Lithium-Ion Batteries

The bq2000 uses a two-phase fast-charge algorithm for

Li-Ion batteries (Figure 3). In phase one, the bq2000

bq2000 then moves to phase two, regulates the battery

charge cycle is started if the cell voltage falls below the

During the current-regulation phase, the bq2000

monitors charge time, battery temperature, and battery

voltage for adherence to the termination criteria. During

the final constant-voltage stage, in addition to the

charge time and temperature, it monitors the charge

current as a termination criterion. There is no

post-charge maintenance mode for Li-Ion batteries.

Charge Termination

Maximum Charge Time (NiCD, NiMH, and

Li-Ion)

The bq2000 sets the maximum charge-time through pin

RC. With the proper selection of external resistor and

capacitor, various time-out values may be achieved. Fig-

ure 4 shows a typical connection.

The following equation shows the relationship between

time (MTO) for the bq2000:

other features of the device. See Tables 2 and 3 for de-

tails.)

For Li-Ion cells, the bq2000 resets the MTO when the

battery reaches the constant-voltage phase of the

charge. This feature provides the additional charge time

required for Li-Ion cells.

Maximum Temperature (NiCd, NiMH, Li-Ion)

placed in thermal contact with the battery, may be used

as a temperature-sensing device. Figure 5 shows a typi-

cal temperature-sensing circuit.

During fast charge, the bq2000 compares the battery

temperature to an internal high-temperature cutoff

termination occurs when voltage at pin TS is less than

this threshold.

Peak Voltage (NiCd, NiMH)

The bq2000 uses a peak-voltage detection (PVD) scheme

to terminate fast charge for NiCd and NiMH batteries.

The bq2000 continuously samples the voltage on the

BAT pin, representing the battery voltage, and triggers

the peak detection feature if this value falls below the

maximum sampled value by as much as 3.8mV (PVD).

As shown in Figure 6, a resistor voltage-divider between

battery voltage measured at pin BAT.

R

R

N

V

V

B1

B2

CELL

MCV

=∗











 − 1

manufacturer-specified charging voltage. The end-to-

end input impedance of this resistive divider network

should be at least 200k

Ω and no more than 1MΩ.

A NiCd or NiMH battery pack consisting of N series-

In a mixed-chemistry design, a common voltage-divider

is used as long as the maximum charge voltage of the

5

Battery Chemistry

Charge Algorithm

NiCd or NiMH

1. Charge qualification

2. Trickle charge, if required

3. Fast charge (constant current)

4. Charge termination (peak voltage, maximum charge time)

5. Top-off (optional)

6. Trickle charge

Li-Ion

1. Charge qualification

2. Trickle charge, if required

3. Two-step fast charge (constant current followed by constant voltage)

4. Charge termination (minimum current, maximum charge time)

Table 1. Charge Algorithm

bq2000