1. Ordinary series charging Currently, lithium-ion battery packs are
generally charged in series. This is mainly because the series charging method
has a simple structure, low cost, and is easier to implement. However, due to
the differences in capacity, internal resistance, attenuation characteristics,
self-discharge and other performance between single lithium-ion batteries, when
charging lithium-ion battery packs in series, the single lithium-ion battery
with the smallest capacity in the battery pack will It is fully charged first,
but at this time, other batteries are not fully charged yet. If the batteries
continue to be charged in series, the fully charged single lithium-ion battery
may be overcharged. Overcharging of lithium-ion batteries will seriously damage
the performance of the battery, and may even cause explosions and personal
injuries. Therefore, in order to prevent overcharging of single lithium-ion
batteries, lithium-ion battery packs are generally equipped with a battery
management system (Battery Management System, Referred to as BMS), each single
lithium-ion battery is protected from overcharging through the battery
management system. During series charging, if the voltage of a single
lithium-ion battery reaches the overcharge protection voltage, the battery
management system will cut off the entire series charging circuit and stop
charging to prevent this single battery from being overcharged, which will cause
other problems. Lithium-ion batteries cannot be fully charged. After years of
development, lithium iron phosphate power batteries can basically meet the
requirements of electric vehicles, especially pure electric cars, due to their
high safety, good cycle performance and other advantages, and the technology has
basically met the requirements for mass production. condition. However, the
performance of lithium iron phosphate batteries is somewhat different from other
lithium-ion batteries, especially its voltage characteristics are different from
lithium manganese oxide batteries, lithium cobalt oxide batteries, etc. The
following is a comparison of the charging curves of lithium iron phosphate and
lithium manganate batteries and the corresponding relationship between lithium
ion deintercalation: When the lithium iron phosphate battery is almost fully
charged, lithium ions are almost completely deintercalated from the positive
electrode to the negative electrode. The voltage will rise rapidly and the
charging curve will rise. This will cause the battery to easily reach the
overcharge protection voltage. Therefore, the phenomenon that some batteries in
the lithium iron phosphate battery pack are not fully charged is more obvious
than the lithium manganate battery pack.
In addition, although some battery management systems have a balancing
function, due to considerations such as cost, heat dissipation, reliability,
etc., the balancing current of the battery management system is generally much
smaller than the current of series charging, so the balancing effect is not very
obvious, and there may also be Some single batteries are not fully charged,
which is more obvious for lithium-ion battery packs that require large current
charging, such as lithium-ion battery packs for electric vehicles. For example,
100 lithium-ion batteries with a discharge capacity of 100Ah are connected in
series to form a battery pack. However, if 99 single-unit lithium-ion batteries
are charged with 80Ah before the group is formed, and the other single-unit
lithium-ion battery is charged with 100Ah, the battery pack will When this
battery pack is charged in series, the single lithium-ion battery with a charge
of 100Ah will be fully charged first, thus reaching the overcharge protection
voltage. In order to prevent this single lithium-ion battery from being
overcharged, the battery management system will The entire series charging
circuit is cut off, which prevents the other 99 batteries from being fully
charged, so the discharge capacity of the entire battery pack is only 80Ah.
Generally, when battery manufacturers test the capacity before leaving the
factory, they first charge the single battery with constant current, then charge
with constant voltage, and then discharge it with constant current to measure
the discharge capacity.
