We now shift our discussion from battery design and construction to battery performance. In this section, we begin our look at performance with a description of battery charging. Charging is the process by which electrical energy is put back into the battery. A variety of different procedures or protocols are used for charging. These protocols depend on the battery chemistry, the application, and, to some extent, the battery manufacturer. We can broadly differentiate the methods used for charging based on cell chemistry, and specifically, whether or not the cell can tolerate overcharging. Lead–acid and NiCd cells are both tolerant of overcharge and, in some instances, even benefit from some overcharge. In contrast, lithium-ion cells cannot be overcharged without causing permanent damage. When a cell is overcharged, this means that more coulombs are passed with charging than are required to fully charge the cell. The ratio is known as the charge coefficient:
(8.20)
Note that this charge coefficient is related to the coulombic efficiency introduced in Chapter 7. Batteries with a coulombic efficiency less than 100% will have a charge coefficient greater than 1. The extra coulombs indicate that a side reaction is occurring, such as hydrogen and oxygen evolution in aqueous systems or SEI formation and electrolyte decomposition for lithium-ion cells.
The two basic methods of adding charge are charging at constant current (CC) and charging at constant voltage. In the first of these, the cell is charged at a constant rate (e.g., C/2–C/8 depending on the battery). During the charge, the potential of the cell rises, and charging is allowed to continue until a specified voltage is reached. The second basic method is to hold the potential of the cell constant and allow the current to vary. Constant voltage (CV) is seldom used as the sole means of charging a cell because very large currents are possible at the beginning of the charge if the voltage is held constant at its final value.
Often, these two basic methods are combined: constant current charge followed by a constant voltage charge (CCCV) as shown in Figure 8.10. In this combined method, charging is done at constant current until a specified maximum voltage is reached. Once that voltage is reached, charging continues at a fixed potential and the charging current decreases with time. This decrease in current with time is known as a current taper. The current is allowed to decrease until it reaches a specified value at which the device is considered to be fully charged. Using this protocol, the majority of charge is added in the constant current mode, but the time spent charging in each mode is roughly equal. For a lithium-ion cell, which cannot tolerate overcharge, charging is stopped once the specified current is reached (C/20 for example), and the charge coefficient is very close to one.
Two other charging protocols deserve mention. The first is pulse-charging (Figure 8.11). Here the charging current is not constant, but rather is interrupted by brief periods of rest. In some cases, the cell may even be discharged briefly. The pulse-charging protocol has been widely applied to different cell chemistries. Although more complex and not without controversy, there are some reported advantages, namely, more rapid charging and a reduced extent of sulfation for lead–acid cells.
The final charging protocol we will consider relates to the use of charging to counteract the effects of self-discharge. Although its magnitude varies greatly, all cells exhibit self-discharge. For some secondary battery applications, for example, standby power applications, the user expects that the battery will always be available and near a full state-of-charge. The most obvious strategy for addressing self-discharge is to occasionally recharge the batteries after a substantial standby period has passed. This strategy is used for lithium-ion cells and other cells that do not tolerate overcharge. An alternative that works for cells that can be overcharged is to put the cell on a float charge following the constant voltage portion of a normal charge cycle. The float charge, commonly used with lead–acid batteries, continues to pass a small current through the cell indefinitely in order to maintain a full state-of-charge. Note that the float charge portion of the cycle is not used in the charge-coefficient calculation.
This section has examined a few of the issues related to battery charging as a subset of battery performance. We next examine battery resistance as a way of evaluating the health of the battery, which is a comparison of its current performance with that achieved initially.
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