Expert Decryption: How to greatly improve the battery life of electric vehicles with active balancing technology

  Regardless of how many cars are sold each year, the electronic systems in the car will continue to increase. The demand for hybrid and all-electric vehicles is further driving the growth of the automotive electronics market. With the increasing popularity of batteries as a power source, the same need has arisen to maximize battery life. Is there a good technology to greatly improve the battery life of electric vehicles?

The automotive market is the terminal market that Linear Technology focuses on. In the last fiscal year, our revenue in the automotive market has grown to 20% of the company's total revenue, and the growth rate continues to be higher than the company's overall growth rate. Most of the innovation and differentiation in the automotive industry comes from automotive electronics systems. The need to improve driving safety, fuel efficiency and comfort creates tremendous business opportunities. The proliferation of hybrid and all-electric vehicles will steadily drive demand for innovative analog electronics. In addition to the increase in automotive electronic systems, the global potential automotive market is expected to grow steadily.

Regardless of how many cars are sold each year, the electronic systems in the car will continue to increase. The demand for hybrid and all-electric vehicles is further driving the growth of the automotive electronics market. With the increasing popularity of batteries as a power source, the same need has arisen to maximize battery life. Battery imbalance (i.e., mismatch in state of charge between the various cells that make up the battery pack) is a problem with large lithium-ion battery packs, which is caused by differences in manufacturing processes, operating conditions, and battery ageing. Battery imbalance can reduce the overall capacity of the battery pack and can damage the battery pack. Battery imbalance prevents the battery tracking process from the state of charge to the state of discharge, which, if not strictly monitored, can result in overcharging or overdischarging of the battery, which permanently damages the battery.

For batteries used in hybrid electric and all-electric battery packs, battery manufacturers classify their capacity and internal impedance to reduce the difference between specific batches of batteries delivered to customers. Then, the battery pack is constructed with carefully selected batteries to improve the overall matching between the batteries in the battery pack. In theory, this should prevent a serious battery imbalance in the battery pack, but nevertheless, a general consensus is that when composing a large battery pack, it is necessary to maintain a large battery capacity during the life of the battery pack. Both monitoring and battery balancing are required. Linear Technology's new battery management system (BMS) product line is designed to meet this demand. These product lines are popular and are used in cars and buses that are currently in production and use. Not one of the many BMS brands.

Currently, large, high-voltage rechargeable battery systems are a common source of power in electric vehicles. These large battery packs consist of a series/parallel battery array that can store large amounts of energy (10 kWh). Lithium polymer or LiFePO4 batteries are a common choice because of their high energy density and strong peak power capability. As with single-cell applications, the battery packs in these battery packs must be carefully controlled and each battery cell must be carefully monitored to ensure safe operation and prevent premature aging or damage to the battery. However, unlike a single-cell battery system, a battery pack connected in series creates an additional requirement: battery balancing.

These batteries are "balanced" when all of the batteries in the battery pack have the same state of charge (SoC). SoC refers to the ratio of its current remaining capacity to its maximum capacity as each battery is charged and discharged. For example, a 10 A-hr battery with a current remaining capacity of 5A-hr is 50% of its SoC. All batteries must be kept within the same SoC range to avoid damage or shorten life. The allowed SoC minimum and maximum values ​​are different, depending on the application. In applications where battery life is most important, all batteries may operate between a 20% SoC minimum and a 100% (full charge) maximum. Applications that require the longest battery life may limit the SoC range from a minimum of 30% to a maximum of 70%. These values ​​are typical SoC limits for electric vehicles that use very large, very expensive batteries and are extremely costly to replace. The primary role of the BMS is to carefully monitor all of the batteries in the battery pack to ensure that all batteries are charged and discharged without exceeding the minimum and maximum SoC limits of the application.

