Smart Chargers

MAIN AIRCRAFT BATTERY "INFLECTION POINT" CHARGING
SIGNIFICANTLY EXTENDS BATTERY MAINTENANCE INTERVALS
By Richard Lukso 
     It has been a long desired goal of battery designers to use the illusive "Inflection Point" as the criteria for cutting off "fast" charge of a battery. Why? Because it represents the point where maximum fast charge has been achieved with minimum amount of overcharge, resulting in minimum overheating.

     It is well known that overcharge is a major cause of battery problems. It is especially acute for "flooded" or vented batteries because overcharge causes heat buildup, which causes fluid/gas boil-off. This results in frequent battery maintenance, as well as short life.

     So, just exactly what is the "Inflection Point?"  Referencing figure 1, it can be seen that it is the point where the change in voltage rate equals zero. In engineering terms, it is when the second derivative of voltage is zero.
As appealing as inflection point fast charging cutoff should be, it may come as a surprise that few, if any, use it today. The reason is because the zero value of a second derivative is nearly impossible to detect in the face of battery load variations using classical electronic methods. Most main battery aircraft charger technology that exists today uses constant current with a battery voltage point as the cut-off point, rather than the inflection point. Generally, this voltage cut-off point is selected to coincide with a "theoretical" inflection point for the particular battery.
 
     The problem with voltage point cut-off is that a rather serious flaw exists that is prematurely killing many batteries. It occurs when a cell (or more) is reversed in a series string of battery cells as, for instance, a NI-CAD of 20-22 cells. In this case, during the constant current fast charge mode, the charger is waiting for the voltage to rise to the set voltage cut-off point.  But since one or more cells are reversed, the battery may never reach the cut-off voltage due to the reversed cells. The longer it tries the less is it likely to reach it. This is because of a NI-CAD's inherent characteristic of a reduction in cell voltage as its temperature increases. An unattended response to this phenomenon can ultimately result in thermal runaway.

     The good news is that inflection point fast charging cut-off has the inherent capability of preventing battery thermal runaway as described above. The reason is that the inflection point still exists, even though a cell or two may be reversed. Therefore, inflection point fast charge cut-off is a very powerful health and safety device for batteries.

     Securaplane Technologies,Inc., an aerospace company located in Tucson, Arizona, received a US patent (number 5,780,994) in 1998 for "Detection of inflection point in secondary battery charging process by matching voltage response to first derivative of battery's characteristic curve." Simply stated, the patented concept includes a model characteristic curve stored in digital memory to which the battery under charge is continually compared. Once a satisfactory match is found, the time remaining to reach the inflection point is predicted by assuming that the voltage response will track the reference curve to the inflection point, which does indeed occur, even with severe battery loading transients.

     Securaplane Technologies has used the inflection point control law in their chargers during the past three years of charger production, now totaling over (500) units. Proven field results have been very exciting.  As an example, after one year of  airline flying (3,500) hours a Securaplane battery charger charging a new NI-CAD low maintenance battery resulted in depleting only 12 cc of fluid out of a (60) ccl reservoir.