The capacitance method balancing relay works in a continuous rotation mode,
and the auxiliary battery method balancing relay works in a forward and reverse
positioning mode.
The battery era is coming. Whether it is the new energy vehicle industry or
the distributed energy storage industry, the use of batteries has exploded. Due
to the low voltage of a single battery, batteries need to be cascaded in various
applications to increase the total voltage. However, due to the performance
differences of the battery cells, the battery pack will inevitably have
deviations in the voltage of each cascade during use. Concepts come with
applications.
Battery pack balancing is one of the management strategies of BMS (battery
management system). Due to cost considerations, although there are many types of
existing balancing schemes, the simplest lossy discharge method is often used in
application cases, and the efficiency is not high. high. Most of the current
balancing schemes are based on the premise that the consistency of the single
cells in the battery pack is very high. Considering the cascade utilization of
secondary batteries in the future, it is necessary to design better, simpler and
more effective balancing schemes and strategies.
Looking at the current equalization solutions, the resistor discharge
method is the simplest, but has the disadvantage of low efficiency and lossy
equalization; solutions such as inductors, transformers, and voltage boosters
have complex circuits and high costs; an electromagnetic relay group plus
capacitor solution (see figure) 1), its advantages are low cost, simple circuit,
and lossless equalization. Its disadvantages are low conversion frequency and
the risk of short circuit when the electrical contacts are melted.
In Figure 1, assuming that the balancing command is executed, the relay
corresponding to the battery with the highest voltage among the five single
cells is first closed, and the capacitor is charged; the relay is disconnected,
and then the relay corresponding to the battery with the lowest voltage is
selected to be closed, the capacitor is discharged, and the relay is
disconnected. Turn on to complete a balancing cycle. As we all know,
electromagnetic relays are not suitable for frequent switching situations, so
the theoretical equilibrium effect of Figure 1
The rate will be very low. If the electromagnetic relay is changed to an
electronic switch, due to the small voltage difference between batteries, the
electronic switch itself has a certain voltage drop, and there will be almost no
equalization effect when using a simple electronic switch plus capacitor
strategy. In view of the low switching frequency of electromagnetic relays, a
feasible solution is to change the capacitor to a backup auxiliary battery and
add a step-up and step-down circuit.
The larger capacity of the backup auxiliary battery is used to charge or
discharge the relevant battery cells for a longer period of time. The birth of
the concept of backup auxiliary batteries makes it possible to achieve deep
equalization during the cascade utilization of secondary batteries. The relevant
technical solutions are described in detail in the invention patent application
(2018107804736) and will not be discussed here.
Returning to the observation in Figure 1, assuming that the relays J1 to J5
can operate at a higher switching frequency without short circuit failure due to
contact bonding, the circuit in Figure 1 will achieve a good balancing effect.
Transform the circuit in Figure 1 as shown in Figure 2:
Comparing Figure 1 and Figure 2, we can see that the relay group consisting
of five relays in Figure 1 has become a component with only one set of moving
contacts and five sets of static contacts in Figure 2. Assuming that five sets
of static contacts are arranged on a certain disk, one set of movable contacts
is driven by a rotating arm, and the power source is a motor, then the balancing
circuit in Figure 1 has a high-efficiency balancing scheme with fast switching
speed and no fear of inter-stage short circuits. .
This resulted in a completely new type of relay, which can be named the
rotary relay, which is particularly suitable for battery pack balancing
applications. One rotary relay can replace multiple separate electromagnetic
relays, and the cost will also be greatly reduced. Moreover, the rotary relay
has only one motor rotation command and is balanced independently. Compared with
the electromagnetic relay group, it has the advantages of requiring fewer switch
points and no need to frequently sample the battery voltage during the balancing
period. It truly achieves the balancing strategy goals of the simplest circuit
and the simplest calculation.
When the consistency of each single cell in the battery pack is good and
the voltage difference is small, the relay plus capacitor solution can be used
for balancing strategy; when the battery pack consistency is poor, the relay
plus backup auxiliary battery solution should be used (it can also be configured
Buck-boost circuit) for equalization. With the capacitive equalization method,
the equalizing current is small, so a single rotary relay can integrate more
than 20 groups of static contacts, and the single size is small. In the backup
battery balancing method, if the balancing current is large, it is appropriate
to have a smaller number of static contact groups within a single rotary relay.