Generally, the discharge capacity is approximately equal to the constant
current charging capacity plus the constant voltage charging capacity. In the
actual series charging process of the battery pack, there is generally no
constant voltage charging process for the single battery, so the constant
voltage charging capacity will not be available, and the battery pack capacity
will be smaller than the single battery capacity. Generally, the smaller the
charging current, the smaller the constant-voltage charging capacity ratio, and
the smaller the battery pack's capacity loss. Therefore, a battery management
system and charger have been developed to coordinate and cooperate with series
charging. 2. The battery management system and the charger coordinate and
cooperate in series charging. The battery management system is the most
comprehensive device for understanding the performance and status of the
battery. Therefore, establishing a connection between the battery management
system and the charger allows the charger to understand the battery in real
time. information, thereby more effectively solving some problems that occur
when charging the battery. The principle of the battery management system and
the charger to coordinate and cooperate with the charging mode is: the battery
management system analyzes the current status of the battery (such as
temperature, single cell voltage, battery Operating current, consistency,
temperature rise, etc.) are monitored, and these parameters are used to estimate
the maximum allowable charging current of the current battery; during the
charging process, the battery management system and the charger are connected
through communication lines to achieve data sharing. The battery management
system transmits parameters such as total voltage, maximum single cell voltage,
maximum temperature, temperature rise, maximum allowable charging voltage,
maximum allowable single cell voltage, and maximum allowable charging current to
the charger in real time, and the charger can adjust the battery according to
the battery The management system provides information to change its own
charging strategy and output current. When the maximum allowable charging
current provided by the battery management system is higher than the designed
current capacity of the charger, the charger charges according to the designed
maximum output current; when the battery voltage and temperature exceed the
limits, the battery management system can detect it in real time and notify the
charger in time. The machine changes the current output; when the charging
current is greater than the maximum allowable charging current, the charger
starts to follow the maximum allowable charging current, thus effectively
preventing the battery from overcharging and extending the battery life. Once a
fault occurs during the charging process, the battery management system can set
the maximum allowable charging current to 0, forcing the charger to shut down to
avoid accidents and ensure charging safety. In this charging mode, it not only
improves the management and control functions of the battery management system,
but also enables the charger to change the output current in real time according
to the status of the battery, thereby preventing overcharging of all batteries
in the battery pack and optimizing charging. The actual discharge capacity of
the battery pack is also greater than the ordinary series charging method, but
this method still cannot solve the problem of some batteries in the battery pack
being undercharged, especially when the battery pack has a large number of
strings, poor battery consistency, and a large charging current. Big time.
3. Parallel charging In order to solve the problem of overcharging and
undercharging of some single cells in the battery pack, parallel charging has
been developed. However, the parallel charging method requires the use of
multiple low-voltage, high-current charging power sources to charge each single
battery. It has the disadvantages of high charging power cost, low reliability,
low charging efficiency, and thick connection wire diameter. Therefore, there is
currently no large-scale charging method. The range uses this charging method.
4. Series high-current charging and low-current parallel charging. Since the
above three charging methods all have certain problems, I have developed a
charging method that is most suitable for high-voltage battery packs, especially
electric vehicle battery packs, that is, using a battery management system. It
coordinates with the charger to cooperate with series high-current charging and
parallel low-current charging with constant voltage and current limiting. This
charging method has the following characteristics:
(1) Since the BMS of this system has the function of preventing
overcharging, it ensures that the battery will not have overcharging problems.
Of course, if the BMS cannot communicate and control the parallel charging power
supply, since the constant voltage value of the parallel charging power supply
is generally the same as the voltage value of the single lithium-ion battery in
the lithium-ion battery pack when it is fully charged, there will be no
overcharging problem. .
(2) Since parallel charging is possible, there is no need for a balancing
circuit with low reliability and relatively high cost. The charging effect is
better than the series charging method with only a balancing circuit, and its
maintenance and management are also simple and easy.
(3) Since the maximum current of series charging is much greater than the
current of parallel charging (generally more than 5 times), it can ensure that a
higher capacity is charged in a shorter time, thereby exerting the maximum
effect of series charging.
(4) The sequence of series charging and parallel charging and the number of
parallel charging power sources during charging can be flexibly controlled, and
charging can be performed at the same time; parallel charging can be carried out
after series charging is completed; a parallel charging power supply can also be
used according to the voltage in the battery pack. In this case, the battery
with the lowest voltage is charged in turn.
(5) With the development of technology, the parallel charging power supply
can be a non-contact charging power supply (wireless charging power supply) or
solar battery power supply, making parallel charging simple.
(6) When there are a large number of single lithium-ion batteries in the
lithium-ion battery pack, the lithium-ion battery pack can be divided into
several lithium-ion battery pack modules. For each lithium-ion battery pack
module, the BMS and charger are used to coordinate and connect in series.
Charging is carried out by combining current charging with constant voltage and
current limiting parallel small current charging. Its main purpose is to reduce
the shortcomings of the poor charging effect of the coordinated charging method
of BMS and charger when there are a large number of batteries connected in
series in the battery pack, so that the consistency between single cells is
relatively poor. Coordinate with the charging mode for maximum effect. This
method is particularly suitable for high-voltage battery packs that are battery
systems composed of quickly replaceable low-voltage (e.g. 48V) battery module
systems, so that they can be charged or repaired in parallel at battery
replacement stations or charging stations (general users usually charge There is
no need to charge in parallel), and a dedicated person will sort and regroup
according to the actual situation.
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