Battery Systems In
Giant Scale Aircraft
|Contributing Feature Editor: Red Scholefield - AMA 951 -
Parallel Operation = Reliability & More Flight Time
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).
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. You may
experiment and find that you get what appears to be both packs charged,
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 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 expertsput forth the argument that plugging two battery packs into the same receiver with out blocking diodes is NOT a good thing, claiming that this 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-Cad cells. One pack cannot charge another
(equal number of cells) as the discharge voltage of a pack cannever be as high as
the voltage required to charge the other pack.
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
Perhaps the following discussion on the nature of shorts
will better help the modeler understand.
In a charged cell the energy in the cell will blow away any
short as it tries to develop. You've heard about zappingcells. 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
Preflight procedure should involve checking each battery separately. First check each with an ESV through charge jack. You 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.