4

ICS1708

Fast Charge

In the second stage, the ICS1708 applies the charging current in a repetitive sequence consisting of positive current charging pulses followed

by a rest time and voltage acquisition time. The cycle, shown with charge, rest, and data acquisition periods in Figure 4, repeats every 1.1

seconds until the battery is fully charged.

Figure 4: Charge cycle showing fast charge current pulses

The amplitude of the current pulse is determined by system parameters such as the current capability of the charging system, the desired

recharge time, battery capacity and the ability of the battery to accept the charge current in the charging environment. The ICS1708 can be

set for three (3) user-selectable fast charge rates: 15 minutes (4C), 60 minutes (1C), or 150 minutes (C/2.5). The CMN indicator is a

continuous active low during this stage. The charge rate is determined by the voltage level on the CMN indicator when the ICS1708 is first

powered-up.

A voltage acquisition window immediately follows a brief rest time after the charge pulse. No charge is applied during the rest time or

during the acquisition window to allow the battery chemistry to settle. Since no current is flowing, the measured cell voltage is not obscured

by any internal or external IR drops or noise. The ICS1708 takes samples of battery voltage during the acquisition window. The voltage

samples are averaged for comparisons to previous and subsequent averages. This method provides a most 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 equalize cells in a

multiple cell packs. The topping stage is not required before putting the battery into immediate service after fast charge completes.

The topping charge is applied for a minimum of two (2) hours. The current consists of the same pulse technique used during the fast charge

stage; however, the delay time is extended as shown in Figure 5. Extending the delay time between charge pulses allows the same charging

current used as 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 1C (1 hour) fast charge rate will occur every eleven (11) seconds for a topping charge rate of C/11. The MMN

indicator is a continuous active low during the topping and maintenance charge stages.

Figure 5: Representative timing diagram for topping and maintenance charge

Maintenance Charge

The maintenance charge is intended to offset the natural self-discharge of NiCd or NiMH batteries by keeping their cells primed at peak

charge. After the topping charge ends, the ICS1708 begins this charge stage by once again extending the delay time between charge pulses.

The maintenance charge will last for as long as the divided down battery voltage is greater than 0.5V at VIN pin 7. The MMN indicator is a

continuous active low during this stage.

fast charge pulse width

acquisition time

rest time

voltage

cycle time

delay time

time

cycle

time

cycle