Philips Semiconductors

Product data

NE57600

One-cell Lithium-ion battery protection with

over/undercharge and overcurrent protection

2001 Oct 03

7

Charging Lithium Cells

The lithium cells must be charged with a dedicated charging IC such

as the NE57600. These dedicated charging ICs perform a

current-limited, constant-voltage charge, as shown in Figure 7.

The charger IC begins charging with a current that is typically the

rating of the cell (1C) or the milliampere rating of the cell. As the cell

the cell decreases, and the charger IC provides a constant voltage.

When the charge current falls below a preset amount, 50 mA for

example, the charge is discontinued.

If charging is begun below the overdischarged voltage rating of the

cell, it is important to slowly raise the cell voltage up to this

overdischarged voltage level. This is done by a reconditioning

charge. A small amount of current is provided to the cell (50 mA for

example), and the cell voltage is allowed a period of time to rise to

the overdischarged voltage. If the cell voltage recovers, then a

normal charging sequence can begin. If the cell does not reach the

overdischarged voltage level, then the cell is too damaged to charge

and the charge is discontinued.

To take advantage of the larger energy density of lithium cells it is

important to allow enough time to completely charge the cell . When

the charger switches from constant current to constant voltage

charge (Point B, Figure 7) the cell only contains about 80 percent of

its full capacity. When the cell is 100 mV less than its full rated

charge voltage the capacity contained within the cell is 95 percent.

Hence, allowing the cell to slowly complete its charge takes

advantage of the larger capacity of the lithium cells.

SL01554

1.0

0.5

1.0

2.0

3.0

4.0

1.0

2.0

Point B

Vov

TIME (HOURS)

TIME (HOURS)

CONSTANT

VOLTAGE

CONSTANT

CURRENT

Figure 7.

Lithium cell charging curves.