explained in later sections. Following fast charge, the
battery is topped off, if top-off is selected. The charg-
ing cycle ends with a trickle maintenance-charge that
continues as long as the voltage on pin BAT remains
below VMCV.
Lithium-Ion Batteries
The bq2000T uses a two-phase fast-charge algorithm
for Li-Ion batteries (Figure 3). In phase one, the
bq2000T regulates constant current until VBAT rises to
VMCV. The bq2000T then moves to phase two, regu-
lates the battery with constant voltage of VMCV, and
terminates when the charging current falls below the
IMIN threshold. A new charge cycle is started if the
cell voltage falls below the VRCH threshold.
During the current-regulation phase, the bq2000T
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 bq2000T sets the maximum charge-time through
pin RC. With the proper selection of external resistor
and capacitor, various time-out values may be
achieved. Figure 4 shows a typical connection.
The following equation shows the relationship be-
tween the RMTO and CMTO values and the maximum
charge time (MTO) for the bq2000T:
MTO=RMTO ∗ CMTO ∗ 35,988
MTO is measured in minutes, RMTO in ohms, and
CMTO in farads. (Note: RMTO and CMTO values also de-
termine other features of the device. See Tables 2
and 3 for details.)
For Li-Ion cells, the bq2000T 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)
A negative-coefficient thermistor, referenced to VSS
and placed in thermal contact with the battery, may
be used as a temperature-sensing device. Figure 5
shows a typical temperature-sensing circuit.
During fast charge, the bq2000T compares the bat-
tery temperature to an internal high-temperature
cutoff threshold, VTCO. As shown in Table 4, high-tem-
perature termination occurs when voltage at pin TS
is less than this threshold.
∆T/∆t (NiCd, NiMH)
When fast charging, the bq2000T monitors the volt-
age at pin TS for rate of temperature change detec-
tion, ∆T/∆t. The bq2000T samples the voltage at the
TS pin every 16s and compares it to the value mea-
sured 2 samples earlier. This feature terminates fast
charge if this voltage declines at a rate of
V
161
V
Min
CC
Figure 5 shows a typical connection diagram.
Minimum Current (Li-Ion Only)
The bq2000T monitors the charging current during
the voltage-regulation phase of Li-Ion batteries. Fast
charge is terminated when the current is tapered off
to 7% of the maximum charging current. Please note
that this threshold is different for the bq2000.
Initial Hold-Off Period
5
Battery Chemistry
Charge Algorithm
NiCd or NiMH
1. Charge qualification
2. Trickle charge, if required
3. Fast charge (constant current)
4. Charge termination (∆T/∆t, 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, time)
Table 1. Charge Algorithm
bq2000T