I'd like to try the new Lithium Polywhatever cells in my Lite Stick. Yet I know nothing about them or the chargers.
What's a site that sells them? Also, any explanatory info would be handy.
We sell the E-tec brand Li-poly packs. For a Litestick I'd probably recommend a 700 mah 2-cell pack, although a 250 mah pack should work OK if you're more interested in climb performance than flight time. That's a guess on my part, I haven't actually flown that particular setup in a Lite Stik, but I have enough data from NiMH cells in one and from comparisons of NiMH vs. Li-poly batteries in our Roadkill Series models to make a very good guess at the setup for a Lite Stik. A 2-cell Li-poly pack typically has a little more voltage in service than a 6-cell NiMH pack. The E-tec cells are good for continuous currents of about 5C, and up to 7C for brief periods.
As far as chargers, our favorite economical unit for smaller cells is the Plantraco LPC-400, which we sell. If you want to invest in some thing with more capability for larger cells, we've had excellent results from the new Sirius charger for Li-poly batteries (NOT the same as the Sirius charger for NiMH and nicad cells!).
There are a lot of scare stories and general mythology floating around about the alleged dangers of lithium cells. Most of them relate back to the older lithium-metal cells, which are no longer made. They were indeed notorious for inflammatory and generally anti-social behavior. The newer lithium-ion cells use a cylindrical construction like the nickel-based cells we're all used to, and as such can also build up pressure, with the potential for bursting. Lithium-polymer cells are a form of lithium-ion in terms of their chemistry, but they contain their electrolyte in a plastic wafer. They are flat, encased in a plastic envelope, and so if they are abused, they tend to puff up rather than bursting.
Yes, there are reports of fires with any of the lithium types under the right circumstances. With the newer types, particularly the Li-poly cells, from what I've heard it's nearly always the result of improper charging, especially the case of folks who try to charge them using a nicad charger, or in some cases using lithium chargers with inadequate error-checking circuitry and improper settings. With the newer Li-poly chargers such as the ones we sell there is software that double-checks the settings before starting a charge.
Just to satisfy my own peace of mind, I did some abuse tests with a single 250 mah Li-poly cell I had damaged with poor soldering technique. I couldn't get it to take a charge on my LPC-400. When I tried to trick the charger with all manner of incorrect settings, it kicked out and flashed the error LED message at me. I then took the dead cell outside and deliberately tried to get it to catch fire.
I shorted it, with no results. I tore the plastic casing open, exposing it directly to air. Nothing. I tried holding it in an open flame, with the case torn open, with the flame impinging directly on the bundle of reactive stuff inside. Nothing. Wouldn't even start, much less sustain a flame. I finally threw it in a metal trash can with water in the bottom (supposedly lithium reacts with water, similar to but to a less violent extent than its cousins potassium and sodium). Still nothing.
I have seen one pack that was apparently shorted. It was obviously puffed up, but did not burst or catch fire. Healthy Li-poly cells do not get warm when charging. A cheap digital voltmeter can check if the cells are matched with each other, and their state of charge.
The bottom line is that with the proper charger and reasonable care, they seem to be reliable and easy to use. I've heard stories of nicads exploding and starting fires when overcharged, and we aren't scared of using them. For all of our small electric-powered airplanes Joe and I have switched almost exclusively to Li-poly batteries because they're lighter, have far more capacity and/or far more power than the NiMH packs we used to use.
For example, our little Fokker Triplane typically weighted around 3.5+ ounces with a 7-cell NiMH pack, had just about scale power (a loop required a short dive and good technique to make it safely over the top), cruised at nearly full throttle, and flight times were typically around 4-6 minutes. With a 3-cell 250 mah E-tec Li-poly pack and one of our new twin-motor power units it weighs a hair over 3 ounces, cruises at about 40-50% throttle, can loop easily from level cruise flight, can hover briefly, at 3/4 throttle can sustain a 45 degree climb followed by a loop directly from that climb, and typically gets flight times around 15-18 minutes in still air while doing all of these things! Anybody still wondering why we're switching to Li-poly in all of our Roadkill Series models?
Lithium cells can be mortally damaged by discharging them too far. From what our suppliers tell us, the unloaded voltage should never go below about 2.7 volts per cell. It's safe to run them down to about 2.3 volts under load in an electric-powered airplane, since the voltage will jump back to 2.7 when you close the throttle. We use a programmable Electronic Speed Control (the Castle Creations Pixie 7-P is our favorite for these applications) and set its voltage cutoff function in keeping with these numbers, but in actual practice we've found that it's really not necessary unless you're going to do a lot of thermalling. The rpm of an electric motor depends on the voltage you feed it, and the power absorbed by a propeller is proportional to the cube of the rpm. When the battery voltages starts to drop, the power absorbed by the prop declines rapidly with it, and even with a 3-cell pack we've found that there aren't enough watts absorbed by the prop to sustain flight long before the battery voltage is low enough to endanger the cells. If you're planning to be WAAYYY high in a thermal at the end of the flight, it could be an issue, but for any sort of low-altitude motoring around sport flying the plane should naturally tell you to land when the batteries still have an adequate margin left, if your battery and motor system are reasonably well matched to the airplane.
We simply use good equipment and reasonable care and prudence when using and when re-charging these batteries. The horror stories are not something to completely disregard, but not a big prohibitive risk either.
Don Stackhouse @ DJ Aerotech [EMAIL PROTECTED] http://www.djaerotech.com
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