Features

Safely charges two rechargeable

alkaline batteries such as Re-
newal

from Rayovac

Terminates pulsed charge with

maximum voltage limit

Contains LED charge status out-

put

Features a pin-selectable

low-battery cut-off

Pre-charge qualification indicates

fault condition

Available in 8-pin 300-mil DIP or

150-mil SOIC

General Description

The bq2902 is a low-cost charger for
rechargeable alkaline batteries such
as Renewal

batteries from Rayovac

The bq2902 combines sensitive, full-
charge detection for two recharge-
able alkaline cells, with a low-battery
cut-off for overdischarge protection.

Designed for system integration
into a two-cell system, the bq2902
can improve the service life of the
rechargeable alkaline cells by prop-
erly managing the charge and dis-
charge. The bq2902 requires a
voltage limited current source to
generate the proper charge pulses
for the Renewal

cell. Each cell is

individually monitored to ensure
full charge without a damaging
overcharge.

Charge completion is indicated
when the average charge rate falls
below approximately 3% of the fast
charge rate. A status output is pro-
vided to indicate charge in progress,
charge complete, or fault indication.

The bq2902 avoids over-depleting
the battery by using the internal
end-of-discharge control circuit.
The bq2902 also eliminates the
external power switching transis-
tors needed to separately charge
individual Renewal cells.

For safety, charging is inhibited if
the per-cell voltage is greater than
3.0V during charge (closed-circuit
voltage), or if the cell voltage is less
than 0.4V (open-circuit voltage).

Rechargeable Alkaline Charge/Discharge

Controller IC

bq2902

5/99 C

PN290201.eps

8-Pin Narrow DIP

or SOIC

VSEL

BAT1N

BAT1P

CHG

VSS

LRTN

Charging supply input

CHG

Battery status output

BAT

Battery 1 positive input

BAT

Battery 1 negative input

Battery 2 negative input
IC ground

LRTN

System load return

SEL

End-of-discharge voltage
select input

Pin Names

Pin Connections

Pin Descriptions

DC supply input

This input is used to charge the rechargeable
alkaline cells and power the bq2902 during
charge. To charge the batteries, this input
should be connected to a current-source lim-
ited to 300 mA. If the DC input current is
greater than 300mA, the power dissipation
limits of the package may be exceeded. The
DC input should also be capable of supplying
a minimum of 3.3V and should not exceed
5.5V.

CHG

Charge status

This open-drain output is used to signify the
battery charging status and is valid only
when DC is applied.

SEL

End-of-discharge select input

This three-level input selects the desired
end-of-discharge cut-off voltage for the
bq2902. V

SEL

= BAT

selects an EDV of

1.10V. V

SEL

floating selects EDV = 1.0V. V

SEL

= V

selects EDV = 0.9.V

BAT

Battery 1 positive input

This input connects to the positive terminal
of the battery designated BAT

(see Figure

3). This pin also provides power to the bq2902
when DC is not present.

BAT

Battery 1 negative input

This input connects to the negative terminal
of the battery designated BAT

(see Figure 3).

Battery 2 negative input/IC ground

This input connects to the negative terminal
of the battery designated BAT

(see Figure 3).

LRTN

Load return

This open-drain pull-down output is typi-
cally used as a low-side switch. High-side
load switching is also possible with the addi-
tion of an external P-FET.

Functional Description

Figure 1 is a block diagram outlining the major compo-
nents of the bq2902.

Figure 2 illustrates the charge control and display
status during a bq2902 cycle. Table 1 outlines the vari-

bq2902

BD290201.eps

Control/Status

Logic

VSEL

CHG

BAT1P

BAT1N

LRTN

VSS

Figure 1. Functional Block Diagram

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 (V

OCV

) of each cell for an undervoltage condition

MIN

= 0.4V) and begins a charge cycle when the V

OCV

of all cells is above V

MIN

. If V

OCV

of any cell is below

MIN

, 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 V

OCV

of each cell

is above V

MIN

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 (V

CCV

) of any cell

exceeds 3.0V (V

FLT

) 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 BAT

, 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 BAT

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 BAT

pin.

The bq2902 pulse charges the battery for approximately
10ms of every 12.5ms, when conditions warrant. The
bq2902 measures the open-circuit voltage (V

OCV

) of each

battery cell during the idle period. If a single-cell poten-

bq2902

Charge Action State

Conditions

BAT

Input

CHG

Output

DC absent

< V

BAT1P

Low battery detection per V

SEL

Charge initiation

DC applied

Charge pending/
fault

OCV

< 0.4V

or V

CCV

> 3.0V

sec = Low

sec = Z

Charge pulse

OCV

1.63V before pulse

Charge pulsed @ 80Hz per Figure 2

sec = Low

sec = Z

Pulse skip

OCV

> 1.63V

before pulse

Pulse skipped per Figure 2

sec = Low

sec = Z

Charge complete

Average charge rate falls below 3% of

the fast charge rate

Charge complete

Low

Notes:

OCV

= Open-circuit voltage of each cell between positive and negative leads.

CCV

= Closed-circuit voltage.

Table 1. bq2902 Operational Summary

tial of any battery is above the maximum open-circuit
voltage (V

MAX

= 1.63V

±3%), the following pulses are

skipped until all cell potentials fall below the V

MAX

limit. Charging is terminated when the average charge
rate falls below approximately 3% of the maximum
charge rate. Once charging is terminated, the internal
charging FET remains off, and the CHG output becomes
active per Table 1 and Figure 2. With DC applied, the
internal discharge FET will always remain on.

End-of-Discharge Control

When DC is not present or less than the voltage present
on the BAT

pin, the bq2902 power is supplied by the

voltage present at the BAT

pin. In this state, the bat-

teries discharge down to the level determined by the
V

SEL

pin. The bq2902 monitors the cell voltage of the re-

chargeable alkaline cells.

If the voltage across any cell is below the voltage speci-
fied by the V

SEL

input, the bq2902 disconnects the bat-

tery stack from the load by turning the internal
discharge FET off. The discharge FET remains off until
either the batteries are replaced or DC is reapplied, ini-
tiating a new charge cycle. After disconnecting the bat-
tery stack from the load, the standby current in the
bq2902 is reduced to less than 1

µA. The end-of-

discharge voltage (V

EDV

) is selectable by connecting the

SEL

pin as outlined in Table 2. Typically, higher dis-

charge loads (>200mA) should use a lower discharge
voltage cut-off to maximize battery capacity.

bq2902

End-of-Discharge Voltage

Pin Connection

1.10V

SEL

= BAT

1.00V

SEL

= Z

0.90V

SEL

= V

Table 2. bq2902 EDV Selections

TD290201.eps

DC Valid

BAT1P

Charge
Complete

12.5ms

Notes:

Pending

Charging 1

CHG

1. Charging: 0.4 < VOCV 1.63V, VCCV 3.0V.
2. Pulses skipped when VOCV > 1.63V.
3. Charge complete when average charge falls below 3% of fast charge rate

2/3 sec.

1/6 sec.

10ms

Figure 2. bq2902 Application Diagram

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