At the same time, some and most of the time, the capacity and remaining power of each battery in the electric vehicle battery pack must be inconsistent. The main reasons are:
3.1 There will be inconsistencies in the battery manufacturing process,
As a chemical product, subject to the constraints of materials, processes and other factors, single batteries are inherently inconsistent, and different batches of battery raw materials may lead to inconsistencies in the chemical properties of battery materials;
Even with the same batch of raw materials, due to grinding, stirring, and spraying processes, the
particle size and conductivity of the electrode material may be inconsistent, and the SEI film formed near the electrode also has a certain randomness, resulting in inconsistent batteries.,,,,,
Only in the process of group matching, batteries with similar voltage and capacity should be selected as much as possible to form modules.
3.2 The difference in the working environment is also one of the important factors leading to the inconsistency of the battery
Even if the single battery is the same, the performance in different external working environments will be different, which will further promote the occurrence of inconsistency (heat conduction, air convection, and heat dissipation conditions in the battery pack cannot occur uniformly on each battery. ),
Due to chemical reactions and polarization internal resistance conditions, there is inevitably an endothermic and exothermic effect, which makes the temperature field distribution inside the battery pack uneven.
So the inhomogeneity can lead to inconsistency in the cell, and the inconsistency in the cell can also exacerbate the overall inhomogeneity.
Appropriately reducing the charge and discharge depth of the power battery (when the battery pack is not scrapped and the consistency of different monomers is large) can improve the safety of the battery and prolong the battery life.
If active or passive equalization is added during this process, it can improve safety, prolong life and retain more battery performance (as shown by a large number of theoretical analysis and experimental data).
Therefore, it is important for the battery pack to be self-balancing or to be maintained by professionals (including the necessary reduction of the depth of discharge DOD and output energy), which can not only improve the overall effective energy of the power battery pack, but also extend the use of the power battery pack to a certain extent life.
It is relatively difficult to formulate and implement an effective equalization control strategy. (Battery voltage alone is biased as the basis for equalization, that is, it is inaccurate to think that a battery with a higher voltage discharges charge and a battery with a lower voltage replenishes charge. There is no basis for judging the remaining battery power or SOC, voltage detection is simple, but accurate battery remaining power or SOC statistics are very difficult)
It can be seen from the above that the remaining capacity of the battery is limited by factors such as frequent charging and discharging of electric vehicles, battery aging SOH, etc., and there is no way to get it well;
It is also difficult to accurately estimate the SOC of each battery.
Even if the SOC value and remaining capacity of the battery are known accurately, it is not easy to transfer the power of the single battery with high power to the single battery with low power in the N batteries connected in series in the battery pack, under the constraints of cost and space, etc. ( If a poor balance strategy is formulated, the more balanced the more unbalanced the situation may appear).