Smart Batteries and the Intersil X3100

APPLICATION NOTE

AN127 Rev 0.00 Page 1 of 11

July 11, 2005

AN127

Rev 0.00

July 11, 2005

Smart Batteries and the Intersil

X3100

Introduction

Smart Batteries today are a requirement for the latest high

performance portable laptop computers. These new

machines must squeeze more performance into smaller

packages while at the same time increasing battery life. To

achieve these goals the designer must incorporate new

methods of managing system power consumption and

available battery capacity. As battery management

techniques mature and become more common, application

of smart batteries will extend beyond the PC into other

portable devices, such as medical electronics, portable test

equipment and other battery powered systems

Portable Laptop PCs have migrated almost exclusively to

Lithium-Ion (Li-ion) type batteries. These batteries provide

increased capacity per weight and per volume, allowing a

more portable product in a smaller form factor. However,

Li-ion batteries have some characteristics that require

additional circuitry to manage their operation. Extra

electronic content is also required in smart battery designs to

squeeze more capacity out of the Li-ion cells and to provide

the user with better “fuel gauge” information.

This application note examines the Intersil X3100 Li-ion

Battery Protection and Monitor IC and it is used in smart

battery designs for the next generation of PCs, portable

equipment and medical electronics.

Battery Pack Considerations

Battery Pack Design

Battery packs consist of 1 or more cells. Connecting cells in

series provides higher voltage, connecting cells in parallel

provide higher capacity

. Some packs utilize combinations of

serial and parallel cells. Typically there is a trade-off between

available space, capacity (run-time) and required voltage.

A battery powered design should start with the consideration

of battery space and weight of the system. Knowing this, and

having a knowledge of the operating voltage of the various

system components, plus total system power requirements

and desired operational life, the designer determines the cell

capacity, voltage and pack configuration. Finally, to optimize

battery usage and run time, the designer specifies the

requirements for the system power management.

A typical PC might use three series combinations of 2

parallel Li-Ion cells. This is termed a 3S-2P pack. If each cell

is nominally 3.6V with 1350mAh capacity, then the overall

pack provides 10.8V at 2700mAh.

Safety

Lithium Ion batteries are key to achieving increased PC

perfomance in laptop computers in the near future

. They

have very good volumetric and gravimetric energy density.

However, they also have some challenging design and

safety issues that must be observed.

Lithium Ion batteries need special safety circuits in every

battery pack to monitor over-charge, under-charge and short

circuit conditions. If these conditions occur, the battery pack

must be “shut down.” In the worst case, over charging lithium

Ion batteries can result in sudden, automatic, and rapid

dissasembly (explosion). In the best case, overcharging

lithium Ion batteries can result in damage to the cells,

reducing capacity and cycle life.

To be safe, the battery pack safety circuit limits the voltage

on the battery pack cells to prevent unsafe conditions. An

over-voltage limit set too high can result in damage to the

cells and unsafe operation, however an over-voltage limit set

too low significantly reduces run time as capacity is given up.

Similarly, the undervoltage limit must be set accurately, since

overdischarging the Li-Ion battery results in chemical

changes that are irreversible, reducing the capacity and

cycle life, while stopping discharge too soon leaves usable

capacity in the battery. The safety unit must, therefore, have

the correct over-voltage and under-voltage limits and these

limits must be accurate to a very narrow range. Because of

the critical nature of the safety circuits in a battery pack,

some systems require redundant or backup mechanisms for

shutting down the battery pack and removing the load from

the cells.

1. Higher capacity can be obtained from a single cell instead of

parallel devices, however higher capacity devices have a larger

diameter or height. In some applications, such as a PC, there

are more limitations on pack height than on length and width. 2. See Application Note AN126 “Battery Primer” from Intersil.

- +

FIGURE 1. 3S-2P ARRANGEMENT OF CELLS IN A BATTERY

Smart Batteries and the Intersil X3100

AN127 Rev 0.00 Page 2 of 11

July 11, 2005

Cell Balancing

A careful examination of the operation of typical protection

circuits reveals an obstacle that the pack designer needs to

consider. Since the pack stops charging when any ONE of

the cells reaches the overvoltage limit, there may be other

cells that are below the limit and not fully charged.

