A review of balanced charging technology for aerospace lithium-ion
batteries
Lithium-ion batteries represent the development direction of spacecraft
energy storage equipment and are the third generation energy storage device for
spacecraft. It has the advantages of light weight, small size, no memory effect,
and wide adaptability to temperature. It is an alternative to the currently
mainstream nickel-cadmium and nickel-hydrogen batteries. The energy-to-weight
ratio of lithium-ion batteries for aerospace is 90 to 110Wh/kg, which has
obvious advantages over nickel-hydrogen batteries of 45 to 60Wh/kg. However, the
electrochemical characteristics of lithium-ion batteries require that the
charging process must be strictly controlled. Therefore, a special charging
management circuit must be designed to control the charging process of the
spacecraft lithium-ion battery.
Lithium-ion battery charging key technologies
Using lithium-ion batteries to replace nickel-cadmium and nickel-hydrogen
batteries cannot use a simple "plug and play" method. This is because there is
one biggest difference between lithium-ion batteries and nickel-cadmium and
nickel-hydrogen batteries: overcharging of lithium-ion batteries is strictly
prohibited. Therefore, a new charge management circuit must be designed based on
the characteristics of lithium-ion batteries. The key points of the lithium-ion
battery charge management circuit (the main difference from the nickel-cadmium
and nickel-hydrogen battery charge management circuits) mainly include two
aspects: charging method and balanced charging.
In engineering applications, lithium-ion battery cells or battery modules
composed of single cells connected in parallel must be connected in series to
form a battery pack. Therefore, the imbalance of each battery cell or battery
module during the charging process must be considered, and as time goes by, This
imbalance will become more serious, seriously affecting battery life and
reliability. Therefore, balanced charging is also a key technology for
lithium-ion charging.
1Constant current-constant voltage (TApER) type charging control
In satellite power systems using nickel cadmium and nickel hydrogen
batteries, constant current charging is basically used. When the control points
of V-T curve, electronic power, pressure, third electrode and other control
methods are reached, charging is stopped and a charging process is completed. .
Lithium-ion batteries are not suitable for these charging control methods,
because these charging methods cannot guarantee that the final charging voltage
of lithium-ion batteries is always limited to the specified range. Even if the
final charging voltage is guaranteed, charging is often stopped immediately
after reaching the final charging voltage. Lithium-ion batteries still need to
replenish about 30% of their power after reaching the final charging voltage.
Judging from the development of lithium-ion batteries over the years, constant
current-constant voltage charging control is the most common and most suitable
charging control method. In this mode, the charger first performs constant
current charging on the lithium-ion battery. At this time, the battery voltage
gradually increases. When the battery voltage reaches the set value, constant
voltage charging is performed. At this time, the charging current decreases
approximately exponentially, so this This charging method is also called TApER
type charging control.
2Balanced charging technology
Lithium-ion batteries used in aerospace must adopt balanced charging
technology. This view has been fully recognized at home and abroad. Balanced
charging technology mainly solves the electrochemical characteristic deviation
phenomenon of lithium-ion battery cells during the long-term charging process.
Therefore, the advantages and disadvantages of the balanced charging method
require a certain amount of time, money, and manpower investment to be
effectively verified.
Lithium-ion battery equalization charging has not been widely valued and
applied in civilian products because there are few applications of multiple
single cells connected in series and the reliability and life requirements are
not high. In the lithium-ion battery system of electric vehicles, there are a
large number of single cells connected in series. Balanced charging technology
has been adopted. Generally, a single-chip microcomputer system is used to
control and switch on and off the shunt resistor on the single battery to
control the charge amount of the single battery. . This method has complex
control, low efficiency, large heat consumption, and long equilibrium time. This
method was transplanted into early aerospace product solutions, and now domestic
and foreign technicians are exploring more ideal solutions.
The significance of balanced charging is to keep the voltage deviation of
the lithium-ion battery cells within the expected range, thereby ensuring that
each single cell will not be damaged by over-stress shock during the life of the
satellite. Without balanced charging control, the voltage of each single cell
gradually differentiates as the charge and discharge cycles increase.
Under normal circumstances, the deviation of the lithium-ion battery cell
voltage during charging is completely acceptable within 50mV. We can think that
the main reason for the deviation is the difference in charging efficiency and
self-discharge rate of single cells. On the other hand, the influence of the
current consumption of the measuring circuit in a single cell must also be
carefully considered. Sometimes the current consumed by the measuring circuit
has reached the level of the battery's self-discharge current. When doing
lithium-ion battery life experiments, some technicians reported that the first
or last battery pack in series is often damaged first, which is often caused by
the consumption of the measurement circuit.
