Battery balancing and battery redistributionrefer to techniques that improve the available capacity of a battery pack with multiple cells and increase each cell's longevity. A battery balancer or battery regulator is an electrical device in a battery pack that performs battery balancing. Balancers are often found in lithium-ion battery packs for laptop computers, electrical vehicles. etc.
Rationale
Typically, the individual cells in a battery have somewhat different capacities and may be at different levels of state of charge. This is due to manufacturing variances, assembly variances, different histories experienced amongst the cells in a battery pack and must be accounted for to maximize life and service of the particular battery pack in use. Each battery cell will be, for these reasons, somewhat different and so every balancing circuit must be able to accommodate those differences. Without effective and appropriate balancing, discharging during use must stop when any cell first runs out of charge ; this limits the energy that can be taken from and returned to the battery. Without balancing, the smallest capacity cell is a problem, and potentially a serious one. It can be easily overcharged or over-discharged whilst cells with higher capacities are only partially charged. The balance circuit should arrange for higher capacity cells to fully charge/discharge, while smaller capacity cells are charged/discharged suitably—which will necessarily be rather different. In a properly balanced battery pack, the cell with the largest capacity will be filled without overcharging any other cell, and it can be discharged in use without over-discharging any other cell. Battery balancing is done by transferring energy from or to individual cells, until the SOC of the cell with the lowest capacity is equal to the battery's SOC. Battery redistribution is sometimes distinguished from battery balancing by saying the latter stops at matching the cell's state of charge only at one point, so that the battery's capacity is only limited by the capacity of its weakest cell which is also categorized as passive balancing. A full battery management system might include active balancing as well as temperature monitoring, charging, and other features to maximize the life of a battery pack. Lithium ionrechargeable battery cells are rather more sensitive to overcharging, overheating, improper charge levels during storage, and other forms of mistreatment, than most commonly used battery chemistries. The reason is that the various lithium battery chemistries are susceptible to chemical damage by only very slight overvoltages during charging, or more charging current than the internal chemistry can tolerate at this point in its charge/discharge cycle, and so on. Heat accelerates these unwanted, but so far inescapable, chemical reactions and overheating during charging amplifies those effects. Because lithium chemistries often permit flexible membrane structures, lithium cells can be deployed in flexible though sealed bags, which permits higher packing densities within a battery pack. Some of the breakdown products outgas when mistreated; such cells will become 'puffy' and are very much on the way to failure. In sealed lithium ion cylinders, the same outgassing has caused rather large pressures ; such cells can explode if not provided with a relief failure mechanism. Compounding the danger is that many lithium cell chemistries include hydrocarbon chemicals which are flammable. Not only is explosion a possibility with mistreated lithium cells, but even a non-explosive leak can cause a fire. Most battery chemistries have less dramatic, and less dangerous, failure modes. The chemicals in most batteries are often toxic to some degree, but are rarely explosive or flammable; many are corrosive, which accounts for advice to avoid leaving batteries inside equipment for long periods as the batteries may leak and damage the equipment. Lead acid batteries are an exception, for charging them generates hydrogen gas, which can explode if exposed to an ignition source and such an explosion will spray sulfuric acid in all directions. Since this is corrosive and potentially blinding, this is a particular danger.
Technology
Balancing can be active or passive. The term battery regulator typically refers only to devices that perform passive balancing. In passive balancing, energy is drawn from the most charged cell and dissipated as heat, usually through resistors. In active balancing, energy is drawn from the most charged cell and transferred to the least charged cells, usually through capacitor-based, inductor-based or DC-DC converters. Battery balancing can be performed by DC-DC converters, in one of 3 topologies:
Cell-to-battery
Battery-to-cell
Bidirectional
Typically, the power handled by each DC-DC converter is a few orders of magnitude lower than the power handled by the battery pack as a whole.
