The use of lithium-ion (Li-Ion) batteries in portable wireless electronics is a growing reality today. High energy density and long charge-discharge cycle life are two driving reasons for this, in addition to overcoming some of the shortcomings of previous technologies, like memory effects . Typically, these batteries are charged to within ±1% of their full-charge voltage, for maximum capacity and long cycle life (that is, highest energy) , but overcharging them is destructive and therefore prohibitive. As a result, the charging process is often relatively slow and cumbersome, requiring the charger to monitor and carefully manage a winning tradeoff between reliability and capacity.
The Charging Process
The Li-Ion battery usually draws power from an AC wall outlet or existing DC supplies via a charger, which is nothing more than a combination of linear and switching regulator circuits, as generally shown in Fig. 1(a). For safety, efficiency, and long operational life, the charger must carefully traverse through a series of charging phases. First, it may have to pre-condition the battery, if for instance, the battery voltage is below its minimum rated value (for example, below 2.7 V); in which case, a low charging current is driven into the battery until it reaches an acceptable low charge voltage. Then, it enters the charging phase, sourcing a larger, well-regulated current. Both of these steps are realized with what amounts to a constant, but dependent, current-source charging circuit, as represented by IC in Fig. 1(b).
Figure 1. Typical (a) charging system and (b) Li-Ion charging scheme
When the battery voltage nears the full- or end-of-charge voltage limit, the charging current decreases to whatever value is necessary to ensure the battery voltage reaches its targeted value. This is achieved by transitioning from a constant and dependent current source into a voltage-source circuit, as shown in Fig. 1(b), wherein, as the battery voltage approaches the targeted VRef, the charging current gradually decreases. Finally, when the charge current decreases below a pre-determined end-of-charge limit, the charging sequence stops and the charger is disengaged. This foregoing charging scheme is referred in literature as the constant-current to constant-voltage (CC-CV) technique .
Reaching the end-of-charge voltage target accurately (for maximum energy) and smoothly traversing between the various phases of the charging process are key features of the charger. In practice, interconnecting current and voltage feedback loops manage the entire charging process, giving rise to three operational regions: constant current, intermediate current-to-voltage, and constant voltage region. A benign interplay between these two feedback loops is critical for a safe and uninterrupted charge. Relatively complex digital and bootstrapped switching circuits are normally used for this purpose, both in academic and commercial circles [3-5], and the foregoing presentation addresses all these features in a relatively simple and effective form.