After the battery decays, the discharge capacity at the rated discharge
current decreases and the internal resistance increases, and it is usually
scrapped. A large number of scientific research and experiments have shown that
after the battery decays, although the capacity is very small under high current
(high rate), it cannot be continued to be used. However, when the discharge
current is reduced, that is, the discharge rate is reduced, the discharge
capacity of the attenuated battery will increase significantly. Therefore, as
long as the discharge current of the attenuated battery is reduced, it still has
a very high reuse value, and the efficient transfer type The intervention and
automatic intervention of battery balancing technology automatically reduces the
charge and discharge current of the attenuated battery and increases the actual
discharge capacity of the attenuated battery, thus increasing the actual
discharge capacity of the attenuated battery pack and extending its life
cycle.
1. Battery capacity
Battery capacity usually refers to the actual discharge of a fully charged
battery under certain discharge conditions, usually expressed in mAh or Ah.
Discharge conditions usually include temperature during discharge, discharge
current, discharge rate, etc. For example, if a lithium battery is discharged at
a constant current of 1A under a certain temperature condition and continues to
discharge for 72 minutes to reach the discharge end voltage, then the capacity
of the battery is 1.2Ah, and the calculation formula is: 72/60*1=1.2Ah.
All batteries have one thing in common: the capacity decreases with the
increase in the number of charge and discharge cycles, commonly known as
attenuation. However, there are obvious individual differences in the
attenuation rate, which is closely related to the ambient temperature, charge
and discharge rate, and the self-discharge rate of the battery. Battery groups
After that, the position of the battery is basically fixed, especially for
battery packs with very high waterproof performance requirements, such as
lithium battery packs for electric vehicles.
In high-current charging and discharging and completely sealed
environments, the working temperature difference between individuals will always
exist. The higher the temperature, the faster the decay rate, which naturally
results in differences in the battery capacity decay rate. The final
manifestation is the consistency of the battery pack. The performance becomes
worse, the discharge time becomes shorter and shorter, and the discharge
capacity becomes less and less.
2. The relationship between lithium battery capacity and discharge rate
The capacity of the battery is related to the discharge rate. Therefore,
when the battery is marked with the capacity, the discharge rate or hourly rate
is usually marked. Even if it is not marked, the discharge current or discharge
rate range is required. Normally, except for batteries with special high-rate
designs, , the larger the discharge rate, the actual discharge capacity of the
battery gradually decreases. When the discharge rate is reduced, the actual
discharge capacity will gradually increase, but it is usually not higher than
the design capacity.
The evaluation of battery capacity must be based on the discharge rate
index, because the actual release capacity of the battery is different under
different discharge rates. The general trend is that the greater the discharge
rate, the smaller the release capacity of the battery; lowering the discharge
rate, the actual release capacity of the battery Capacity will increase. At the
same time, this changing trend is closely related to the health of the battery.
For new batteries, the impact of discharge rate on capacity is relatively small,
while for decaying batteries, the impact of discharge rate on capacity is
relatively large. The more serious the attenuation, the greater the impact. The
impact is more obvious.
2.1 Capacity performance of normal lithium batteries at different discharge
rates
Due to the manufacturing process, except for specially designed high-power
power batteries, lithium batteries usually limit the maximum discharge current
or discharge rate to ensure maximum capacity release. For ordinary lithium
batteries, the allowable or safe discharge rate is usually limited to 2C. If the
discharge rate exceeds this, the released capacity will be greatly reduced, as
shown in Figure 1. Moreover, the battery will heat up severely due to the
existence of internal resistance, and it is prone to thermal runaway, causing
lithium battery explosions, fires and other dangers.
It can be seen from this discharge capacity curve that the lithium battery
releases its capacity most fully at a lower discharge rate and is closest to the
ideal capacity. At the same time, due to the reduction in discharge current, the
discharge temperature rise of the lithium battery also decreases, and the
discharge safety Also greatly improved.
2.2 Capacity performance of attenuated lithium batteries at different
discharge rates
The following is the detected capacity of a set of 18650 attenuated lithium
batteries at different discharge rates (discharge cut-off voltage 3.0V). As
shown in Table 1, the original design capacity is 2.2Ah, and all of them decay
to varying degrees. At different discharge currents and Under the discharge
rate, the actual discharge capacity changes significantly.
Through the various measurement data and regular analysis in the table, it
can be seen that the discharge rate has a great influence on the actual capacity
of the attenuated battery. The greater the discharge rate, the smaller the
available capacity. As the discharge current and discharge rate increase,
Decrease, the available capacity gradually increases to varying degrees. The
more serious the attenuation of the battery, the more obvious the increase in
small rate discharge capacity. Figure 2 shows the discharge curve comparison
chart of 1# under three discharge currents. Through the discharge curve
comparison chart, we can clearly find the obvious differences in the attenuation
battery under different discharge currents, including changes in the discharge
voltage platform, discharge capacity changes, etc.
According to this regular characteristic, as long as the discharge current
or discharge rate of the attenuated battery is reasonably reduced, the
attenuated battery still has good utilization value and can still use its waste
heat.
3. The significance and application of different discharge rate
capacities
Through the analysis and comparison of the discharge experimental curves of
lithium batteries with different attenuation degrees, it can be found that under
the same discharge current, the slope of the discharge curve of the attenuated
battery shows a negative increase as the degree of attenuation increases, and
the discharge rate also gradually increases. , that is, when the load current
remains unchanged, as the battery attenuation increases, the discharge time and
discharge capacity gradually decrease.