For series/parallel battery arrays, it is assumed that the batteries connected in parallel are automatically balanced with each other and are generally safe. That is to say, as time changes, the state of charge of the parallel connected batteries is automatically balanced as long as the conduction path between the battery terminals exists. It is assumed that the state of charge of the battery connected in series tends to deviate over time, resulting in a variety of factors that cause deviation. The temperature change rate or impedance of the battery in the battery pack is different, the self-discharge rate is different, or the loading of each battery is different, which may cause the SoC to gradually change. Although these differences between cells tend to be less important than the charge and discharge currents of the battery pack, the charge state mismatch will gradually accumulate without diminishing unless all cells are periodically balanced. The SoC of each battery will change gradually and need to be compensated. This is the most fundamental reason for balancing the battery in series. In general, for battery packs with tightly matched capacities, passive or consumable balancing methods are sufficient to deal with SoC rebalancing issues.

The passive balancing method is simple and inexpensive. However, the passive balancing speed is very slow, generating unwanted heat inside the battery pack, which is achieved by lowering the remaining capacity of all the batteries to match the lowest battery of the SoC in the battery pack. Due to another common problem, capacity mismatch, the ability of passive balancing methods to effectively cope with SoC errors is also insufficient. All batteries experience a decrease in capacity with aging, and for similar reasons as described above, the rate of capacity decline tends to be different. Since the battery currents of all series batteries are equal, the battery with the lowest capacity in the battery pack determines the available capacity of the battery pack. Only the active balancing method can redistribute the charge across the entire battery pack, compensating for the capacity drop caused by mismatch between the batteries.

Whether the capacity or the SoC mismatch between the batteries will seriously reduce the available capacity of the battery pack unless the batteries are balanced. To maximize battery capacity, it is required to balance the battery when charging and discharging the battery pack.

For example, there is a battery pack consisting of 10 batteries in series, each battery is 100A-hr (nominal value), from the smallest battery to the largest battery, the capacity error is +/-10%, for this The battery pack is charged and discharged until the preset SoC limit is reached. If the SoC value is limited to 30% to 70% and no cell balancing is performed, then after a full charge/discharge cycle, the actual available battery pack capacity is reduced by 25% relative to the theoretical available capacity of these cells. When the battery pack is being charged, the passive balancing method can theoretically balance the SoC of each battery, but when the battery pack is discharged, this method cannot prevent the No. 10 battery from reaching the 30% SoC value before other batteries. Even if passive balancing is used while the battery pack is charging, there is still a large amount of "lost" (unavailable) when the battery pack is discharged. Only with an active balancing solution, “capacity recovery” can be achieved by redistributing charge from a high SoC battery to a low SoC battery while the battery pack is discharging.

With the "ideal" active balancing method, the "lost" capacity due to battery mismatch can be recovered 100%. When using this method in a steady state, when the battery pack is discharged from its 70% SoC "full" state of charge, the charge stored in the No. 1 battery (the largest battery) must be effectively taken out and transferred to the No. 10 battery. (The battery with the smallest capacity), otherwise the No. 10 battery will reach its 30% minimum SoC value before the remaining batteries, then the battery pack discharge must be stopped to prevent the battery pack life from further shortening. Similarly, when charging, the charge must be transferred from the No. 10 battery and redistributed to the No. 1 battery. Otherwise, the No. 10 battery will reach its 70% SoC limit before other batteries, and the charging cycle must be terminated. At some point during the life of the battery pack, differences in the degree of battery aging will inevitably lead to capacity mismatch between the batteries. Only an active balancing solution can achieve “capacity recovery” by redistributing charge from a high SoC battery to a low SoC battery on demand. To achieve maximum battery capacity over the life of the battery pack, an active balancing solution is required to efficiently charge and discharge the cells to maintain SoC balance throughout the battery.

Linear Technology's LTC3300 (see Figure 1) is a new product specifically designed to meet the high performance active balancing needs of electric vehicles. The LTC3300 is a high efficiency, bidirectional active balance control IC that is a key component of a high performance BMS system. Each IC can simultaneously balance up to 6 series connected Li-Ion or LiFePO4 batteries.