Charging control circuit
1Bypass charging control
As shown in Figure 1, the solar cell charging array directly charges the
lithium-ion battery pack through diodes during the light period, and each
battery in the battery pack is equipped with a charging bypass circuit. When the
voltage of a certain single cell reaches the set value, the power transistor in
the charging bypass circuit begins to conduct, diverting part of the charging
current to keep the single cell voltage constant within a very narrow range. The
characteristics of the battery determine that the charging current gradually
decreases (approximately exponential law) until the end of the light period.
This charging method can ensure balanced charging of each single cell, but the
power consumption of the bypass circuit is large and the charging current is
difficult to measure.
2 split charging control
The single cell cycle detection circuit samples the voltage of each battery
separately, extracts the maximum value of the single cell voltage through the OR
gate circuit, and compares it with the reference signal in the signal conversion
circuit to generate an error signal. The error signal is sent to the shunt
regulator circuit to control the lithium-ion battery pack. The cell voltage in .
When the voltage of any battery reaches the set value, the average charging
current of the battery pack gradually decreases. If a switching shunt regulator
is used, the charging current will pulsate during constant voltage charging of a
single unit. Therefore, using this charging control method requires the
lithium-ion battery pack to be able to adapt to the pulsating charging
current.
The main error amplifier (MEA) samples the bus voltage signal, generates an
error signal and sends it to the shunt regulator. That is to say, the shunt
regulator is controlled by both the bus voltage and the battery cell voltage.
The shunt lithium-ion battery charging control circuit is shown in Figure 2.
3-step charging control
The single cell cycle inspection and comparison circuit samples the single
cell voltage. If the voltage of any single cell exceeds the set value, the OR
gate circuit will generate an overvoltage signal and disconnect one charging
array through the lock circuit, reducing the charging current by 1/ 3. When an
overvoltage signal is generated again, turn off the second charging array until
the last charging array is turned off. When the pulse load comes or enters the
shadow period, the unlocking circuit generates an unlocking signal, allowing the
charging control circuit to proceed with the next charging process. Obviously,
when charging at constant voltage, the charging current is not approximately
exponential, but decreases step by step. The segmented lithium-ion battery
charging control circuit is shown in Figure 3.
4Single cell peak voltage limiting linear charging control
The single cell cycle detection circuit samples the voltage of each single
cell respectively, extracts the maximum single cell voltage through the OR gate
circuit, and sends it to the voltage limiting control circuit through the signal
conversion circuit. The voltage limiting control circuit controls the
lithium-ion battery pack by dynamically adjusting the impedance of the power
tube. single cell voltage in . When the voltage of any single cell has not
reached the set value, the solar array charges the lithium-ion battery pack
through the power tube in the voltage limiting control circuit with a relatively
stable current, and the impedance of the power tube is close to zero; when any
When the single cell voltage reaches the set value, the impedance of the power
tube gradually increases, and the charging current of the battery pack gradually
decreases. The charging current reduction rule is determined by the
characteristics of the lithium-ion battery pack (approximate exponential law).
The advantage of this circuit is that the charging current continuously
decreases during the constant voltage stage of charging, which is basically an
exponential law. It is more suitable for the charging habits of lithium-ion
batteries, and the power consumption of the charging circuit is not large. The
single battery peak voltage limiting linear charging control circuit is shown in
Figure 4.
Several balanced charging technologies
1Constant shunt resistor balanced charging
The principle of resistive shunt equalization charging is shown in Figure
5.
Each lithium-ion battery cell is connected in parallel with a shunt
resistor. It can be seen from the circuit that the shunt current on the resistor
must be much larger than the self-discharge current of the battery in order to
achieve a balanced charging effect. Generally, the self-discharge current of
lithium-ion batteries is about C/20000, so it is more appropriate for the
current flowing through the shunt resistor to be C/200.
In addition, the deviation of each shunt resistor is also an important
factor affecting the equalization effect. After a certain number of charge and
discharge cycles, the deviation of a single cell can be determined using the
following formula:
V battery voltage deviation = R shunt × I self-discharge + 2 × V single
cell × K resistance deviation
If the shunt resistance is 20Ω±0.05%, the battery voltage deviation can be
controlled within the range of 50mV. The average power of each resistor is
0.72W, but the shunt resistor always consumes power regardless of the battery
charging process or the battery discharging process.
