I was pulled up sharply the other day when I told one of our pilots to "make up
your own battery" "How? He asked." After a couple of false starts at an
explanation, realized that construction of a dependable battery pack requires some
knowledge in a number of varied areas.
What size cells should you choose? As mentioned last time in "The Flight
Battery," flight of a carefully set up sailplane in the usual soaring mode demands
very little from the flight battery. A 270-mAh battery should enable you to fly more than
1 hour if the battery is fully charged and the plane is set up properly. The smaller
batteries can shave a little weight by using lead for balance instead of a larger battery.
Regardless of choice of capacity, the flight battery never gets the high drain demand of
the electric airplane. The elevated temperatures produced during over charge with a 10%
capacity charger are just enough to notice if you hold the pack in your hand for a few
seconds. Your choice of battery is similar to choosing a puppy as a pet, you will only
know after it has been around for a while and you get to know it. The cheapest 500-mAh
cells will give good service if you cycle them before placing them in service and keep a
close check during use. I don t recommend using a new battery without recycling.
What of choice of wiring harness for your new battery? Never use the old plug or wires
because the old plug will be worn and dirty and the wires may have the "black wire
disease." This disease it caused by the Ni-cad (nickel cadmium) electrolyte
(concentrated potassium hydroxide paste) leaking from the vent or seals and wicking up the
wiring harness, usually the positive. This corrodes the copper wire to a black copper
oxide, weakening the wire, decreasing its current carrying capacity and making it almost
impossible to "tin" with solder. I like to have a supply of #26 gauge, 19
strand, R/C wire and a few 8-pin Deans plugs and heat shrink to make new harnesses for
battery packs.
Common wire from Radio Shack will not work. The #26 gauge is large
enough to carry the load of even a "big bird" radio and servos but because of
the amount of movement the harness may receive, we need all the flex resistance possible.
It is not funny to have a battery harness wire break during fight. Therefore, plan on
having some sort of strain relief over the wires where the solder joint ends and the wire
begins. This is the most likely place of failure. Any flexible plastic tubing that can be
secured over the solder joint will do fine. I have used hot glue guns for strain relief
over solder joints to switches, where the wires leave the battery wrapping and plugs, as a
last resort. Heat shrink tubing works best and Radio Shack has a good assortment for this
purpose.
Try to use a black wire for the anode (negative) end of the battery and a colored wire,
red if possible, for the cathode (positive). It may be easier to use an aileron extension,
cutting off the plug not needed, for the harness. Note that Airtronics persists in coding
their plugs with the positive (red) on the outside and the black wire in the center. Just
the opposite of almost all other radios. Most builders cut the plug from the aileron servo
and splice a long wire down to the receiver. It is a good idea to leave at least 1 inch of
wire on the plug so that it can be used later for a flight battery.
Figure 1 shows how to solder the tabs of the cells so that all are in series. Before
soldering, I like to tape the cells together temporarily then use thick CA glue between
each cell. (Note: Silicone works nicely). The proper size heat shrink tubing for the
battery is difficult, so just cover it with some clear shipping tape when finished. (Note:
Heat shrink for helicopter blades will sometimes do the trick). The soldered contact is
fairly simple to make with a 25 watt pencil tip iron and rosin core (NOT acid core) solder
about the diameter of the #26 gauge wine. Practice until you get a nice glossy smooth
surface. A frosty or dull surface indicates that you have moved the wire while the solder
is wet and caused the solder to crystallize forming a weak "cold solder joint."
I like to tin (wet) both parts of an intended joint with solder, then, with one part in a
vice or clamp, hold the other part very steady and touch the joint with the iron until I
see the solder wick to the iron. A few seconds later, a nice joint is made. Finish by
slipping the plastic strain relief over the joint securely.
If you have no other way to cycle the battery, after full charge, turn on the radio and
receiver and move two or more controls continually until you notice failure. This should
occur in about 1 hour, just enough to watch a couple of 30-minute TV programs (Note: This
is not the recommended procedure for cycling - There are many inexpensive cyclers on the
market - use one.) Keep a check on the transmitter battery at the same time. If it fails
first, you only know the capacity of the transmitter (approximately 200-mAh drain).
Regular, continuous control movement will average about 250 mAh from the night battery, so
use this figure with the time to calculate the capacity of this battery. Multiply time in
hours by 250 mAh to obtain capacity of the battery pack. This constant movement is
approximately 2 1/2 times the movement you would nominally use in flight. If you obtain an
hour before failure using constant movement, then you should get at least 2 hours of
flight from your new pack, depending on how you use your controls.
