|Model Aviation Home|
by Bob Aberle
(Editor's note: Battery technology has advanced since this article was posted. This contains valuable information but more current information is available in Bob Aberle's article on the Sport Aviator Web site. The article is titled "These Things We Call Batteries - Part Three.")
MY APOLOGIES. I had every intention this month of publishing my new Radio Control (RC) electric sailplane model that I hope beginners will build from scratch. (It's not a kit or an Almost Ready-to-Fly [ARF]/Ready-to-Fly [RTF] model!) As I learned, designing, building, flying, photographing, and writing about it takes more than four weeks. I promise to publish my "Scratch-One" design next month.
This month (the eighth installment I've written) will be devoted to the all-important battery basics. In the previous seven chapters I've referred to the RC-system batteries and the electric-motor batteries on many occasions. I will probably repeat some of those points to emphasize their importance.
The battery power to your RC system is like putting gas in your car; if you run out of gas, the car doesn't run. If your RC system runs out of electrical power, it doesn't work (fly)! During this presentation I will concentrate mostly on the RC-system batteries, but I will get into some aspects of the electric-motor batteries as they are used in electric-powered flight.
Types of Batteries: In the RC hobby today, modelers typically use Nickel Cadmium (Ni-Cd) or Nickel Metal Hydride (NiMH) cells. Both are rechargeable batteries. Under normal operation they can be recharged hundreds of times and have been known to last an average of three to five years and more.
From a beginner's perspective Ni-Cd and NiMH cells provide roughly the same kind of service, so you need not worry at the start about what kind of cells were supplied with your particular RC system.
The NiMH cell is the newer of the two. It can offer more capacity than the Ni-Cd cell of the same physical size. More capacity means that it can operate your system longer or fly your airplane longer. NiMH cells have a slightly lower characteristic voltage under load than Ni-Cd cells. From an RC-system standpoint, that difference is of little concern because the load is relatively low.
However, when using NiMH cells for electric-powered flight, the difference can be important. If you fly a model with a seven-cell Ni-Cd pack, then substitute a pack with NiMH cells, you might have to go up one more cell to a total of eight to achieve comparable performance. With every passing day, advancements in NiMH battery technology are removing this deficiency.
Some inexpensive (economy-type) RC systems are sold with no batteries included. If that is the case, you will have to purchase 12 AA-size alkaline (nonrechargeable) batteries (eight for the transmitter and four for the receiver or airborne side). Although these cells will last a long time (possibly 10 hours or more!), they must eventually be replaced since they can't be recharged.
Balsa Products sells a charger and eight individual NiMH cells which must be removed from the transmitter and placed in the charger. After charging is complete, the user inserts the cells back in the transmitter. The charger is $22.95 and a set of eight 1300 milliampere-hour (mAh) NiMH cells is $8.50. It's an interesting solution to what could become a costly long-term battery-replacement problem.
There is a new type of battery technology called Lithium Polymer (Li-Poly), which has emerged thanks to the cellular-telephone industry. There isn't much application for this type of battery cell for RC systems at this time, but you will be hearing and reading more about the great advantages of low weight and high capacity for electric-powered flight. There will be more about that in another installment.
Battery-Capacity Ratings: All batteries have a capacity or rating. The capacity can tell you how much power the battery can supply in a period of time, or how long the charge will last while powering your system, appliance, etc.
Battery capacity for our purposes is usually stated as mAh, and ampere-hours (Amp-hr) are used for larger-capacity cells. Most RC systems through the years have been powered by Ni-Cd cells of approximately 600 mAh capacity. Thanks to the newer NiMH technology, higher-capacity cells are being offered with some systems. I'll use that 600 mAh capacity for this discussion.
For most purposes, the Ni-Cd cell's nominal (average) voltage is 1.2 volts. If you applied a load of 600 milliamps (mA) to a fully charged Ni-Cd cell, it should take exactly one hour for that cell to reach 1.0 volt. At that 1.0-volt level, the cell, by definition, would be considered discharged. If you continued to load the cell, the voltage would rapidly head for 0.0.
We usually see RC transmitters with eight-cell battery packs. Using the nominal-voltage rule, eight of these cells in series would produce a total of 9.6 volts. If we used a commercially available battery tester, we would apply a load until that pack reached 8.0 volts (8 x 1.0 volts), at which point the testing device would cut off and the capacity in mAh could be read off of a monitor, meter, or Liquid Crystal Display (LCD) screen. I'll get into more specifics about battery testing later.
Model Aviation Home