Analysis of battery balance control for electric-Lithium - Ion Battery Equipment

Analysis of battery balance control for electric vehicle -Lithium - Ion Battery Equipment

In the battery pack of electric vehicle (EV), the inconsistency of single battery will reduce the use level of battery pack and affect the performance of EV. Research on advanced battery balance control technology to reduce the differences in the use of single batteries will be able to maximize the efficiency of batteries, extend the service life, and increase the safety of EVs.

The equilibrium theory and control technology studied by the author in this paper can not solve the performance difference caused by the manufacturing process of the battery. The production process and screening criteria are the decisive factors for the consistency of the battery pack before use.

1 Control model of intelligent balance

As there may be batteries with low and high residual energy in the battery pack, a high-performance equalization management system should have a charge discharge equalization function, a good charge discharge equalization matching and control strategy, and a control of the recovery and reuse of discharge energy.

On the premise of ignoring the difference between the charging and discharging efficiency of the battery, as for the single battery with the discharge current equal to the charging current, the discharge energy will be equal to the charging energy, and its energy will maintain a dynamic balance; For a single battery with a discharge current less than the charging current, the discharge energy will be less than the charging energy, and the energy will continue to increase. The smaller the discharge current, the faster the energy will increase, and vice versa.(Lithium - Ion Battery Equipment)

In the process of energy flow, on the one hand, a balanced energy of discharge occurs through the discharge of the whole group of batteries. In the discharge equalization process, although the discharge current of all batteries is the same, the batteries with high residual energy actually release more energy due to their high electromotive force, that is, the higher the proportion of the discharge energy of the batteries, and vice versa.

On the other hand, the energy released by the whole group of batteries is converted to supplement the low energy batteries through independent charging. In the process of charging equalization, the battery with low residual energy has high charging current due to low electromotive force, and the higher the charging energy obtained by the battery, that is, the higher the proportion of the charging energy of the battery, and vice versa.

If the charge discharge equalization lines and parameters used by all single batteries are identical, the distribution and flow of equalization energy only depends on the energy state of the single battery. The less the remaining energy, the more energy the battery will charge and the less energy it will release, and vice versa. There will be neither a drop in all battery energy nor a rise in all battery energy. For the battery with good consistency, the residual energy state always keeps good consistency dynamically; Among the batteries with poor consistency, the charging energy of the battery with high residual energy is less than the released energy, or even the charging energy is equal to zero. As a result, the energy is released quickly, which is close to the battery with good consistency; The charging energy of the battery with low residual energy is greater than the released energy. As a result, the energy is quickly supplemented, and thus tends to be similar to the battery with good consistency. The actual equalization effect is that the discharge energy flows from the high energy battery to the low energy battery. The macroscopic performance is that the energy of the battery pack is evenly distributed and adjusted among all single batteries. The model can realize the automatic and proportional flow and distribution of battery pack energy according to the difference of individual battery energy state, and the energy balance process is highly intelligent.

2 Control strategy of intelligent balance

In this model, the parameters of all charging units are identical, so all secondary windings of T are identical in design, and the charging voltage Ui~Un is equal. According to the working principle of half bridge inverter circuit, the charging voltage is:

Uch is the charging voltage of single battery; Ut is the total voltage of the battery pack in the balanced charge discharge state; Is the duty cycle of the inverter circuit, that is, the ratio of the opening time of the power switch T1 or T2 to the switching cycle; Np is the number of turns of the primary winding of the high-frequency transformer; Ns is the number of turns of secondary winding of high-frequency transformer.



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