Philips Semiconductors

Product data

NE57600

One-cell Lithium-ion battery protection with

over/undercharge and overcurrent protection

2001 Oct 03

6

TECHNICAL DISCUSSION

Lithium Cell Safety

Lithium-ion and lithium-polymer cells have a higher energy density

than that of nickel-cadmium or nickel metal hydride cells and have a

much lighter weight. This makes the lithium cells attractive for use in

portable products. However, lithium cells require a protection circuit

within the battery pack because certain operating conditions can be

hazardous to the battery or the operator, if allowed to continue.

Lithium cells have a porous carbon or graphite anode where lithium

ions can lodge themselves in the pores. The lithium ions are

separated, which avoids the hazards of metallic lithium.

If the lithium cell is allowed to become overcharged, metallic lithium

plates out onto the surface of the anode and volatile gas is

generated within the cell. This creates a rapid-disassembly hazard

less than approximately 2.3 V), then the copper metal from the

cathode goes into the electrolyte solution. This shortens the cycle

life of the cell, but presents no safety hazard. If the cell experiences

excessive charge or discharge currents, as happens if the wrong

charger is used, or if the terminals short circuit, the internal series

resistance of the cell creates heating and generates the volatile gas

which could rupture the battery.

The protection circuit continuously monitors the cell voltage for an

overcharged condition or an overdischarged condition. It also

continuously monitors the output for an overcurrent condition. If

any of these conditions are encountered, the protection circuit opens

a series MOSFET switch to terminate the abnormal condition. The

lithium cell protection circuit is placed within the battery pack very

close to the cell.

Charging control versus battery protection

The battery pack industry does not recommend using the pack’s

internal protection circuit to end the charging process. The external

battery charger should have a charge termination circuit in it, such

as that provided by the SA57611. This provides two levels of

overcharge protection, with the primary protection of the external

charge control circuit and the backup protection from the battery

pack’s protection circuit. The charge termination circuit will be set to

stop charging at a level around 50 mV less than the overvoltage

threshold voltage of the battery pack’s own protection circuit.

Lithium Cell Operating Characteristics

The internal resistance of lithium cells is in the 100 m

Ω range,

compared to the 5–20 m

Ω of the nickel-based batteries. This makes

the Lithium-ion and polymer cells better for lower battery current

applications (less than 1 ampere) as found in cellular and wireless

telephones, palmtop and laptop computers, etc.

The average operating voltage of a lithium-ion or polymer cell is

3.6 V as compared to the 1.2 V of NiCd and NiMH cells. The typical

discharge curve for Lithium cell is shown in Figure 6.

SL01553

100

50

2.0

3.0

4.0

NORMALIZED CELL CAPACITY (%)

Figure 6.

Lithium discharge curve.