For a long time, poor consistency of individual lithium battery cells has been a design challenge for lithium battery packs. Here, consistency refers not only to parameters such as capacity and voltage in the traditional sense, but also to factors such as the capacity decay rate of individual cells, the internal resistance decay rate, and the temperature distribution of the battery pack.
Ideally, lithium batteries from the same batch should have the same electrochemical performance. However, due to errors in the manufacturing process, there are inconsistencies between individual lithium-ion cells. Battery packs are often made up of hundreds or even thousands of individual cells connected in series and parallel. Therefore, the capacity of the battery pack is greatly affected by the inconsistency of individual cells (the inconsistencies that have the greatest impact on the performance of the battery pack include inconsistencies in coulombic efficiency, inconsistencies in self-discharge rate, and inconsistencies in the rate of increase in internal resistance). Studies have shown that even if the cycle life of individual cells reaches more than 1,000 cycles, the life of the battery pack may be less than 200 cycles after being assembled into a battery pack [1].
Therefore, balancing equipment is essential for a battery pack composed of numerous individual cells. Currently, the most common balancing methods on the market mainly rely on electronic devices to achieve voltage balancing between individual cells, and thus the technologies are largely similar. Recently, Alexander U. Schmid and colleagues at the University of Stuttgart in Germany achieved electrochemical balancing of the battery pack using Ni metal hydride (NiMH) and Ni-Zn batteries, providing a new approach to battery pack balancing.
Due to the limitations of lithium-ion batteries' working principle, their overcharge resistance is very weak, potentially leading to electrolyte decomposition and lithium plating under overcharge conditions. In NiMH batteries, however, under overcharge conditions, the H2O in the electrolyte decomposes into O2 and H2 at both the positive and negative electrodes. O2 and H2 can then recombine to form water with the aid of a catalyst, thus completing a cycle. At low rates (C/3-C/10), the rate of gas formation is almost identical to the rate of recombination, resulting in excellent overcharge resistance for NiMH batteries. Based on this principle, Alexander U. Schmid used NiMH batteries and similar Ni-Zn batteries to balance lithium-ion battery packs. Using this electrochemical balancing method, traditional voltage monitoring and electronic balancing units can be omitted, effectively reducing the complexity of battery pack management and improving battery pack reliability.
Alexander U. Schmid chose LiFePO4 and Li4Ti5O12 as experimental materials because both materials have a certain tolerance to overcharging, and the voltage rises rapidly after complete delithiation. At this time, NiMH and Ni-Zn batteries take on the role of current bypass, and the excess current flows into NiMH and Ni-Zn batteries, thereby preventing the lithium battery from overcharging.
Its working principle is shown in the figure below. The NiMH or Ni-Zn batteries used for balancing are connected to the lithium battery in parallel. When a group of low-capacity batteries in the battery pack is fully charged, the voltage reaches the threshold. At this point, the NiMH batteries connected in parallel take on the role of current diversion, with almost all the current flowing through the NiMH batteries and no longer flowing through the lithium battery, thus preventing overcharging of the lithium battery. The changes in voltage and current of the lithium battery and NiMH batteries during this process are shown in Figure b below. Under perfect matching conditions, the lithium battery current is shown as the red curve.
The table below shows the information of the batteries used in the experiment. The main batteries used in the experiment were LFP/graphite, LMO/LTO, LFP/LTO, Ni-Zn and NiMH batteries.
The figure below shows the capacity-voltage curves of several batteries used in the experiment. "2´NiZn" indicates two Ni-Zn batteries connected in series. It can be seen that the maximum voltage of two Ni-Zn batteries connected in series is 3.95V (I=150mA), which is suitable for use with LFP/C batteries, preventing overcharging. A single Ni-Zn battery can be connected in parallel with an LFP/LTO battery to prevent overcharging, or two NiMH batteries can be connected in series with an LMO/LTO battery in parallel. In this case, the maximum voltage will reach over 3V, while the maximum voltage of an LMO/LTO battery is around 2.8V. However, as long as the LMO/LTO battery voltage does not exceed 3.2V, it is acceptable. Furthermore, the capacity increase of an LMO/LTO battery from 2.8V to 3.2V is only 0.65Ah, approximately 6.5% of its room-temperature capacity, so the impact on battery performance is minimal.
The diagram below illustrates the operation of an LMO/LTO battery and two NiMH batteries connected in series. During charging, the LMO/LTO battery is fully charged first. At a certain point, the current changes: the current flowing through the LMO/LTO battery decreases, while the current flowing through the NiMH battery increases. Eventually, the current flowing through the LMO/LTO battery drops to zero. All current flows through the NiMH battery, so the battery voltage no longer increases. During discharge, both types of batteries begin discharging simultaneously. Due to the smaller capacity of the NiMH battery, its current quickly drops to zero, and the LMO/LTO battery completes the discharge.
At the start of charging, almost all the current flows to the LFP/C battery, with only about 80mA flowing through the NiZn battery. Then, at t=1.2h, the current flow completely reverses, with the current primarily flowing through the NiZn battery. Therefore, to prevent the NiZn battery from overheating, the module's charging current is divided into several steps: first 1.1A, then 0.75A, then 0.3A, and finally 0.15A. At the start of the discharge process, the NiZn battery supplies the maximum current, which then begins to decrease, while the current from the LFP/C battery gradually increases.
The table below summarizes the effects of several types of batteries connected in parallel with NiZN and NiMH batteries. The first column shows that all parallel connection methods ensure the maximum voltage of the battery pack is lower than the maximum limiting voltage of the lithium battery, preventing overcharging. The second column shows that, except for LFP/LTO-NiZn batteries which cannot fully utilize the lithium battery capacity, the other two parallel connection methods can fully utilize the lithium battery capacity, thus achieving battery pack balancing (third column). The fourth column shows that, due to the influence of the parallel NiZn and NiMH batteries, the maximum discharge current of the battery pack is lower than the maximum current of the lithium battery. Therefore, in practical applications, high-power NiZn and NiMH batteries should be selected to ensure that the battery pack performance is not degraded.
The figure below shows the charging and discharging operation of two LFP/C-2NiZn batteries connected in series. The initial capacity difference between the two LFP/C batteries is 200mAh. After one charge and discharge cycle, the capacity difference between the two battery packs is reduced to 100mAh. This means that 8% of the capacity of the two series battery packs is balanced in one cycle.
Alexander U. Schmid's work provides a new approach to battery pack balancing. Due to their design characteristics, NiMH and NiZn batteries decompose water in the electrolyte at the positive and negative electrodes respectively during overcharging, producing O2 and H2. With the help of the catalyst inside the battery, O2 combines with H2 to form water, completing a cycle. Therefore, NiMH and NiZn have excellent overcharge resistance. We can take advantage of this by connecting one or more NiMH/NiZn batteries in series with lithium batteries in parallel. When the charging voltage reaches its upper limit, almost all the current will flow through the NiMH/NiZn batteries, thus preventing the lithium batteries from overcharging. We can also use this to balance lithium battery packs. By continuously charging the battery pack, we can ensure that all batteries are fully charged without worrying about some batteries overcharging, thereby improving the consistency of the battery pack's capacity. Experiments have also confirmed that one charge-discharge cycle can achieve 8% capacity balancing (LFP/C-2NiZn). The biggest advantage of this method is that the voltage of individual cells in the battery pack does not need to be monitored throughout the process; it is all done automatically. This greatly simplifies the structure of the battery pack and improves its reliability.
