5

ICS1700A

ICS1700A

The discharge current pulse amplitude is typically set to about 2.5

times the amplitude of the charging current based on 1.4V/cell. For

example, if the charge current is 4 amps, then the discharge current

is set at about 10 amps. The energy removed during the discharge

pulse is a fixed ratio to the positive charge rate. The amplitude of

the discharge pulse does not affect the operation of the part as

described in this section.

A voltage acquisition window immediately follows a brief rest time

after the discharge pulse. No charge is applied during the rest time

or during the acquisition window to allow the cell chemistry to

settle. Since no current is flowing, the measured cell voltage is not

obscured by any internal or external IR drops or distortions caused

by excess plate surface charge. The ICS1700A makes one

continuous reading of the no-load battery voltage during the entire

acquisition window. The voltage that is measured during this

window contains less noise and is a more accurate representation

of the true state of charge of the battery.

Topping Charge

The third stage is a topping charge that applies current at a rate low

enough to prevent cell heating but high enough to ensure a full

charge.

The topping charge applies a C/10 charging current for two hours.

The current consists of the same pulse technique used during the

fast charge stage; however, the duty cycle of the pulse sequence

has been extended as shown in Figure 5. Extending the time

between charge pulses allows the same charging current used in the

fast charge stage so that no changes to the current source are

necessary. For example, the same charge pulse that occurs every

second at a 2C fast charge rate will occur every 20 seconds for a

topping charge rate of C/10. The CMN indicator flashes at a one

second rate during this stage.

Maintenance Charge

The maintenance charge is intended to offset the natural self-

discharge of NiCd or NiMH batteries by keeping the cells primed

at peak charge. After the topping charge ends, the ICS1700A

begins this charge stage by extending the duty cycle of the applied

current pulses to a C/40 rate. The maintenance charge will last for

as long as the battery voltage is greater than 0.5V at the VIN pin.

The CMN indicator flashes at a one second rate during this stage.

Figure 5: Representative timing diagram for topping and maintenance charge

delay time

time

cycle

time

cycle