BQ2902 Datasheet, PDF (3/12 Pages) Texas Instruments – Rechargeable Alkaline Charge/Discharge Controller IC

bq2902
ous operational states and their associated conditions
which are described in detail in the following section.
Figure 3 is an application example.
Charge Initiation
The bq2902 always initiates and performs a charge cycle
whenever a valid DC input is applied. A charge cycle
consists of pulse charging the battery and then checking
for a termination condition. The charging section ex-
plains charging in greater detail.
Charge Pre-Qualification
After DC is applied, the bq2902 checks the open-circuit
voltage (VOCV) of each cell for an undervoltage condition
(VMIN = 0.4V) and begins a charge cycle when the VOCV
of all cells is above VMIN. If VOCV of any cell is below
VMIN, the bq2902 enters a charge-pending mode and in-
dicates a fault condition (see Table 1). The bq2902 re-
mains in a charge-pending mode until VOCV of each cell
is above VMIN.
Charge Termination
Once a charge cycle begins, the bq2902 terminates
charge when the average charge rate falls below 3% of
the maximum charge rate. The bq2902 also terminates
charge when the closed-circuit voltage (VCCV) of any cell
exceeds 3.0V (VFLT) during charge and indicates a fault
condition on the CHG output (see Table 1).
Charge Re-Initiation
If DC remains valid, the bq2902 suspends all charge
activity after full-charge termination. A charge cycle is
re-initiated when all cell potentials fall below 1.4V. The
rechargeable alkaline cells, unlike other rechargeable
chemistries, do not require a maintenance charge to
keep the cells in a fully charged state. The self-discharge
rate for the Renewal cells is typically 4% per year at
room temperature.
Charge Status Indication
Table 1 and Figure 2 outline the various charge action
states and the associated BAT1P, and CHG output
states. The charge status output is designed to work
with an LED indicator. In all cases, if DC is not present
at the DC pin, or if the DC supply is less than the volt-
age at the BAT1P pin, the CHG output is held in a high-
impedance condition.
Charging
The bq2902 controls charging by periodically connecting
the DC current-source to the battery stack, not to the in-
dividual battery cells. The charge current is pulsed
from the internal clock at approximately a 80Hz rate on
the BAT1P pin.
The bq2902 pulse charges the battery for approximately
10ms of every 12.5ms, when conditions warrant. The
bq2902 measures the open-circuit voltage (VOCV) of each
battery cell during the idle period. If a single-cell poten-
Table 1. bq2902 Operational Summary
Charge Action State
DC absent
Charge initiation
Charge pending/
fault
Conditions
VDC < VBAT1P
DC applied
VOCV < 0.4V¹ or VCCV > 3.0V²
BAT1P Input
Low battery detection per VSEL
-
-
Charge pulse
VOCV ≤ 1.63V before pulse
Charge pulsed @ 80Hz per Figure 2
Pulse skip
Charge complete
VOCV > 1.63V
before pulse
Average charge rate falls below 3% of
the fast charge rate
Pulse skipped per Figure 2
Charge complete
Notes:
1. VOCV = Open-circuit voltage of each cell between positive and negative leads.
2. VCCV = Closed-circuit voltage.
CHG
Output
Z
-
2
3
sec
=
Low
2
3
sec
=
Z
1
6
sec
=
Low
1
6
sec
=
Z
1
6
sec
=
Low
1
6
sec
=
Z
Low
3