By Red Scholefield
The use of redundant parallel fight packs (packs may be of different capacity but MUST be of an equal number of cells) is an excellent way to increase the available flight time and significantly improve the reliability of the on power system. The simplest means is to run two complete wiring harness, switches and charge jacks from each pack and plug one into the normal battery port and the other into an extra channel on the receiver. No diodes or isolation is required (see below). This is simpler and more reliable than some of the complex battery backup systems being offered on the market. Whether you are using 4 or 5 cells is your option, remembering that a 5 cell pack will provide more power to the servos but at the same time discharge faster giving you less flight time.
Parallel charging of Ni-Cds is not recommended due to the tendency of the cells to have the voltage drop off after they reach full charge. Should one pack have a slightly different capacity than the other then it will reach full charge sooner and the voltage will start to drop off allowing more current to flow into this pack. The other pack may not then reach a full state of charge. Repeated charge/discharge cycles under this parallel arrangement causes additional charge unbalance. While you may experiment and find that you get what appears to be both packs charged you will eventually run into problems with this arrangement. As an extreme, take the case of two packs, one having 250 mAh capacity and one having 600. The smaller capacity pack will reach full charge much sooner assuming that there is at least an equal "sharing" of charge current. As it peaks and the voltage declines slightly due to the heating of the battery as the oxygen is recombined it will begin to take more and more current to maintain a voltage equal to the as yet uncharged pack and the voltage tries to drop further and demands even more current to keep it up. This pack will then be taking nearly all the charge current leaving the larger pack woefully short during what would be perceived as a normal charge time like 16 hours.
Many pseudo battery "experts" put forth the argument that plugging two battery packs into the same receiver with out blocking diodes is NOT a good thing, claiming that his creates a host of problems and the two packs will end up fighting each other or "cross charging".
These concerns show a lack in the understanding of the charge and discharge potentials involved in Ni-Cd cells. One pack cannot charge the another (equal number of cells) as the discharge voltage of a pack can never be as high as the voltage required to charge the other pack. For the doubters here is an experiment: completely discharged one pack to 4.0 volts and then connected to a fully charged pack having an equal number of cells. There will be less than a 10% transfer of charge in a 24 hour period. Since shorts rarely occur in fully charged packs the risk of one pack "dumping" into one with a shorted cell are insignificant. A simple ESE preflight test would detect a pack with a shorted cell.
While it is a fact that the typical failure mode of a battery is for a cell to fail shorted there are some subtleties here that escape many people. First,one of the major causes of "battery" failure has nothing to do with the batteries themselves but rather with a switch or connector in the battery circuit. The dual redundancy concept is to protect against the failure having the highest probability - that being the circuit path from the battery to the power buss in the receiver. Adding more components to this path, like regulators and/or diodes isn't going to help the matter but rather adds to the probability of failure.
Perhaps the following discussion on the nature of shorts will better help the modeler understand.
While it is agreed that shorts are the failure mode in Ni-Cds batteries one has to look further into the "when" of the failure.
A short develops in a Ni-Cd when conductive particulate bridge the separator or the separator itself deteriorates to the point where it allows the positive and negative plates to touch. Rarely does the short occur all at once but rather building up a very small conductance path termed "soft shorts". In a charged cell the energy in the cell will blow away any short as it tries to develop. You've heard about "zapping" cells. The cell actually zaps itself before the short can develop. Only in cases of severe overcharge at high rates can the separator melt down to the point where the plates contact each other (hard short). In this case the energy in the cell then dumps and we have what is referred to as a hot steamer, the electrolyte boils, nylon in the separator melts down and is forced by the steam through the vent. On some occasions the vent is clogged by the molten nylon separator and becomes inoperative causing the cell to rapidly disassemble. So under normal circumstances a cell maintained at some state of charge is much less likely to short than a cell that is completely discharged. It should be noted however that the self discharge increases rapidly in cells where there is a short building (high resistance -soft short) due to separator deterioration and/or cadmium migration. One other shorting mechanism is a manufacturing defect where the positive or negative collector tab bridges the opposite plate. These usually fall out before the cells are shipped or assembled into batteries.
Preflight procedure should involve checking each battery separately. First check each with ESV through charge jack. Youi should get nearly identical readings, then switch one on, check controls, switch off and then switch on the other battery, check controls again, then turn both systems on and fly with confidence.
Summary: Diodes are not required. Packs must be of the same number of cells. Packs may be of different capacities. Individual charge jacks must be provided for each pack (and not interconnected). Total capacity available will be the sum of the individual capacities. Specialized chargers are not required since standard packs (600-800 mAh AA packs) can be charged employing regular system wall chargers (1200 to 1600 mAh should cover most giant size projects).
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