Revision B 5-17
CPU Theory of Operation: Theory of Operation
MAC 5000 resting ECG analysis system
2000657-002
avoid premature termination, which would give a false "full" reading on
the gas gauge).
Battery Pack
The MAC 5000 uses a 15-cell nickel metal hydride (NiMH) battery pack
with integral thermal sensor for charge termination detection and self
resetting fuse for short circuit protection. Charge current and normal
system operating power are obtained from the AC power supply. The
charger circuitry monitors both battery charge current and power
supply output current. The battery is always charged at the maximum
rate possible but system power demands take precedence over charger
demands. The charger automatically reduces charge current as required
to keep the AC power supply output current within specified limits. In
the extreme (during printing) charging ceases and energy is taken from
the battery to meet peak system demands. When system power draw
declines, all excess power supply capacity is once again delivered to the
battery.
Battery Thermistor
The thermal sensor is used by Moe for termination of charge. During
normal charge, the electrical energy obtained from the power supply is
stored in chemical reactions in the battery. When the battery reaches
full charge there are no more reactants available in which to store
chemical energy and the supplied charge power is converted directly to
heat. The sudden rise in pack temperature caused by this release of heat
is an indicator of full charge. When the rate of pack temperature rise
exceeds a certain threshold, charge is terminated. This is the only
normal charge termination mechanism, though abnormal conditions
(battery or ambient temperatures beyond spec or excessive pack
voltage) may also terminate charge. Once fully charged, the battery is
maintained by low duty cycle charge current pulses.
Charge Light
Moe provides power to the amber charge light in the power supply
compartment. Moe communicates the current battery/charger state via
this light.
Four conditions may be indicated:
■ Battery charged (light is off)
■ Battery needs charge (light blinks once per second)
■ Battery is critically low (light flashes furiously)
■ Battery is charging (light on continuously)
Battery Gauge
Current flow into and out of the battery pack is monitored by Moe via a
MAX472 current sensor (IMON). By integrating the current flow, Moe is
able to maintain a reasonable estimate of the battery pack’s charge. The
current sensor hasn’t sufficient dynamic range to cover the full scale of
system current draw so some compromises are made. The very highest
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