Charging finishing current

VRLA products can experience thermal runaway due to the high finishing currents available, as with CV charging. 

Electrostatic spraying is used in shops to coat conductive objects. It is very useful for odd-shaped objects such as wire fence, cables, and piping. An electrostatic potential of 60,000 volts on the object attracts oppositely-charged paint particles the spray can wrap around and coat the side of the object opposite to the spray gun. This technique produces very uniform finishes and has the least paint loss of the three methods. However, it is slow, requires expensive equipment, produces only thin coats, and is sensitive to wind currents. 

It has been assumed that the decline of the current with an increase in time (not shown in the figure) is due only to the onset of a degree of diffusion control, and that the method for obtaining the desired iF depends on this assumption. However, there are two other reasons for a decline in current. First, as already stated, the effect of double-layer charging may not be finished in the early part of the f( — t plot (between B and C in Fig. 8.9) so that it may be that a straight line between 1/i and t is observed only if the earlier points are neglected. 

Constant voltage-constant current combinations. It is also possible to combine the CV and CC approaches in fact, one of the most eflbctive methods for charging VRLA batteries is the so-called lUI algorithm. This is simply a current-limited, CV charge with a CC finishing step at some low current level an example is shown in Fig. 9.6. In this example, a CV charge at 2.45 VPC is first applied with a... 

Fig. 9.6. Example of CV charge followed by CC finish (lUI). The finish can be either a voltage-limited, low-current step (solid line) or a high-current step with no voltage limit (dotted line).

Fig. 9.17. End-of-charge voltage (CC finishing step) and end-of-charge current (CV step) as function of cycle-life for 12-V/50-Ah VRLA battery using the standard Optima lUI recharge algorithm.

Fig. 9.22. Stepped CC (upper curve) and current-limited CV (lower curve) charge algorithms with a current-interrupt finish. Cl can be initiated anywhere between 80 and 100% charge return. The y-axis is charge current in both cases.

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