lesson5page2

Charging: The last item in the electric power package is the essential battery charger. There are
many choices, simple and complex, available at varied prices. Kirk Massey's favorite for a beginner
is the Global (WattAge brand) PF-12 Park Flyer AC/DC peak-detect charger. The key words are
"peak detect"!

Some of the simplest battery chargers come with just a rotary-crank type timer. You rotate the timer
switch to 15-20 minutes and wait for the timer to run down. The trouble with this is that the charger
can't sense the amount of charge already in the battery. This can easily result in overcharging,
which can cause excessive heat buildup in the battery. An overheated battery can quickly be ruined.

A peak-detect charger is essentially an automatic charger. All Nickel Cadmium (Ni-Cd) and NiMH
batteries have a characteristic where the voltage applied during charging increases until the point of
full charge (full capacity) is reached, at which point the voltage peaks then begins to drop off. At the
peak, or slightly thereafter, the charge cuts off automatically.

Since this is a sensing cutoff and not a timed cutoff, only what is necessary to reach full charge is
put into the battery. If your battery had residual charge in it, the charge period would be reduced
accordingly.

The one thing you must do manually is tell the charger the amount of charge current required. For
NiMH batteries it is suggested that you apply a charge current equal to two times the rated capacity
of the battery. We refer to this as "2C."

In this case the battery is rated at 1100 mAh (which is the same as saying 1.1 amp per hour).
Mulitiply 2.0 by 1.1 amps, which equals 2.2 amps. This particular Global charger has a maximum
charge current of 2.0 amps, so you would set the charger to that maximum current. At that
approximate 2C charge rate, a fully depleted NiMH battery pack would take roughly 30 minutes to
reach full charge.

Each battery will vary according to the number of cells, the type of the cells, and the capacity rating.
You will pick up on this concept as you progress with electric power. For this first choice I've selected
a charger that can't overcharge the battery pack because its maximum current of 2.0 amps is close
to what this battery requires.

When you purchase this or any charger, it is generally your responsibility to prepare a cable that will
allow the charger to connect to your battery pack. Kirk Massey has prepared a cable with APP
connectors to attach between the Global charger and our battery pack. Note the output connectors
on the front of the Global charger. Press down on each connector, insert the wire, then release. Do
that for the positive (red) and negative (black) connectors.

Later you might want to purchase a more sophisticated charger that has a higher current rating or
can handle battery packs with more cells. (The Global is capable of handling four to 12 cells.)

There are several fully automatic chargers on the market. When you attach a battery to one for
charging, it will sense all of the necessary parameters and set the charger accordingly. The concept
involves a computer sensing system and has a menu that must be accessed for regular operation.
These chargers work well, but they are more complicated to use than this basic Global unit and they
cost much more.

It is typically a good idea to remove the battery pack from the model when charging. Immediately
after a flight the battery may be quite warm--even hot. A basic rule is to never charge a hot battery.
You should cool it off before attempting to recharge. Putting a hot battery in your soda cooler isn't
the right method. It will result in uneven temperatures throughout the pack.

The better approach is to buy a RadioShack 12-volt electric fan (part number 273-243). You can
mount it at one end of a length of 3-inch-diameter PVC (polyvinyl chloride) plumbing pipe. Power the
fan from your 12-volt car battery. Put your hot battery pack at the other end of the PVC tube. This
will allow the air the fan blades generate to pass over the battery pack and cool it. Approximately five
minutes in this tube will reduce the temperature to a safe level for charging.

This Global charger can be powered by 115VAC for indoor (shop) use or from 12 volts DC from your
car battery. At the field, most of us just raise our car hoods and attach the charger input cables
(alligator clips) to the battery terminals.

The polarity is important. Make sure that the red alligator clip goes to the car-battery positive (+)
terminal and the black goes to the ground or negative (Ð) terminal. The Global charger has a built-in
timer circuit that cuts off the charger after a 90-minute period. If you left the charger unattended and
something went wrong, the charger would safely cut off after 11Ú2 hours.

That's the full electric power system. Remember that this article has been prepared as a starting
point for a rank beginner who is entering electric flight. There is a shopping list in the accompanying
table that you can use to purchase all of the necessary equipment. You can purchase all of the
items from Kirk Massey at New Creations R/C, Box 497, Willis TX 77378; Tel.: (936) 856-4630. (Kirk
prefers telephone calls to E-mail correspondence.)

