Under the influence of the consistency problem, the actual discharge
capacity of the battery pack depends on the battery with the smallest capacity
in the battery pack. The greater the number of battery packs connected in
series, the greater the impact on the discharge capacity of the battery pack,
and the lower the utilization rate of the battery pack. It not only affects the
charge and discharge capacity and battery life, but also easily causes thermal
runaway and other faults, especially for high-power power and energy storage
battery packs. The intervention of a real-time, high-efficiency battery
equalizer not only intelligently adjusts the charge and discharge current and
charge and discharge rate of batteries of different capacities, but also
significantly improves the capacity utilization of the battery, and the effect
of controlling the attenuation of the temperature rise of the battery is also
very obvious. This article fully proves the importance of a high-efficiency
battery equalizer in stabilizing the battery life and capacity by comparing and
analyzing the experimental data of conventional discharge and equalization
discharge of a set of 13 strings of 48-volt scrapped lithium batteries with
seriously deteriorated consistency.
1 Causes of battery pack consistency problems
Ideally, the battery pack should have the following characteristics: when
charging or discharging, the voltages of all batteries rise or fall
simultaneously, and the capacity, voltage, self-discharge rate and internal
resistance between batteries are very close, that is, the performance of all
batteries is basically the same and consistent. The performance is very good,
all batteries can be fully charged or discharged almost at the same time, and
there will be no problem of overcharge or overdischarge of the battery.
However, in reality, most battery packs perform very poorly and have
prominent consistency issues. The cycle life of the battery pack is usually only
1/3 to 1/5 of the design life of a single battery, which greatly affects the
service life and endurance of the device. In serious cases, thermal runaway
failure may occur, causing damage to equipment or personnel.
Through a large amount of experimental research and operation data
analysis, it can be found that there are two main reasons for consistency
problems in conventional battery packs: One reason is caused by differences in
battery production processes, referred to as internal causes. Once the battery
packaging is completed, the differences between batteries There are differences
in capacity, self-discharge rate and internal resistance parameters, but the
degree of difference is different.
The second reason can be called external factors, which are mainly caused
by differences and fluctuations in charging and discharging voltage parameters,
current parameters, and ambient temperature. These external factors will
gradually accumulate and amplify the differences caused by internal factors, and
the amplification of this difference will It exhibits exponential amplification
characteristics, which is why consistency problems in the battery pack will
quickly worsen once they occur.
2 Common Ways to Solve Consistency Causes
In terms of solving the consistency problem of the battery pack, there are
two main technical solutions based on the main reasons for the battery
consistency problem. One solution is to make a fuss about the battery production
process and improve the battery by improving the production process level.
Consistency at the factory, this solution has a certain effect and can slow down
and delay the occurrence of consistency problems to the greatest extent, but it
cannot be eradicated;
Another solution is to use a battery equalizer to intervene. Battery
balancing includes passive balancing and active balancing. Passive balancing is
also called energy consumption balancing. The balancing current is small and the
balancing efficiency is zero. It is only suitable for applications with good
consistency and uniform heat dissipation. , and the battery pack capacity is
small; the typical representative of active balancing is transfer battery
balancing, the balancing efficiency and balancing current are much higher than
passive balancing.
It is foreseeable that even if active balancing is the mainstream of future
development, the design architecture and implementation methods are diverse.
This article will not discuss it, but one thing is certain. All design goals are
to support larger balancing currents and balanced Develop in the direction of
high efficiency and balanced speed.
3Real-time, high-speed battery balancing technology and shunt
characteristics
In battery balancing technology, the most difficult thing to solve is the
matching and balancing of balancing current and balancing efficiency. It must be
able to provide a larger balancing current and have a higher balancing
efficiency. The reason for this requirement is mainly because of the existence
of The balancing equipment generates heat under high current balancing
conditions and affects the temperature rise of the battery pack.
In order to solve this contradictory problem, the author has developed a
unique bidirectional synchronous rectification technology [1] after years of
research, which not only supports large current balancing, but also has high
balancing efficiency. Under full load operation, the temperature of the
equipment is reduced. The temperature rise is also low and will hardly increase
the temperature rise of the battery pack.
Reasonable distribution and optimization of power is achieved through
high-speed voltage equalization [2]. In terms of discharge equalization, this
technology automatically analyzes and determines the capacity of the battery by
detecting the relative voltage difference between adjacent batteries in real
time, and automatically adjusts the battery capacity. Batteries with high
voltage (high voltage during discharge usually have large capacity) increase the
discharge current, and the increased discharge current is transported to both
ends of the low-voltage battery through efficient conversion by the equalizer.
For low-voltage batteries (low voltage during discharge usually has small
capacity) batteries Reduce the discharge current to make up for the lack of
discharge capacity of small-capacity batteries, so that batteries with different
capacities can be discharged at approximately the same rate.
In terms of charge balancing, the charging current is automatically reduced
for batteries with high voltage (high voltage during charging means small
capacity). The reduced charging current is efficiently converted by the
equalizer and transported to both ends of the low-voltage battery. Low voltage
during the period means large capacity) The battery increases the charging
current, so that batteries with different capacities can be charged at
approximately the same rate; this technology can also support high-speed static
equalization at the same time, increasing the effective equalization time. The
unique pulse technology is very useful for stably attenuating battery capacity.
Obviously, the battery equalizer used in the example of this article adopts the
newly developed bidirectional synchronous rectification technology [3].
4 Thirteen-string lithium battery pack discharge experiment
The experimental battery pack is shown in Figure 1. It is assembled from
scrapped lithium battery packs after disassembly, selection and echelon
utilization. They are all 18650 model lithium batteries. The longest storage
time is more than 8 years. The initial voltage of the disassembled battery is
only The voltage ranges from a few tenths of a volt to a few tenths of a volt,
and most of them are in a scrapped state. The original design capacity of a
single cell is between 2200mAh and 2500mAh. Most of the batteries have heavy
leakage and generate serious heat during charging.
When the capacity is tested for 2 hours after being fully charged, the
actual remaining capacity of the 1A discharge test is only between 550mAh and
2350mAh. See the remaining capacity column in Table 1. It can be seen from the
detected capacity that the capacity difference is very large, with the maximum
difference reaching 1.8Ah, there are 13 experimental batteries in total, and the
remaining power histogram of all batteries is shown in Figure 2.
The battery with the words "meter power supply" on the far right side of
the experimental bench is only responsible for powering the high-precision
voltmeters under the 7# and 13# batteries, and does not participate in the
charge and discharge experiment. The other voltmeters are powered in cascade
mode. The bottom of each battery corresponds to a high-precision voltmeter,
which displays the current voltage of the upper battery in real time. The
experimental platform (the platform has been modified to facilitate battery
replacement) supports up to 2 and 14 strings of 18650 battery experiments. The
example in this article only connected 13 strings. Battery, nominal voltage 48
volts.
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