Electronic enthusiasts provide you with how to solve the electrochemical
balancing problem of lithium-ion battery packs. The work of Alexander U. Schmid
provides a new idea for battery pack balancing. Due to the design
characteristics of NiMH and NiZn batteries, when overcharge occurs , the water
in the electrolyte will decompose at the positive and negative electrodes
respectively, producing O2 and H2. Under the action of the catalyst in the
battery, O2 will combine with H2 to produce water, completing a cycle.
Therefore, NiMH and NiZn have very good resistance to overheating. We can take
advantage of this by connecting a single or several series-connected NiMH and
NiZn batteries in parallel with the lithium-ion battery. When the charging
voltage reaches the upper limit, almost all the current will flow through the
NiMH and NiZn batteries, thus avoiding the risk of lithium-ion batteries.
Battery packs are generally composed of hundreds or thousands of battery
cells, so the capacity of the battery pack is also affected by the single cells.
Research shows that even if a single battery has a cycle life of more than 1,000
times, the composition After the battery pack is replaced, the battery pack's
life may be less than 200 times. This shows that the balance of the battery pack
is very important.
The poor consistency of lithium-ion battery cells has long been a problem
in the design of lithium-ion battery packs. The consistency we are talking about
here not only refers to parameters such as capacity and voltage in the
traditional sense, but also includes the capacity decline of single cells.
Factors such as speed, internal resistance decay rate and temperature
distribution of the battery pack.
Ideally, lithium-ion batteries from the same batch should have the same
electrochemical performance, but in practice there will be inconsistencies
between lithium-ion cells due to errors in the manufacturing process. Battery
packs are often composed of hundreds or even thousands of single cells connected
in series and parallel, so the capacity of the battery pack is greatly affected
by the inconsistency of the single cells (the inconsistency factors that have
the greatest impact on battery pack performance include Coulomb Inconsistency in
efficiency, inconsistency in self-discharge rate, inconsistency in internal
resistance increase rate, etc.), research shows that even if a single battery
has a cycle life of more than 1,000 times, after being assembled into a battery
pack, the life of the battery pack may be less than 200 times.
Therefore, balancing equipment is necessary for a battery pack composed of
a large number of single cells. The common balancing methods currently on the
market mainly use electronic equipment to achieve voltage balance between single
cells, so they are technically Much the same. Recently, Alexander U. Schmid of
the University of Stuttgart in Germany and others used Ni metal hydride
batteries (NiMH) and Ni-Zn batteries to achieve electrochemical balancing of the
battery pack, providing a new idea for the balancing of the battery pack.
Due to the limitations of the working principle of lithium-ion batteries,
their ability to resist overcharge is very weak. In the case of overcharge,
problems such as electrolyte decomposition and lithium precipitation may occur.
When a NiMH battery is overcharged, the H2O in the electrolyte will decompose at
the positive and negative electrodes to produce O2 and H2, and O2 and H2 can
recombine under the action of the catalyst to form water, thus forming a
complete cycle. At small rates of C/3-C/10, the rate of gas generation is almost
the same as the rate of its recombination, so the NiMH battery's anti-overcharge
performance is very good. Based on the above principles, Alexander U. Schmid
used NiMH batteries and similar Ni-Zn batteries to balance lithium-ion battery
packs. When using this electrochemical balancing method, the traditional voltage
monitoring and electronic balancing units can be omitted, effectively reducing
the complexity of battery pack management and improving the reliability of the
battery pack.
AlexanderU.Schmid selected LiFepO4 and Li4TI5O12 materials as experimental
objects because both materials have a certain tolerance to overcharging, and the
voltage will rise rapidly after complete delithiation. At this time, NiMH and
Ni-Zn batteries bear the burden Acts as a current bypass, and excess current
will flow into NiMH and Ni-Zn batteries to prevent overcharging of lithium-ion
batteries.
Its working principle is shown in the figure below. NiMH batteries or Ni-Zn
batteries used for balancing are connected in parallel with lithium-ion
batteries. When a group of series-connected low-capacity batteries in the
battery pack is fully charged, the voltage reaches the threshold. , at this
time, the NiMH battery connected in parallel with it assumes the role of
shunting. Basically, all current flows through the NiMH battery and no longer
flows through the lithium-ion battery, thereby avoiding overcharge of the
lithium-ion battery. During this process, the changes in voltage and current of
the lithium-ion battery and NiMH are shown in Figure b below. In the case of
perfect matching, the lithium-ion battery current is shown as the red curve.
The following table shows the information of the batteries used in the
experiment. The experiments mainly used LFp/graphite, LMO/LTO, LFp/LTO, Ni-Zn
and NiMH batteries.
