Portable
PLUS
Computer
Functional
Description
5·17
A2CRSl, A2CRS2,
and
A2CSI form a dc rectifier for
A2USO
. This prevents
A2USO
from being
powered by the battery
when
a recharger is
not
connected to the computer.
If
A2USO
were powered
from
the
main dc rectifier
and
the recharger disconnected,
A2QSO
would stay
on-operating
in re-
verse mode.
Current
would flow from
VBAT
through
A2QSO
to the main dc rectifier, powering
A2USO, which would keep
A2QSO
turned
on
.
A2QSl
is in series with the resistor-divider string,
which
provides the feedback for the
battery
charger
regulator.
When
a recharger
is
connected
and
providing power, the voltage
at
A2CSI
turns
on
A2QSl
after being divided by A2R118
and
A2R60.
When
the recharger is disconnected,
A2QSl
turns
off so
that
the
feedback resistor divider string does
not
load the battery.
A2F1 is connected in series
with
the
battery for safety.
If
shorted, the battery can provide a current of
more
than
100A,
but
the fuse limits this to a safe value. A2CR68 is included in the circuit to protect
the
computer
circuitry
if
the battery is connected backwards.
If
this happens, current flows for a
short
time through A2CR68,
and
VBAT
is clamped to the forward diode voltage below ground. Eventually
the
fuse
A2Fl
blows.
Jumper
A2Jll
acts as a switch for the battery.
When
a
shunt
is placed between pins 1
and
2,
the
battery
is connected.
If
the
shunt
is
moved
to pins 2
and
3, the battery is disconnected
and
VBAT
is
shorted
to
ground
.
If
a recharger is connected
when
VBAT
is
shorted
to ground,
the
battery charger
circuit enters the low-current charge mode,
and
power dissipation is minimized.
Constant-voltage charging provides the fastest battery recharge. A battery charged this
way
accepts
the
most
charge current
when
its voltage is low. During battery recharging, charge current tapers off
exponentially
until
100% charge is reached.
The
computer's
battery charger regulator attempts to constant-voltage charge the battery,
but
this does
not
occur during
much
of
the charge cycle. The computer's recharger
has
an
output
impedance
of
about
11
ohms, so as its
output
current goes up, its
output
voltage goes
down
. A low battery accepts
much
more
current
at
the
desired charge voltage
than
the
recharger is able to supply
under
normal
line conditions. The
net
effect
is
that
the recharger is unable to pull
up
the battery voltage to
the
desired
point
until
the
battery
is
about
80% charged. Thus, during most of
the
charge cycle,
the
opera-
tion
of
the systein is more like constant-current charging
than
constant-voltage charging. The
battery
voltage starts
out
below the target charge voltage
and
gradually increases as the charge level increases.
When
the
80% charge level is reached, the charge current
has
tapered off to the point
where
the
recharger
can
provide
enough
current
at
the
desired
battery
voltage. From this point
on
the
battery
is
constant-voltage charged
by
the
battery
charger regulator.
The
PPU
maintains a "fuel gauge" variable
that
estimates the state
of
the
battery charge
at
any
time.
This variable is displayed for
the
user
in
the
main PAM screen. A displayed value
of
100% represents
2.S ampere-hours, the full capacity
of
the
battery.
No
actual
measurement
of
battery capacity is
made
by
the
PPU
in
the
maintenance of this variable, however.
When
no
recharger
is
connected,
the
PPU
decrements
the
fuel gauge value
at
various rates
that
correspond to
the
currents
that
the
computer
is
drawing
from
the
battery. A series
of
constants
is
provided
which
represent battery current drain for
the
computer
when
it
is
in
sleep mode,
awake
mode,
when
the serial interface is
in
use,
and
so forth.
When
a recharger is connected,
the
PPU
continues to decrement
the
fuel gauge according to
battery
current drain,
but
it also
adds
a constant
that
corresponds to charge current provided
by
the
battery
charger. Battery charger current is usually
more
than
the
battery
drain current, so the fuel gauge incre-
ments
when
a recharger is connected.
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