NiCd/NiMH Battery

Fast-Charge Controllers

8

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_______________Detailed Description

The MAX712/MAX713 fast charge NiMH or NiCd batter-

ies by forcing a constant current into the battery. The

MAX712/MAX713 are always in one of two states: fast

charge or trickle charge. During fast charge, the

current level is high; once full charge is detected, the

current reduces to trickle charge. The device monitors

three variables to determine when the battery reaches

full charge: voltage slope, battery temperature, and

charge time.

Figure 1 shows the block diagram for the MAX712/

MAX713. The timer, voltage-slope detection, and temper-

ature comparators are used to determine full charge

state. The voltage and current regulator controls output

voltage and current, and senses battery presence.

Figure 2 shows a typical charging scenario with batteries

already inserted before power is applied. At time 1, the

MAX712/MAX713 draw negligible power from the bat-

tery. When power is applied to DC IN (time 2), the

power-on reset circuit (see the

POWER

-

_ON

-

_RESET sig-

nal in Figure 1) holds the MAX712/MAX713 in trickle

charge. Once

POWER

-

_ON

-

_RESET goes high, the device

enters the fast-charge state (time 3) as long as the cell

voltage is above the undervoltage lockout (UVLO) volt-

age (0.4V per cell). Fast charging cannot start until (bat-

tery voltage) / (number of cells) exceeds 0.4V.

When the cell voltage slope becomes negative, fast

charge is terminated and the MAX712/MAX713 revert

to trickle-charge state (time 4). When power is removed

(time 5), the device draws negligible current from the

battery.

Figure 3 shows a typical charging event using tempera-

ture full-charge detection. In the case shown, the bat-

tery pack is too cold for fast charging (for instance,

brought in from a cold outside environment). During

time 2, the MAX712/MAX713 remain in trickle-charge

state. Once a safe temperature is reached (time 3), fast

charge starts. When the battery temperature exceeds

the limit set by THI, the MAX712/MAX713 revert to trick-

le charge (time 4).

0

A

mA

µA

1

0.4

TIME

VOLTAGE

1.4

1.5

1.3

2

4

5

3

1. NO POWER TO CHARGER

2. CELL VOLTAGE LESS THAN 0.4V

3. FAST CHARGE

4. TRICKLE CHARGE

5. CHARGER POWER REMOVED

TEMPERATURE

Figure 2. Typical Charging Using Voltage Slope

A

mA

µA

1

TIME

TLO

THI

24

3

1. NO POWER TO CHARGER

2. CELL TEMPERATURE TOO LOW

3. FAST CHARGE

4. TRICKLE CHARGE

Figure 3. Typical Charging Using Temperature

A

1.5

1.4

1.3

mA

µA

1

TIME

VREF = VLIMIT

24

3

1. BATTERY NOT INSERTED

2. FAST CHARGE

3. TRICKLE CHARGE

4. BATTERY REMOVED

Figure 4. Typical Charging with Battery Insertion