell

imbalance is not created by this mechanism. There is a small

reversible loss associated with this irreversible loss but it is

proportional. The amount of reversible loss created in the

factory is recharged before capacity sorting. Therefore, loss is

the field is very small and consistent from cell to cell. Again it is

no cause for cell imbalance.

Another source of pack cell imbalance occurs when cells

remain on the shelf for long periods of time prior to assembly.

This is compounded if cells from different manufacturing lots

are mixed within a pack. In this case, cells with varying

reversible losses will have had time to accumulate significant

differences after capacity sorting operations, but before pack

assembly. Even after assembly, imbalances can develop over

time within the pack, if cells within a pack vary significantly –

especially if the pack remains “on the shelf” for a significant

period.

Soft Shorts

Soft shorts are the primary cause of cell imbalance in some

cells. Due to tiny imperfections in cell construction the cell can

have very high resistance shorts on the order of 40,000 or

more. The self discharge rate due to this higher resistance is

on the order of 0.1 milliamperes or 3% per month. Most cells

do not have this condition and can hold much of their capacity

for years. Some cells which meet specifications when they

leave the factory may sometimes develop this condition later.

This is strictly an electromechanical condition. Used in a single

cell pack, this cell can just be recharged and shows no

capacity loss. But, in a series pack, a cell with soft sorts could

lose 3% per month, while another cell loses none at all. This

differential SOC in the pack reduces overall pack capacity.

Without a corresponding capacity degradation, a conditioning

operation primarily needs to be done once before the device is

used, then occasionally again, if put in to storage. This

recovers the initial capacity loss, then a less aggressive

SOC

SOC

CELLN

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SOC

TOTAL percent

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