The picture below shows the capacity-voltage curve of several batteries
used in the experiment. 2´NiZn means two Ni-Zn batteries connected in series.
You can see that the maximum voltage of two Ni-Zn batteries connected in series
is 3.95V. (I=150mA), it can be used on LFp/C batteries to avoid overcharging. A
Ni-Zn battery can be connected in parallel with an LFp/LTO battery to avoid
overcharging of the battery, or two NiMH batteries can be connected in series
with LMO/LTO in parallel. At this time, the maximum voltage will reach more than
3V, while the maximum voltage of the LMO/LTO battery is 2.8 V, but as long as
the LMO/LTO battery voltage does not exceed 3.2V, it is acceptable, and the
increased capacity of the LMO/LTO battery from 2.8-3.2V is only 0.65Ah, which is
about 6.5% of the normal temperature capacity, so it has a negative impact on
the battery performance Has little effect.
The picture below shows how an LMO/LTO battery works with two NiMH
batteries connected in series. It can be seen that during the charging process
of the battery pack, the LMO/LTO battery is first fully charged. When a certain
point is reached, the current begins to change. , the current flowing through
the LMO/LTO battery begins to decrease, and the current flowing through the NiMH
battery increases. Finally, the current flowing through the LMO/LTO battery
drops to 0, and all current flows through the NiMH battery. Therefore, at this
time, the battery pack The voltage no longer increases. During the discharge
process, the two batteries start to discharge at the same time. Since the NiMH
battery has a smaller capacity, the current quickly drops to 0, and the
discharge is mainly completed by the LMO/LTO battery.
The picture below shows the working condition of the LFp/C-2NiZn battery
module. It can be seen that when charging starts, almost all the current will
enter the LFp/C battery, and only about 80mA of current will pass through the
NiZn battery. Then at t=1.2h, the flow direction of the current completely
changed, and the current began to mainly flow through the NiZn battery.
Therefore, in order to avoid overheating of the NiZn battery, the charging
current of the module was divided into several steps, first 1.1A, and then 0.75
A, then 0.3A, then 0.15A. At the beginning of the discharge process, the NiZn
battery provided the maximum current, then its current began to decrease, and
the current of the LFp/C battery began to gradually increase.
The following table is a summary of the effects of several types of
batteries connected in parallel with NiZN and NiMH batteries. From the first
column, you can see that several parallel connection methods can make the
maximum voltage of the battery pack less than the maximum limit voltage of
lithium-ion batteries to avoid lithium-ion batteries. The ion battery has been
overcharged. As can be seen from the second column, except that the LFp/LTO-NiZn
battery cannot fully utilize the capacity of the lithium-ion battery, the other
two parallel connection methods can fully utilize the capacity of the
lithium-ion battery, so it can also achieve balance of the battery pack. (third
column). As can be seen from the fourth column, affected by the parallel
connection of NiZn and NiMH batteries, the maximum discharge current of the
battery pack is smaller than the maximum current of the lithium-ion battery.
Therefore, in actual use, high-power NiZn and NiMH batteries need to be
selected. Ensure that the performance of the battery pack is not degraded.
The figure below shows the charging and discharging operation of two
LFp/C-2NiZn batteries in series. The initial capacity difference of the two
LFp/C batteries in series is 200mAh. After the following charge and discharge,
the capacity difference of the two battery packs Reduced to 100mAh, which means
that 8% of the capacity of the two series-connected battery packs is balanced in
one cycle.
The work of Alexander U. Schmid provides a new idea for battery pack
balancing. Due to the design characteristics of NiMH and NiZn batteries, when
overcharge occurs, the water in the electrolyte will decompose in the positive
and negative electrodes respectively, producing O2 and H2. Under the action of
the catalyst in the battery, O2 will combine with H2 to produce water,
completing a cycle. Therefore, NiMH and NiZn have very good anti-overcharge
properties. We can just take advantage of this to pass single or several NiMH
and NiZn in series. The battery is connected in parallel with the lithium-ion
battery. When the charging voltage reaches the upper limit, almost all the
current will flow through the NiMH and NiZn batteries, thereby preventing the
lithium-ion battery from overcharging. We can also use this to balance the
lithium-ion battery pack. As long as we continue to charge the battery pack, we
can ensure that all batteries can be fully charged without worrying that some
batteries will be overcharged, thereby improving the battery pack. The internal
capacity is consistent, and experiments have also confirmed that a capacity
balance of 8% (LFp/C-2NiZn) can be achieved in one charge and discharge cycle.
The biggest advantage of this method is that the entire process does not require
voltage monitoring of the individual cells in the battery pack. It is completely
automated, thus greatly simplifying the structure of the battery pack and
improving the reliability of the battery pack.
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