You can buy the components on the list from other sources; for simplicity's sake I specified one
source and the exact equipment necessary. The addition of the connectors made this a custom
order. If you are lucky to live near experienced electric modelers, by all means solicit their help; it
can save you a great deal of time. However, the theme of this series is to get you going on your
own--with little or no help!

Next month I will start with a basic electric-powered ARF trainer: the AeroCraft Pogo. It was
specifically selected to use the electric power equipment I have described and the Hitec Neon
three-channel RC system, which I discussed in part two of this series (in the April 2003 Model
Aviation, starting on page 54).

I expect to take the Pogo from the kit box to the flying field, which includes final assembly of the
model and installation of the electric power and RC system. MA




*page 1* *back to AMA* *Articles*



	The ESC is probably the heart of the electric power system. The ESC in electric-powered flight takes the place of the throttle (or engine) servo used on a fueled model. The ESC in this instance weighs roughly 3Ú4 ounce and has two pairs of wires: a servo-type cable and a cable with a switch on the end, all exiting the case. Two wires, with APP connectors already attached, will plug into the motor wire connectors. Polarity is important, so it is red to red and black to black. Two more wires will have APP connectors attached, and they will plug into the battery pack. The third cable has a servo cable connector on the end. That cable is plugged into the throttle port (usually the number-three position) on your RC receiver. The switch will be mounted on the side of the fuselage and must be manually turned on to activate the entire electric power system. There are all kinds and sizes of ESCs on the hobby market. The one chosen for this project is the Jeti 110. The ESC will be rated for current; in this case it is 11 amps continuous operation. For our application we need 8-9 amps, so there is a margin of safety. If the rating wasn't high enough, you could overheat and possibly burn out the ESC. The rating must also take into account the number of cells in the battery pack. The Jeti 110 can handle six to 10 cells. We will be using an eight-cell battery. Many small ESCs (such as the Jeti 110) will contain what is called a "Battery Eliminator Circuit" (BEC). It will permit the main motor battery pack to also power the onboard RC system (receiver and servos) on a shared basis. This saves the weight of an extra airborne battery pack. It is also a convenience because this one battery is recharged for every flight. In actual practice, the BEC has a special circuit that provides a regulated 5 volts to the RC system via that cable that is plugged into the receiver throttle port. When the battery gets down near 5 volts the circuit cuts off the motor, but it still provides the necessary power to operate the RC system so that you can safely land the model. Most ESCs with this BEC feature (the Jeti 110 included) will allow you to briefly restart the motor after the first shutdown. You do this by moving the throttle stick on the transmitter all the way to idle, then back up. That resets the ESC and will allow a few more seconds of power so that you can line up on the runway for a safe landing. Most modern ESCs (including the Jeti 110) employ "smart" circuitry via a microchip, which can add considerably to the safety of electric motor operation. Remember what I wrote earlier: when the battery is attached to the motor, it could start instantly. If you have the throttle stick at full or partial with the entire system plugged in, the motor and propeller could start turning. The microchip in the ESC will sense anything other than a dead idle position and prevent the motor from starting. To start the motor you must physically move the transmitter throttle stick down to idle then go back up. The motor will then start, and its speed will be proportional to the control-stick position. The Battery: This is also an important part of the electric power system. Batteries come in all types, sizes, weights, and capacities. The choices are critical to the model's performance. For this project the choice of battery has been made for you; it is a Nickel Metal Hydride type (NiMH) consisting of eight cells made up as a pack. Each cell has a capacity rating of 1100 milliampere-hours (mAh). The nature of these ratings will be explained in later articles. APP connectors have thoughtfully been attached to the two wire cables. As in the case of the motor connections, the polarity is critical to the system's correct operation. It is always positive to positive and negative to negative. If the color-code convention is followed, it is usually the usual red to red and black to black. However, not everyone uses that color convention. Do not mix up connections between the motor and battery. The connectors going from the ESC to the motor are generally connected once at the time of initial installation and not touched thereafter. Placing a piece of masking tape on each connector can remind you not to touch them until such a time as you transfer the equipment to another aircraft. Estimating that this direct-drive Speed 400 motor will have a current of 8-9 amps, this particular battery pack should be capable of providing six to seven minutes of electric power at full throttle. In reality you will have much more power than you need; therefore, you will be able to throttle back during a normal training flight. Throttling back reduces the motor current, and, as a result, increases the flight time. With average throttling back it will be possible to extend your flying times to 10 minutes or so. Owning more than one battery pack will allow you to fly on one while the other is on charge. That will provide you with more flying time and less waiting time.

		*top of page* *page 1* *back to AMA* *Articles*