Hill-Rom VersaCare P3200, Service Manual

Page: 127 / 379

Electrical System
Chapter 3: Theory of Operation
power components of the circuit board and secondarily provides for mounting
the assembly inside the box. The PACM supplies the following functions:
• Battery charger
• Battery gas gauge
• Deployment control
• Enable switch logic & relay
• Drive motor over-temperature protection
• Battery shut-off
Battery Charger
The battery charger operates from a nominal 40 VDC produced by the P3200
bed whenever the bed is plugged in. First, a voltage regulator provides a
regulated 34 VDC supply. The 34 VDC supply powers the charger. This
charger has three modes of operation: trickle charge, bulk charge, and float
charge.
The trickle charge mode produces a current of 20 to 30 milliamps to bring the
battery voltage up to 22.5 volts. In the event a battery has a defective cell, this
low level current will not produce a hazardous situation. If the battery is
capable of taking a charge, its voltage will eventually come up to 22.5V. (If the
battery has been deeply discharged, this may take several days.) Once the
voltage is above the trickle threshold, the charger circuit changes to mode 2. In
this mode, bulk charging occurs during which current is limited to a constant
level. The charger stays in this mode until the battery voltage is approximately
29 - 30 volts. At this point the current slowly drops off as the battery nears full
charge. When the current drops to 10% of the constant current level, the
charger enters mode 3, the float charge mode. In this mode the charger output
drops to 27.8 volts to keep the battery topped off. A STATUS output of the
charger goes high indicating that charging is complete.
The 34 VDC powers the PACM's logic which, upon sensing correct battery
polarity, energizes the battery relay, thereby connecting the battery to the
charger and to the IntelliDrive® System circuitry.
Battery Gas Gauge
The battery gas gauge monitors the voltage across a low value resistor in series
with the battery as a function of time, interpreting positive voltages as current
into the battery (charging) and negative voltages as current out of the battery
(discharging). The gas gauge indicates battery capacity on five output signals,
each representing approximately 20% of the nominal battery capacity.
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VersaCare™ Bed Service Manual (MAN333 REV 2) Page 3 - 13