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Fast-Charge Phase and Termination---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.

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