Expert Decryption: How to greatly improve the battery life of electric vehicles with active balancing technology

Figure 1: Linear Technology's LTC3300 is a two-way active battery balancer for electric vehicles

SoC balancing is achieved by redistributing the charge between a given battery and up to 12 or more adjacent cells. The balance decision and balance algorithm must be handled by a separate monitor component and system processor that controls the LTC3300. Redistribute charge from a designated battery to 12 or more adjacent cells to discharge the battery. Similarly, charge is transferred from a 12 or more adjacent cells to a designated battery to charge the battery. All balancers can operate in the same direction of charge transfer to minimize battery pack balancing time. All balance control commands are provided to each IC via a stackable, noise-prone serial SPI interface with no restrictions on stack height.

Each balancer in the LTC3300 uses a non-isolated, boundary-mode synchronous flyback power stage for efficient charging and discharging of each cell. Each of the 6 balancers requires its own transformer. The "main" side of each transformer is connected across the balanced battery, and the "secondary" side is connected across the battery pack of 12 or more adjacent cells, including the balanced battery. The number of batteries connected to the secondary side is limited only by the breakdown voltage of the external components. Corresponding to the adjustment range of the external switch and transformer, the charge and discharge current of the battery can be set to a value of up to 10A or more by an external sense resistor. Whether to sort and whether to detect the IPEAK/IZERO current through the primary and secondary components depends on whether the balancer is activated to charge or discharge a battery. High efficiency can be achieved by running synchronously and properly selecting components. Each balancer is activated by the BMS system processor, and these balancers will remain active until the BMS command requires a balance to be stopped or a fault condition is detected.

One of the biggest challenges of battery packs is heat. High ambient temperatures can quickly shorten battery life and reduce its performance. Unfortunately, in high current battery systems, the balancing current must also be large to extend run time or to achieve fast charging of the battery pack. An inefficient balancer can cause unwanted heat in the battery system. This problem must be solved by reducing the number of balancers that can be run at a given time or by expensive heat reduction methods. The LTC3300 achieves >90% efficiency in both charging and discharging directions, which allows the balancing current to be more than doubled in the case of an 80% efficiency solution with equal balancer power consumption. In addition, the higher the balancer efficiency, the more efficient the charge distribution, which in turn makes capacity recovery more efficient and charging faster.

Expert Decryption: How to greatly improve the battery life of electric vehicles with active balancing technology

Figure 2: How the LTC3300 transfers charge across the entire battery pack

The transfer of charge throughout the battery pack is accomplished by a secondary interleaved connection, as shown in Figure 2. Interleaved in this manner allows any battery pack of charge from 6 cells to be transferred to an adjacent battery pack. Please note that the adjacent battery can be the upper battery or the lower battery in the battery pack. This flexibility is very useful when optimizing the balancing algorithm. A common misconception about staggered systems is that the redistribution of charge from the top to the bottom of a very long battery pack must be very inefficient because moving the charge from top to bottom requires a large amount of conversion. In practice, however, most of the balance can be accomplished by simply redistributing the charge to or from those cells that are closest to the battery that needs to be balanced. In a secondary battery pack consisting of 10 or more cells, by operating only one balancer, the weaker battery is allowed to recover more than 90% of the "lost" capacity, otherwise the battery will limit the entire battery. The running time of the group. Therefore, with the LTC3300's staggered topology, there is no need to move charge from the top of the battery pack to the bottom, and most of the balancing work is done locally by adjacent cells.

In addition to providing excellent electrical performance, the LTC3300 bidirectional active balancer offers a wide range of driving safety features to prevent flashover during balancing and maintain maximum reliability. Data integrity check (CRC check for all incoming and outgoing data, watchdog timer, and readback data) Prevents the balancer from responding to unexpected or erroneous commands. Programmable volt-second clamps ensure that current sense faults during balancing do not cause current runaway. Battery-by-battery overvoltage and undervoltage verification and secondary side overvoltage detection prevent sudden battery line faults from damaging the balancing circuit during balancing.

These features enable the LTC3300 to deliver high performance in a series battery system and provide reliable active balancing, such as the tandem battery system commonly found in electric vehicles. As batteries in such systems age or need to be replaced, it is becoming increasingly important to compensate for the resulting battery capacity mismatch to prevent further damage to run time, charging time or battery pack life.

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