2 on-off shunt resistors for balanced charging
The difference between on-off shunt resistor equalization charging and
resistor shunt equalization charging is the addition of an on-off switch. The
control of this switch can be realized by the microcontroller system software or
by a simple logic circuit. The balancing circuit using this control method only
works during the constant voltage charging period of TApER charging. The on-off
switch is always turned off at other times, so that when the battery pack needs
to be discharged, the shunt resistor does not consume precious energy. During
the light period, the solar cell generates sufficient power. At this time, it is
reasonable for the power system to consume a certain amount of energy in the
balancing circuit. In the LEO track, the working time of this balancing circuit
only accounts for about 10%, so to achieve the balancing effect discussed above,
the resistance value needs to be reduced by 10 times. It can be seen that the
peak thermal power consumption is quite large, which is the characteristic of
this circuit. Major Disadvantages. In addition, the actual effect of the on-off
switch is a fatal failure, so redundant means must be used.
3 switched capacitor equalization charging
The principle of balanced charging of switched capacitors is shown in
Figure 7. It can be seen from the figure that the sequential switch driving
circuit is mainly composed of a clock circuit, which drives multiple switches to
close sequentially, and sequentially connects the lithium-ion battery cells to
the transmission capacitor. Unbalanced energy between single cells achieves the
purpose of balanced charging. At the same time, the voltage of each single cell
is monitored by measuring the voltage on the transfer capacitor. If a
short-circuit fault occurs in a single cell, the low-voltage comparator outputs
a switch prohibition signal and prohibits the short-circuited single cell from
connecting to the transmission capacitor to prevent affecting the normal
operation of other single cells. At the same time, the low voltage of the
battery is sent to the constant current and constant voltage converter. The
alarm signal enables the constant current and constant voltage converter to
determine the correct constant voltage according to the short circuit condition
of the single battery. The biggest advantage of this balancing circuit is
extremely low energy waste. The disadvantage is that the circuit is complex. The
on-state resistance of the multi-channel switch and high common mode limitations
will affect the realization of balanced charging. On the other hand, parameter
selection is difficult. For different power system configurations, circuit
parameters require detailed design and verification, which is detrimental to the
development cycle.
4 step-down converter equalization charging
The buck converter balancing charging scheme is also a low-consumption
balancing scheme. Its idea is very clear. The main circuit is a standard buck
regulator, and multiple sets of identical secondary windings are added to the
energy storage inductor for auxiliary charging of battery cells. Obviously, the
single battery with low voltage will get more energy from the secondary winding,
and the single battery with high voltage will get less energy, thus achieving
the purpose of balanced charging. In order to obtain a good balancing effect,
the consistency of the secondary winding needs to be strictly controlled.
However, the consistency of the inductor winding is very difficult to control,
so this is one of the biggest shortcomings of this control method. The research
on this charging method has just started, and further in-depth research on
charging efficiency, equalization effect, reliability analysis, etc. is
needed.
5 average battery voltage equalization charging
The principle of average battery voltage equalization charging is shown in
Figure 9. The figure only shows the equalization circuit of one single cell. The
other single cells are also equipped with the same equalization circuit. Among
them, the amplifier is powered by the single cell.
The idea of this balanced charging control circuit is: compare the single
cell voltage with the average single cell voltage, and control the power switch
to shunt the single cell whose battery voltage is higher than the average
voltage. Therefore, all cell voltages tend to the average cell voltage under the
action of the balancing circuit.
This circuit appears to be an open-loop control at first glance. In fact,
due to the internal resistance of the battery, the balancing circuit works in a
closed-loop state with negative feedback characteristics. In order to prevent
the balancing circuit from working when the battery pack is discharging, a Zener
diode can be connected in series at the lower end of the power switch. In this
way, when the battery is discharging, the battery voltage is low and the shunt
circuit is lost.
The average battery voltage balancing charging circuit model has been
studied in depth and is considered to be a very effective solution. This circuit
is listed as the preferred solution for LEO orbit lithium-ion battery
applications, and has applied for French and European patents.
Conclusion
The key technologies of the lithium-ion battery charge management circuit
are discussed above: constant current-constant voltage (TApER) charging method
and balanced charging technology. Through comparison, we believe that the
"single battery peak voltage limiting linear" charging control scheme is more
suitable for the use of small satellites, and can avoid the "bypass type"
control of huge heat consumption, the "shunt type" control of huge pulsating
charging current, and the "split type" control of huge pulsating charging
current. The "segmented" constant voltage charging stage has the disadvantage of
discontinuous reduction in charging current; the average battery voltage
balancing charging circuit has strong adaptability and ideal indicators in all
aspects.
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