E - 8 Desktop fast charger June 2003 IPN: M5000-00-105
The discharge circuit is based around a
constant current sink. This uses an N-channel
MOSFET, Q6, controlled by an operational
amplifier, IC3:B. R41 and R42 are the current
sense resistors that allow the op amp to set the
current. The nominal discharge current is
400 mA.
The microprocessor needs to be able to
monitor the battery voltage. A coarse voltage is
provided by the voltage divider R48, R49 and
R50. This voltage divider brings the range of
battery voltage into the same range as the
microprocessor A/D input (0-5 V). This
enables the microprocessor to monitor the
general battery voltage. An expanded battery
voltage interface is based around operational
amplifier IC3:C. It expands a small window of
battery voltage over the range of the micro-
processor A/D input. This enables the micro-
processor to detect small changes in battery
voltage and accurately pick when the battery
voltage drops below peak.
The interface to the battery is through four
contacts, positive (+BATT), temperature
(TEMP), capacity (CAP) and ground (GND).
The positive and ground contacts are the main
connections for charging. In the battery pack,
the temperature contact is connected to
ground via a thermistor. This allows the charg-
er to monitor the temperature of the battery
and if the battery temperature is outside the
range +5°C to +40°C, the charger will not
charge the battery. The charger also uses the
temperature line for detecting the presence of
a battery; for example, detecting whether the
battery has been inserted or removed from the
charger. In the battery pack, the capacity
contact is connected to ground via a resistor. If
there is a capacitor in parallel with the capacity
resistor, then the resulting time constant
indicates to the charger that the battery pack is
NiMH. Consequently, battery packs that do
not have a capacitor in parallel with the capac-
ity resistor are considered to be NiCd.
Software operation
The charging sequence is as follows.
■ Battery discharge (optional);
■ Battery flat test;
■ Battery type detection;
■ Battery open circuit test;
■ Battery short circuit test;
■ Battery temperature test;
■ Rapid charge, with multiple end-of-charge
detection methods;
■ Rest;
■ Trickle charge (optional);
■ Standby charge;
■ Maintenance charge.
If during trickle or standby charge the battery
voltage falls to the discharged battery thresh-
old, the charger restarts the charge cycle as if
the battery has just been inserted.
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