Since increasing the discharge rate will accelerate the decay rate of the
attenuated battery, and reducing the discharge rate will weaken the decay rate
of the attenuated battery, then as long as the discharge current of the
attenuated battery is scientifically and reasonably reduced, the actual
discharge time and Discharge capacity. In order to verify this idea, the author
conducted continuous cycle discharge tests of attenuated batteries at different
discharge rates. The results show that reducing the discharge current of the
attenuated battery, that is, the discharge rate, can indeed extend its discharge
time and increase its actual discharge capacity. , and slow down the decay
rate.
Continuous cycle test data shows that when the discharge rate is reduced,
the actual discharge capacity of the attenuated battery increases to varying
degrees. Especially for lithium batteries with very small discharge capacity at
high discharge rates, when the discharge rate is greatly reduced, The available
capacity has been greatly increased, and this test result and conclusion is of
great practical significance.
Its most important significance and application value lies in the echelon
utilization of retired battery packs. Although most of the batteries in the
retired battery pack still have echelon utilization value, due to the impact of
decay and scrapped batteries in the pack, the performance of many batteries has
dropped a lot. Whether it is the decrease in remaining capacity or the increase
in internal resistance, it has It is no longer possible to charge and discharge
at a larger rate, and can only be used safely by reducing the charge and
discharge rate. This is one of them;
Secondly, the load of the battery pack that is used in series usually
requires the output of larger current and power. Therefore, as long as more
batteries of the same model and type are combined in multiple parallels and
multiple series, appropriate voltage and power output can be achieved. This This
method can not only minimize the discharge current and discharge rate of each
ladder battery, but also achieve the purpose of extending the service life of
the ladder battery.
In the case where the consistency technical problem of the battery pack has
not been completely solved, cascading utilization will be the best solution to
extend the battery life and improve the utilization value.
4. How to achieve low-rate charging and discharging of attenuated
batteries
The newly assembled battery pack has the best discharge time and power
performance. This is because the consistency of the new battery pack is usually
very good, and the output power of each unit cell is basically the same, thus
ensuring the efficiency of the battery pack. Although battery pack merchants all
say that battery packs have been strictly screened and matched before leaving
the factory, and the consistency is very good, the reality is cruel. No matter
how consistent the battery pack is when it leaves the factory, after several
charging cycles Consistency issues will occur after discharge cycles.
With at least dozens of charge and discharge cycles and as many as hundreds
of charge and discharge cycles, the consistency problem will be very serious and
become more and more serious. Theory and a large amount of experimental data
show that for decaying batteries, reducing their charge and discharge current or
charge and discharge rate is the most effective way to slow down the rapid decay
of decaying batteries.
When the current battery production process cannot solve the problem of
battery pack consistency during use, the only feasible solution is to solve it
through battery management technology. Specifically, it is to use battery
balancing technology to adjust the charge of batteries with different
capacities. The discharge current allows it to charge and discharge at the same
rate throughout the entire period.
Over the years, battery balancing technology has attracted much attention
and triggered a development boom. Today, there are only three types, namely
passive resistive discharge balancing, active charge balancing and
high-efficiency transfer battery balancing. According to the principle of
equalization, the key point to solve is the rapid shunt function of current.
Resistor discharge equalization, the current is very small, generally within 100
mA, the effect is limited for large-capacity battery packs, and discharge
equalization cannot be performed, and the battery is prone to over-discharge
problems. .
The balancing current of charge balancing technology can be relatively
large, reaching several amps or more, but the main drawback is that it cannot
solve the problem of battery over-discharge; transfer battery balancing
technology has achieved a breakthrough in balancing technology, not only has a
large balancing current, but is also applicable It is the most ideal battery
pack balancing technology in all stages of battery operation.
At the beginning of the research and development of battery balancing
technology, the author chose the most difficult and promising transfer battery
balancing technology. After years of unremitting technical research and
continuous optimization, he successfully achieved double breakthroughs in
balanced current and balanced efficiency. In addition, The unique bidirectional
synchronous rectification technology has been developed to achieve a maximum
balancing current of more than 10A and a maximum balancing efficiency of about
97%.
For example, the prototype of a 20A single 2V lead-acid battery equalizer
currently developed has an equalization efficiency of about 80% even at 20A full
load under the adverse conditions of a very low voltage platform; while for
lithium battery packs , under the same 20A balancing current situation, the
balancing efficiency reaches about 90%.
The substantial increase in the balancing current means that the shunting
ability of the attenuated battery is enhanced, the balancing speed is faster,
and the protection ability of the attenuated battery is stronger; the
improvement of the balancing efficiency means that the utilization rate of
electric energy is higher and the loss of electric energy is smaller. Small, the
balancing module generates less heat and will not increase the temperature rise
of the battery pack. Such a high-efficiency battery balancing module is designed
to provide efficient and safe operation of high-power power battery packs and
energy storage battery packs (including ladder utilization battery packs)
technical guarantee.
5. Outlook
With the improvement of production technology, the quality of single cells
has gradually increased, and the consistency of capacity, internal resistance,
and voltage has gradually improved. However, when used in groups, they show
various differences, especially in capacity, internal resistance, Differences in
voltage and other aspects lead to consistency problems in various battery packs
with a high probability, which can lead to thermal runaway, leading to
explosions, fires and other accidents. With the emergence of a large number of
retired batteries for electric vehicles and the emphasis on tiered utilization,
it seems that A solution has been found for the reasonable placement of retired
batteries.
However, because the consistency of tiered batteries is worse, and the
consistency problem has not been able to achieve a technical breakthrough, the
consistency problem of tiered batteries will occur earlier. If the management
technology cannot keep up, the same will happen. In the event of accidents such
as thermal runaway, explosion, and fire, the successful development of
high-efficiency battery balancing technology has greatly improved the
consistency of battery pack operation, thereby improving operational safety and
significantly extending the cycle life of the battery pack. It can be said that
this efficient battery balancing technology provides an intelligent management
method for the efficient and safe operation of the battery pack.
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