Some people say that the internal resistance of BYD’s blade battery is too high, it generates a lot of heat, and thermal management is difficult, so the top speed of the vehicle is also limited. If you want to solve it, you need to use a larger battery to reduce the discharge rate of each cell. For LFP batteries, the blogger recommends at least 70kWh to meet the power and life requirements. For ternary batteries, there is no requirement for the number of kilowatt-hours of electricity, and cars above 50kWh are acceptable. Power-type battery packs, such as the Lilith L9 ternary battery pack, have a total power of 330kW and a battery energy of only 42kWh. So why is the internal resistance of the blade battery relatively large? This article will take you to find out.
1.Current collector resistance of blade battery
Compared with ordinary batteries, the biggest difference between blade batteries is that they are extremely long, close to 1m in length. For comparison, the length of CATL’s ordinary square aluminum-shell batteries generally does not exceed 0.2m because the tabs are in the height direction. However, the length of the soft-packed batteries provided by LG for the Volkswagen MEB platform reaches 0.54m, which is almost the limit. . The length of the battery core mainly affects the resistance of the current collector. The calculation method is as shown in the figure below (only a single pole is considered, located at one end of the pole piece or in the middle):
In fact, if it is assumed that the length of the current collector is L, the distance between the pole welding position is xL from one end and (1-x)L from the other end, when x=0 or x=1, it means that the pole is welded at the end; when When x=0.5, it means that the tab is welded in the geometric middle of the length direction. After a series of derivation, the calculation formula of current collector internal resistance is obtained:
L in the formula is the length of the pole piece. The path that electrons take from the pole piece to the pole ear has a linear impact on the current collector resistance. The blade battery is similar to the design with the tab at one end. For the 138Ah blade battery, the length and width of the positive aluminum foil are 944mm83mm respectively. Considering the process stability, assuming that the aluminum foil is 16µm thick, the resistivity of the aluminum foil is 2.8310-8Ωm, single The resistance of the pole piece is R=ρL/3S=2.8310-894410-3/(3831610-9)=0.00671Ω=6.71mΩ. The entire blade battery is composed of 26 positive electrodes. Equivalent to 26 parallels, the total resistance is 6.72/26=0.26mΩ. Similarly, the resistivity of copper foil is 1.7210-8Ωm, the thickness is 8µm, and the negative electrode is 94685mm. There are 27 layers in total. The calculated single-chip resistance is 7.98 mΩ and the total resistance is 0.31 mΩ (the calculation is still based on 26, because One piece does not participate in the reaction). In this way, the combined ohmic resistance of the positive and negative pieces is 0.57 mΩ. At 50% SOC, the DCR of the entire blade battery is 1.1±0.2mΩ (30s, 1.5C), and the ohmic internal resistance of the positive and negative electrodes accounts for half, which cannot be ignored. It can be seen that the ohmic resistance of the blade battery is indeed large because the pole piece is too long. Referring to the internal resistance ratio of each part of the small soft-pack battery below, the current collector resistance only accounts for about 10% of the overall DCR DC resistance. This is still the data for small batteries. If it is a large battery, its proportion will be lower, because as the number of parallel layers increases, the resistance of the pole pieces and current collectors decreases proportionally, but the resistance of the tabs is not. changing. Therefore, the proportion of the latter will increase and the proportion of the former will decrease.
For comparison, CATL had an LFP battery in the early years with a capacity of about 120Ah, which was lower than the 138Ah blade battery (which indicates that the internal resistance should be larger). Its model is 48175165mm, the size of the positive electrode sheet is 136135mm, the thickness is 16µm, a total of 32 pieces; the size of the negative electrode sheet is 141137mm, the thickness is 9µm, a total of 33 pieces. For the convenience of analysis, it is assumed that it is in the form of lamination , full pole design, the resistance of each pole piece is R=ρL/3s, then the total resistance of the positive pole piece is: R=ρL/3S/32=2.8310-813510-3/(3136* 1610-9)/32=0.586mΩ/32=0.0183mΩ. In the same way, the total resistance of the negative electrode can be calculated as R=ρL/3S/32=1.7210-813710-3/(3* 1419109)/32=2.926mΩ/32=0.0193mΩ, the total resistance of the positive and negative electrodes is 0.0376mΩ, the total AC resistance of the battery is about 0.271mΩ, the DC internal resistance is generally a little more than twice the AC internal resistance , from this point of view, the resistance of the pole piece accounts for only about 7% of the entire DC internal resistance. It can be seen that as the thickness of the battery increases and the height decreases, its resistance will decrease accordingly, so short and thick is better than slender.
2.A short knife idea to reduce the internal resistance of the current collector
In addition to large internal resistance, batteries with a length of about 1m also face many problems in terms of production efficiency and yield rate. In addition to being suitable for its own automobile products, BYD’s blade batteries are not suitable for other car companies. not good. At the same time, the blade battery patent applied by BYD covers cells with a length of 600-2000mm. In order to improve adaptability and avoid patent disputes, Honeycomb Energy prefers cells with a length of about 600mm, and uses cells with a length of 300~600mm. It is commonly known as “dagger”. According to the above calculation method, if the length of the short knife battery is 480mm, the total resistance of the positive and negative electrodes will be halved to about 0.285mΩ. At the same time, in order to increase the capacity, it is necessary to increase the thickness of the battery core and increase the number of parallel electrodes. The final pole piece resistance will be halved, which is about 0.143mΩ. It can be seen that reducing the length and increasing the thickness can effectively reduce the resistance of the battery core. This also shows that even if all-solid-state batteries are realized in the future, we cannot expect only a few batteries to form a battery pack. In that case, the electronic transmission path will become longer, which will cause the overall internal resistance of the battery pack to be much larger, which is even more detrimental to the battery. performance.
3.Summary for resistance of blade battery cell
BYD is both a car company and a battery company. Its batteries have been designed with the needs of its own cars in mind. The main idea is to increase the volume energy density so that the car can be equipped with more batteries to achieve long battery life. BYD was the first to launch it in Shenzhen The electric taxis e5 and e6 have this idea. However, although the blade battery improves the volumetric energy density, its internal resistance is indeed too large, which is not conducive to high-power performance release. LFP batteries are safer, more resistant to high temperatures, and their thermal management is better. It will be more difficult to replace them with ternary batteries. It is estimated that ternary system blade batteries will not be installed in cars in the short term. The above analysis also shows that although thin and long battery cells have better heat dissipation, the resistance is greater; thick and short battery cells have difficulty in heat dissipation, but have lower resistance. Taking comprehensive considerations into account, the size of the current 4C charging battery cells It still adopts a thick and short (44194112~143Ah) design, and adopts double-sided heat dissipation and large-surface heat dissipation to greatly improve the cooling capacity. For the DM-i model, BYD also adopts an internal series connection method to reduce the internal resistance and increase the power by reducing the length of the battery cells. Therefore, the high resistance of the blade battery is indeed a problem, but it does not necessarily lead to safety issues. Most of the safety accidents that occurred in the past two years were DM-i models. The batteries connected in series inside are packaged in soft packages. Once leakage occurs, it is easy to cause a short circuit and catch fire. However, BYD should solve this problem in subsequent models.