Taking the automotive power supply as an example, when the number of static
contact groups is set to 4 groups, an effective configuration can be
achieved.
The capacitance method balancing relay works in a continuous rotation mode,
and the auxiliary battery method balancing relay works in a forward and reverse
positioning mode.
Taking a certain type of Tesla car battery as an example, the total number
of single cells is 7104, and the single stage is 74 parallel, so the number of
cascade strings is 96. According to the capacitive balancing scheme, each relay
can balance 204/8 battery strings. , then at least 5 continuously rotating
relays can complete the balancing work. The balance here refers to the battery
string corresponding to each relay. Based on this calculation, 96 battery
strings will have up to 5 different voltage values.
Theoretically, it is also possible to design an additional configuration of
a continuous rotating relay with a smaller number of contacts and stronger
overcurrent capability to eliminate the five different voltage values, but this
design is not recommended. In particular, when some car battery strings have
hidden dangers, they are extremely harmful to driving safety.
Therefore, adding a backup auxiliary battery balancing solution based on
the capacitance balancing method is a reliable means for safe use of automotive
batteries. Still taking the above-mentioned Tesla battery as an example, on the
basis of 96 strings, the total battery pack is subdivided into 16 groups (series
connection), each group contains 6 packages (series connection).
We use 16 groups as a standard. Each four groups corresponds to a
four-channel forward and reverse positioning relay, which requires 4 positioning
relays. These 4 positioning relays are then cascaded with a four-channel
positioning relay with a stronger load capacity. Then lead a pair of balanced
contacts to the outside of the battery box, for example, a two-core socket
embodied on the surface of the battery box. In addition, when we insert the plug
of a backup battery with a larger capacity (including a step-up or step-down
circuit) into the balanced socket, which set of battery voltages among the 16
sets of batteries is low, and the 5 positioning relay combinations (4+1) are
selected When the backup battery circuit is working, adjustable electrical
energy is supplied to it.
It can be seen from this that a large-capacity backup battery not only
plays a balancing role, but also plays a range-extending role. When the barrel
effect is triggered, the backup battery can replace the short board to ensure
safe driving of the car.
Therefore, the practical application of the above two types of rotary
relays will play a positive and effective forward-looking role in the current
balancing scheme. The common point in the structural design of the two types of
rotary relays is that they both use a disc-type static contact group and a
rotating arm-type movable contact group. The difference is that the contact area
and number of groups are designed according to the contact capacity, and the
contact area and group number are designed according to the contact capacity.
Balance method to determine whether continuous rotation or forward and reverse
positioning. Two examples are discussed below.
For example, there is currently a certain 17-cell lithium battery pack with
a nominal voltage of 60V, which is mostly used in two-wheel electric vehicles.
Adding a compact continuous rotation relay to the battery pack and reprogramming
a relay motor start command on the BMS board perfectly achieves the balancing
goal of this type of battery. Specifically, it is recommended that the battery
is always balanced when charging, so that the charge reaches the maximum
value;
During battery cycling and discharge, when the voltage difference between
each battery string exceeds the design value, the equalization will stop for a
certain period of time and the voltage difference will be re-monitored to
determine whether to equalize again. The design of its relay disk and rotating
arm is shown in Figure 3.
Four circles of conductive loops are laid out on the front of the relay
disk. The inner two circles are continuous sliding contact wires. The sliding
contact wires lead out wires on the back of the disc and are connected to a
capacitor with high capacitance, high resistance and low leakage; the outer two
circles are equal parts. 17 sets of sliding contacts. On the back of the disc,
the negative end (blue block) of the first set of sliding contacts (shown as red
and blue blocks) is short-circuited with the positive end of the second set.
Operate in sequence until the negative end of the 17th set is terminal
terminated. Each group of positive terminals leads to wires, plus the 17th group
of negative terminals, for a total of 18 wires, which are connected to the 18
nodes of the battery pack in sequence. When the rotating arm on the right shown
in Figure 3 continues to rotate, the capacitor will be cyclically connected in
parallel with each single cell of the battery pack, and the capacitor will
continue to charge and discharge, eventually making the voltages of each single
cell in the battery pack consistent.
