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 by Bob Aberle |
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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 Sources: Balsa Products
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