User Guide

After Sales
Technical Documentation
Desktop Charger CHH–6
Page 12 Original, 11/94
Circuit Description
Charging the Spare Battery
Charging current for the spare battery is fed via switching transistor V130 and
Schottky diode V131. The transistor is controlled by the processor (BSW
line). When the transistor is on, a constant current is supplied to the battery.
This is the rapid charging mode. In the pulsed charging mode, charging
current is adjusted by pulsing the rapid charge current.
Battery voltage is measured through resistor divider R143/R144 (100/121
kohm). The range measured is 0 to 11 V; a voltage variation of 43 mV can
be detected. For improved resolution, i.e. rapid charge cut off, operational
amplifier N140 is added. This amplifies the battery voltage by a factor of (1
+ 100 kohm/56 kohm) and compares the result to a +5 V reference supply.
The A/D converter receives a battery voltage range of 7.0 to 11 V; a better
resolution of 16 mV results.
Battery size is determined by reading the BSI line state. This is pulled to
+5 V reference voltage by R147 (100 kohm). In the battery pack a ”size”
resistor is connected between BSI and GND.
Temperature is measured over the BTEMP line. This line is pulled to +5 V
reference voltage by R145 (27 kohm). In the battery pack an NTC resistor
is connected between BTEMP and GND.
Because the phone and the spare battery cannot be charged at the same
time the charging current of the phone must be detected. This is done at
the A/D input AN2 (PDET). The charging current for the phone is directed
through series diode V114. If the voltage drop across the diode is higher
than 100 mV (corresponding to an A/D value of 5), charging current is
flowing to the phone.
The Desktop Charger has two dual–colour LEDs to indicate the charging
states of the phone and the spare battery. See ”Charge Indication to
User”.
Discharging the Spare Battery
The Desktop Charger is also provided with a discharge function. This is
activated by pressing a discharge switch on top of the charger. The spare
battery is then discharged before charging. Resistors R181–R188
determine the discharge current which is fed through switching transistor
V134 which is controlled by the processor over the DISW line (pin 64).
The battery is discharged to 5 V and thereafter charged normally. The
state of the discharge switch is read via the DBIN line (pin 63).