Additionally, the pack will turn off when ONE of the cells

reaches the minumum voltage, even though other cells may

not be at the minimum value.

The mismatch of the voltage between cells does two things.

First, it reduces the overall capacity of the pack. The cells

are not all fully charged nor discharged, even though the

electronics sense that the pack is fully charged or

discharged. This leads to reduced run time. Second, having

cells charged or discharged to different values leads to

increasing pack imbalances and reduced cell life.

Cells become unbalanced in two ways. First, although

quality control is improving, manufacturing variations can

contribute errors of a few percent across all cells in a pack.

Second, cell imbalances can be accelerated by temperature.

This is especially true in newer PCs that have a high

performance CPU that generates more heat than

surrounding circuits. Placing a battery pack in close

proximity of the processor could result in one cell being

heated disproportionately. The heat applied to one side of

the pack causes one or more cells to charge or discharge

more slowly, accentuating cell disparities.

Fuel Gauging

Lithium ion batteries need very precise electronic monitors

and safety circuits built into the battery pack, but the pack

also needs other electronic content. The pack and system

designer must implement cost effective hardware and

software to provide the user with the greatest possible run

time and the most accurate information possible on the

status of the battery. To do this requires the ability to monitor

the current put flowing into and taken out of the battery over

a wide dynamic range

. The circuit must factor in

temperature, cycle history, battery chemistry,

charge/discharge state, application usage and other

conditions to achieve the highest accuracy gauge of

remaining capacity.

Finally, battery pack electronics must be very small, to fit

within ever smaller battery pack geometries. Space

requirements depend on the desired level of pack

functionality and the level of electronic integration.

The X3100 Safety Unit

Intersil designed the X3100 to meet the specific needs of a

three or four cell (series configuration) Lithium-Ion battery

packs, such as found in a laptop computer. Special features

provide flexibility in dealing with safety parameters for

various Li-Ion chemistries and the X3100 provides building

blocks for implementing fuel gauging, cell balancing and

pack monitoring hardware. This section describes some of

the key circuits and how they increase the safety,

performance and flexibility of the battery pack design.

Pack Architectures

All Li-Ion battery packs with three or four series cells require

basically the same functions, however there are several

ways to partition the electronics. The major components

consist of a safety unit, monitoring circuitry, a fuel gauge, an

EEPROM nonvolatile memory and a controller. The X3100

integrates the safety unit with the EEPROM and provides

circuits that allow a low cost microcontroller to monitor

various battery voltages and load current. A general purpose

microcontroller, through software and the analog building

blocks in the X3100, provides fuel gauge operation, cell

balancing, redundant pack monitoring and control (if desired)

and a communication link to the host CPU.

The X3100 operates at battery pack voltages of up to 24V

maximum, and provides voltage regulation and voltage

reference to the microcontroller. This combination of

functions isolates the microcontroller from the high battery

pack voltages, gives the flexibility of a fully programmable,

but low end microcontroller, and offers an overall cost effective

solution. This system partition is shown in Figure 3

Other battery pack configurations group the components

differently. This choice of pack architecture is based on two

primary considerations. The first is technology. Fuel gauging

and controller circuits are predominatly digital, while safety

circuits are analog and EEPROMs are a mixture of analog

and digital. Combining all of these functions on a single chip

is difficult and requires non-standard semiconductor

processes. These processes have historically lead to higher

cost components.

Voltage

Low voltage battery shut down

Minimum usable battery voltage

Unused Capacity

Maximum charge voltage

High voltage battery shut down

Time

Unused Capacity

Lost run

time

Lost run

time

Usable capacity

FIGURE 2. EFFECTIVE BATTERY CAPACITY

3. A system in idle state might consume as little as a few milli-

amps, while pulses of several amps are not uncommon in

graphic subsystems or during spin-up of disk drives.