Active balancing scheme for connecting batteries in series

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Overview:
Large, high-voltage rechargeable battery systems are now common in a variety of applications, from electric cars to grid load balancing systems. These large battery packs consist of a number of single-cell serial/parallel arrays that store large amounts of energy (tens of kilowatt-hours). Lithium polymer or lithium iron phosphate (LiFePO4) batteries are a common technology choice because of their high energy density and high peak power. In single-cell applications, careful control of battery charging and monitoring of the battery is key to ensuring safe operation and preventing premature aging or damage to the battery. However, unlike a single-cell battery system, a battery pack connected in series presents an additional requirement, which is “battery balancing”.

All series connected batteries must maintain charge balance. When each cell in a battery pack has the same state of charge (SoC), these cells are "balanced". SoC refers to the remaining capacity of an individual battery as it is being charged and discharged, currently relative to its maximum capacity. For example, a 10A-hr battery with a remaining capacity of 5A-hr has a 50% SoC. All batteries must remain within a certain SoC range to avoid damage or shorten life. The allowable SoC minimum and maximum values ​​vary by application. In applications where battery life is most important, all batteries can operate between a 20% SoC minimum and a 100% maximum (full charge). For applications that require the longest battery life, the SoC range may be limited to between 30% minimum and 70% maximum. In electric vehicles and grid storage systems, these values ​​are typical SoC limits, and electric and grid storage systems use very large and very expensive batteries, which are extremely expensive to replace. The primary function of the Battery Management System (BMS) is to carefully monitor all of the batteries in the battery pack to ensure that each battery does not charge or discharge beyond the minimum and maximum limits of the application's state of charge.

In the case of series/parallel battery arrays, the parallel connection of the batteries will automatically balance each other. This assumption is generally correct. That is to say, as time passes, as long as there is a conduction path between the battery terminals, the state of charge is automatically balanced between the batteries connected in parallel. The assumption that the charge state of a series connected battery will differentiate over time is correct, so there are several reasons. The SoC will gradually change due to the difference in temperature change rate across the battery pack, or the impedance, self-discharge rate, or load between different batteries. Although the charge and discharge currents of the battery pack tend to make these differences between the batteries less important, the cumulative mismatch will become larger and larger unless the battery is periodically balanced. The most basic reason for achieving charge balancing in a series connected battery is to compensate for the gradual changes in the SoC of each cell. Typically, in a battery pack with tightly matched capacities for each cell, a passive or dissipative charge balancing scheme is sufficient to bring the SoC back into equilibrium.

As shown in Figure 1A, passive balancing is simple and inexpensive. However, passive balancing is very slow, producing unwanted heat inside the battery pack, and balancing is achieved by lowering the remaining capacity of all batteries to match the battery with the lowest SoC value in the battery pack. Due to another common problem of "capacity mismatch," passive balancing also lacks the ability to effectively cope with SoC errors. As aging, the capacity of all batteries decreases, and the rate at which battery capacity decreases tends to be different for reasons similar to those previously described. Since the battery current flowing into and out of all series batteries is equal, the available capacity of the battery pack is determined by the battery with the smallest capacity in the battery pack. Only active balancing methods (such as those shown in Figures 1B and 1C) can be used to redistribute charge throughout the battery pack and to compensate for capacity reduction due to mismatch between different batteries.

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