IEEE - Aerospace and Electronic Systems - January 2022 - 50

Power System Design and Performance for Low Earth Orbit Spacecraft
Figure 8.
BCR Block Diagram.
BATTERY CHARGE REGULATOR
The function of the Battery charge regulator (BCR) is to
convert the raw fluctuating power from the solar arrays
into a regulated, practical output power. In order to harness
as much of the power from the solar arrays as possible
the BCRs have been made as efficient as possible,
although care has been taken not to compromise output
voltage ripple with efficiency.
BATTERY CHARGE REGULATOR OPERATING
The MPP of the solar panel is tracked by the BCR by
mean of temperature compensation thermistors that are
housed on the rear of each solar panel. The solar panels
are more exposed to the huge temperature variations in
orbit than practically any other part of the spacecraft, the
MPP can vary wildly over the temperature range. With
this in mind, it is essential that the MPP is tracked by the
BCR for optimum efficiency [9].
The BCR has two working modes:
■ Current Mode (MPP tracking mode).
■ Voltage Mode (EoC voltage tracking mode).
The BCR operates in current Mode when the battery is in
state of being discharged. In current mode, the BCR tracks
(predicts with temperature) the MPP (or Vmpp)ofthe solar
panels and all the available array power is channelled into
charging the battery and meeting various spacecraft power
requirements. Once the BCR has determined that the battery
has reached its EoC voltage, the BCR backs offthe current to
50
MPPT METHOD
An alternative method of tracking the MPP of the solar
arrays and the EoC voltage of the battery is by using
hardware control also known as Computer Control
(CC). This gives the on board computer (OBC) the
facility to over-ride the control hardware of the BCR
and directly control the BCR pulsewidth modulation
(PWM). The advantage of software control is that it
compensates for solar array and battery degradation,
therefore optimising efficiency over the lifetime of the
spacecraft. The hardware control is, however, essential
for fully autonomous operation of the spacecraft power
system. The power supply of the IC TL494ID is provided
by start-up circuitry [10].
The BCRs performs MPPT to maximise the power
from the array to charge the battery, when the battery is
not fully charged. The BCRs use temperature compensation
data from PT2000 thermistors on the solar arrays to
track the MPP as shown in Figure 9.
IEEE A&E SYSTEMS MAGAZINE
JANUARY 2022
the battery and keeping the voltage constant, initiates voltage
mode operation. The BCR continues backing off the current
to the battery until the battery is fully charged. The BCR is
now considered to be operating in voltage mode, adjusting
the solar arrays operating voltage point between Vmpp and
Voc (open circuit voltage), thereby avoiding the MPP match,
reducing the input power to the BCR. The EoC voltage is
indirectly proportional to the temperature of the battery. The
BCR tracks the temperature ofthe battery. The block diagram
ofthe BCR is illustrated in the Figure 8.
IEEE - Aerospace and Electronic Systems - January 2022

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