Fast-charge and termination methods used depend on cell chemistry and other design factors. The following discussion covers fast-charging techniques widely used for today's common battery chemistries. For specific guidelines and recommendations, consult the battery manufacturer's applications department.

NiCd and NiMH Cells

Fast-charging procedures for NiCd and NiMH batteries are very similar; they differ primarily in the termination method used. In each case, the charger applies a constant current while monitoring battery voltage and other variables to determine when to terminate the charge. Fast-charge rates in excess of 2C are possible, but the most common rate is about C/2. Because charging efficiency is somewhat less than 100%, a full charge at the C/2 rate requires slightly more than two hours.
While constant current is applied, the cell voltage rises slowly and eventually reaches a peak (a point of zero slope). NiMH charging should be terminated at this peak (the 0DV point). NiCd charging, on the other hand, should terminate at a point past the peak: when the battery voltage first shows a slight decline (-DV) (Figure 2). Cell damage can result if fast charge continues past either battery's termination point.

Figure 2. NiCd battery-charging characteristics at C/2 rate.
At rates exceeding C/2 (resulting in a charge time of no more than two hours), the charger also monitors the cell's temperature and voltage. Because cell temperature rises rapidly when a cell reaches full charge, the temperature monitor enables another termination technique. Termination on this positive temperature slope is called DT termination. Other factors that can trigger termination include charging time and maximum cell voltage. Well-designed chargers rely on a combination of these factors.
Note: Because certain effects that appear when a cell first begins charging can imitate termination conditions, chargers usually introduce a delay of one to five minutes before activating slope-detection termination modes. Also, charge-termination conditions are difficult to detect for rates below C/8, because the voltage and temperature slopes of interest (DV/Dt and DT/Dt) are small and comparable to other system effects. For safety during a fast charge, the hardware and software in these systems should always err on the side of earlytermination.

Lithium-Ion Cells

Li+ battery charging differs from the nickel-chemistry charging schemes. A top-off charge can follow to ensure maximum energy storage in a safe manner. Li+ chargers regulate their charging voltage to an accuracy better than 0.75%, and their maximum charging rate is set with a current limit, much like that of a bench power supply (Figure 3). When fast charging begins, the cell voltage is low, and charging current assumes the current-limit value.

Figure 3. Li+ battery voltage vs. charging current.
Battery voltage rises slowly during the charge. Eventually, the current tapers down, and the voltage rises to a float-voltage level of 4.2V per cell (Figure 4).

Figure 4. Li+ battery-charging profile.
The charger can terminate charging when the battery reaches its float voltage, but that approach neglects the topping-off operation. One variation is to start a timer when float voltage is reached, and then terminate charging after a fixed delay. Another method is to monitor the charging current, and terminate at a low level (typically 5% of the limit value; some manufacturers recommend a higher minimum of 100mA). A top-off cycle often follows this technique, as well.
The past few years have yielded improvements in Li+ batteries, the chargers, and our understanding of this battery chemistry. The earliest Li+ batteries for consumer applications had shortcomings that affected safety, but those problems cannot occur in today's well-designed systems. Manufacturers' recommendations are neither static nor totally consistent, and Li+ batteries continue to evolve.

Lead-Acid Cells

PbSO4 batteries are usually charged either by the current-limited method or by the more common and generally simpler voltage-limited method. The voltage-limited charging method is similar to that used for Li+ cells, but high precision isn't as critical. It requires a current-limited voltage source set at a level somewhat higher than the cell's float voltage (about 2.45V).
After a preconditioning operation that ensures that the battery will take a charge, the charger begins the fast charge and continues until it reaches a minimum charging current. (This procedure is similar to that of a Li+ charger). Fast charge is then terminated, and the charger applies a maintenance charge of VFLOAT (usually about 2.2V). PbSO4 cells allow this float-voltage maintenance for indefinite periods (Figure 5).

Figure 5. PbSO4 battery-charging profile.
At higher temperatures, the fast-charge current for PbSO4 batteries should be reduced according to the typical temperature coefficient of 0.3% per degree centigrade. The maximum temperature recommended for fast charging is about 50°C, but maintenance charging can generally proceed above that temperature.

Optional Top-Off Charge (All chemistries)Chargers for all chemistries often include an optional top-off phase. This phase occurs after fast-charge termination and applies a moderate charging current that boosts the battery up to its full-charge level. (The operation is analogous to topping off a car's gas tank after the pump has stopped automatically.) The top-off charge is terminated on reaching a limit with respect to cell voltage, temperature, or time. In some cases, top-off charge can provide a run life of 5% or even 10% above that of a standard fast charge. Extra care is advisable here: the battery is at or near full charge and is therefore subject to damage from overcharging.

Optional Trickle Charge (All chemistries except Li+)Chargers for all chemistries often include an optional trickle-charge phase. This phase compensates for self-discharge in a battery. PbSO4 batteries have the highest rate of self-discharge (a few percent per day), and Li+ cells have the lowest. The Li+ rate is so low that trickle charging is not required or recommended. NiCds, however, can usually accept a C/16 trickle charge indefinitely. For NiMH cells, a safe continuous current is usually around C/50, but trickle charging for NiMH cells is not universally recommended.
Pulsed trickle is a variation in which the charger provides brief pulses of approximately C/8 magnitude, with a low duty cycle that provides a typical average trickle current of C/512. Because pulsed-trickle charging applies to both nickel chemistries and lends itself well to the on/off type of microprocessor (µP) control, it is used almost universally.

Generic Charging SystemBefore looking at specific circuit implementations, designers should become familiar with generic blocks and features (Figure 6). All fast chargers should include these block functions in some form. The bulk power source provides raw dc power, usually from a wall cube or brick. The current and voltage controls regulate current and voltage applied to the battery. For less-expensive chargers, the regulator is usually a power transistor or other linear-pass element that dissipates power as heat. It can also be a buck switching supply that includes a standard freewheeling diode for average efficiency or a synchronous rectifier for highest efficiency.

Figure 6. Generic charging-system block diagram.
The blocks on the right in Figure 6 represent various measurement and control functions. An analog current-control loop limits the maximum current delivered to the battery, and a voltage loop maintains a constant voltage on the cell. (Note that Li+ cells require a high level of precision in the applied charging voltage.)
A charger's current-voltage (I-V) characteristic can be fully programmable, or it can be programmable in current only, with a voltage limit (or vice versa). Cell temperature is always measured, and charge termination can be based either on the level or the slope of this measurement. Chargers also measure charging time, usually as a calculation in the intelligence block.
This block provides intelligence for the system and implements the state machine previously described. It knows how and when to terminate a fast charge. Intelligence is internal to the chip in stand-alone charger ICs. Otherwise, it resides in a host µC, and the other hardware blocks reside in the charger IC. As mentioned previously, this latter architecture is the one preferred today.