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 by Bob Aberle |
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Memory: Since Ni-Cd cells were first used in the early 1960s, we have been told that they can acquire a certain capacity memory. It was said that if a modeler made six flights each week, his/her batteries would get accustomed to supplying that level of energy. If that modeler made eight flights during a particular week, he or she might find the batteries incapable of supplying the extra power. Why? The batteries supposedly developed a "memory" for that amount of power. This subject has been greatly debated through the years. The newer NiMH cells don't seem affected, and the even newer Li-Poly cells are not affected at all. Regularly discharge testing (or cycling) your battery packs would eliminate the problem if it actually ever existed. RC-system batteries tend to be used at roughly the same rate, so periodic testing (cycling) can't hurt in that regard. Batteries used for electric-powered flight are usually taken down to the minimum charge level on each flight; as such, they are essentially cycled on every flight. It would be difficult to conclude that batteries used in this manner require additional cycling in your shop. When your electric-powered model refuses to take off or support flight, then you know you have a battery problem and can investigate further. But don't bother to cycle these batteries; it's unnecessary. Li-Poly cells will be the subject of a separate article sometime in the future. Progress in this area is being measured in weeks—not in months. Considerable recent development work is gradually producing cells that are capable of supplying more capacity at higher current (load) levels, with ever-decreasing weight. At the same time, the cost of Li-Poly cells is coming down. Right now Li-Poly use is limited to roughly 10-amp loads, which can handle up through Speed 400 power (such as the Aero Craft Pogo in last month's article). However, these load-level limitations are constantly improving, so it is just a matter of time. Although I don't see the need right now, Li-Poly batteries may eventually be used to power our RC systems. Don't ever attempt to charge Li-Poly batteries with peak-detect chargers; they have a completely different chemistry and should only be charged with their own chargers. You might be tempted to write in that there are chargers available—such as the Great Planes Triton—that can peak-detect charge Ni-Cd and NiMH cells and have a separate function for charging Li-Poly cells. That is correct; the Triton is new, and it works well. But don't leave it set on peak detect and try to charge Li-Poly cells. I've written many articles about batteries throughout the years. No matter how basic and thorough I am, these articles generate the most inquiries. Readers have been relatively quiet to date when it comes to questions. Now is your chance! Please write in and let us know how we can continue to help you. What else would you like to see in this series? Address suggestions to Bob Hunt at Box 68, Stockertown PA 18083, or to robinhunt@rcn.com. Next month look for the Pogo follow-up: a new design which I hope you can build from basic materials without a kit. This will be your first scratch-built RC model. I think you can do it! MA
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