The 17-channel continuous rotating relay is also suitable for battery packs
with less than 17 series series. It is suitable for brand-new battery packs with
good single-cell consistency. If applied to secondary batteries, uncontrollable
balancing current may cause contact ablation and oxidation, losing the balancing
effect. In particular, in order to prevent certain contacts from failing, the
same set of batteries can be connected to two relays for time-sharing operation.
The probability that the contacts of two relays on the same group number will
fail at the same time is extremely low, so full autonomous balance can be
achieved.
The example design goal of the forward and reverse positioning relay is
large current balancing, usually with a range extension (capacity supplement)
function, and the balancing component is a backup auxiliary battery. When
applied to automobile battery packs, a feasible design solution is to configure
four sets of switching contacts. When the backup auxiliary battery is always
connected to the main battery pack and the balancing goal is the main purpose,
the auxiliary battery does not need to be configured with a step-up and
step-down circuit; when the backup battery is often replaced offline and charged
separately for range extension purposes, the auxiliary battery needs Comes
standard with boost or buck circuitry.
The contact design area of the 4-channel positioning relay is large, and
the two moving contacts of the swing arm are designed on both sides of the
rotation center to balance the joint force of the two pairs of moving and static
contacts. In order to reduce contact resistance, the positioning relay cancels
the concept of sliding contact line in the continuous rotation relay, and uses a
flexible wire method to deal with the rotation angle difference of the arm. The
design of its relay disk and rotating arm is shown in Figure 4.
In Figure 4, two conductive rings are arranged on the front of the disk.
The conductive rings are divided into four equally spaced groups of sliding
contacts. The inner ring is thick and short, and the outer ring is slender. The
first set of contacts (red and blue) Color block) The outer circle is marked
A1+, and the inner circle is marked A3-, in order. Then on the back of the disc,
short-circuit A1- and A2+, A2- and A3+, A3- and A4+, and A4- to lead wire 5,
plus wires 1, 2, 3, and 4 as shown in the figure, a total of 5 The cable can be
connected to four battery packs with the same voltage level in the battery pack
(single string or multi-string combination).
Each movable contact on the swing arm of the positioning relay is designed
to be composed of multiple spring pieces to ensure over-current capability. The
access terminal of the auxiliary battery is directly led out from the spiral
arm. The risk of bending and breakage of the lead wire is eliminated by two
measures. One is that the spiral arm only rotates back and forth within a range
of 325 degrees. The other is that a flexible wire is used to circle one or two
times on the spiral arm layer. to weaken the wire bending force.
Because positioning relays require precise positioning, they require more
components and circuits such as angle detection or position detection than
continuous rotation relays. Still taking the application of 4-channel
positioning relays in automobile batteries as an example, a total of 5
four-channel positioning relays, large, 4 small, and 4 small, are configured in
the main battery pack, and are cascaded in two levels in 1+4 mode. The maximum
number of target battery strings is 16. Decompose the total number of strings in
the target battery pack into integer parts less than or equal to 16, and each
part has the same voltage level. Assuming 16 equal parts, when the voltage of
each part is 24V, the total voltage of the battery pack can reach 384V, and when
the voltage of each part is 48V, the total voltage of the battery pack can reach
768V. These two battery pack voltage levels basically cover the range from
electric cars to electric vehicles. Bus application.
Therefore, according to this design, a two-core balancing (extended range)
interface is set up on the battery pack box, and an external 24V or 48V backup
auxiliary battery with optional capacity is connected, which not only achieves
the partial balancing goal of the main battery pack (balance between battery
packs) ), also has the effect of emergency response and range extension. The
above two application examples are discussed based on the use of vehicle
batteries. The requirements for vehicles are small and delicate components. When
the application environment is transferred to an energy storage environment, the
structural design of the above two types of rotary relays is allowed to be
greatly changed. Adapt to the specific requirements of the energy storage
environment.
The mixed application or separate application of the two types of rotary
relays will play an efficient balancing role in the diversified use of new
batteries or secondary batteries. Moreover, with the advancement of technology,
the concept of balance will inevitably fade away, especially with the proposal
of auxiliary battery solutions and the safe application of battery packs. The
future goal is controllable regulation of energy flow.
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