2 - APPLICATION CONTEXT AND PRINCIPLE OF OPERATION

The Li-Ion Battery

Rechargeable battery using Lithium have high voltage, big capacity and light weight, yielding an extraor-

dinary energy density, more than twice the one of the NiCd. The maximum load current is not as high as

for the NiCd, but is still sufficient for many applications such as cellular phone and camcorder.

To improve lifetime and keep the battery in safe operating conditions, some protection circuitry is always

added inside the battery pack that can disconnect the electrochemical cell from the external connectors.

This protection circuitry is designed to trigger in case of overcurrent (both when charging and discharging),

overvoltage (when charging) and undervoltage (when discharging). The cell temperature is also monitored.

Charging Principle

The charging principle of the Lithium-Ion batteries is very different from the Nickel type. Figure 2 shows the

different stages in the charging process. Time values are only indicative and depend on battery type and

speed of charge.

Cell voltage rises

to voltabe limit

Maximum cell voltage

is reached

Occasional topping charge is

applied if battery voltage drops

15 30 45 60 75

90 105 120 135 150 165 180 minutes of charge

Maximum charge current

is applied until the set

voltage limit is reached.

Charge current starts

to drop as the battery

gets saturated.

The Li-ion battery cannot

absorb over-charge. Trickle

charge would be harmfull.

CONSTANT

CURRENT CHARGE

CONSTANT

VOLTAGE CHARGE

NO TRICKLE

CHARGING

1h

2h

100% charged

70% charged

I

V

Figure 2 : Li-Ion Charging Scheme

During Stage 1, the battery is charged at constant current. The battery voltage is slowly increasing from

original voltage up to the maximum cell voltage, which depends on the battery electrode technology

(4.2V/cell for coke electrode, 4.1V/cell for graphite electrode).

Should this maximum voltage be exceeded and the battery could suffer significant damage and the

protection circuitry may trigger.

Thus during Stage 2 a constant voltage charge is applied. Battery chargers manufacturers recommend a

highly precise voltage supervision of

±0.05 V/cell.

The current is slowly decreasing as the battery gets closer to full capacity.

End of charge can be detected by the charge current getting lower than a fixed threshold value (usually

one fifth to one tenth of the constant current charge value).

The dual Li-Ion Batteries charger :

In this application which requires ever increasing performances in more and more reduced space, the

silicon triplet VIPER20, ST62T25C and TSM102A provides an attractive solution in terms of performance,

cost efficiency and versatility.

AN1051 - APPLICATION NOTE

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