Electrifying breakthrough!: How one flyer ingeniously used Li-poly cells to increase flight time and save weight

Model Airplane News, Nov 2003 by Aberle, Bob

If you think that Lithium-polymer (Li-poly) batteries are just for small electric planes, you haven't seen Dave Grife's giant-scale Travel Air Mystery Ship. With Li-poly power, this plane achieves up to 20minuti: flights!

Dave has flown his 1/4-scale plane on electric power for about nine years using an AstroFlight 90 cobalt motor and up to 40, 240OmAh Ni-Cd or NiMH batteries; that setup weighed 19 pounds! The system produced approximately 45 amps of motor current, and flight times lasted about 4 to S minutes with some throttling. At the Mid-America E-FLY in Livonia, MI, last summer, he showcased his new and improved power system: a prototype Hacker brushless motor (Aero CSO geared 6.7:1) and 3, 3S4P Thunder Power Li-poly packs. Each pack consists of 12, 195OmAh cells wired in both series and parallel. These substitutions reduced the weight of the Travel Air Mystery Ship from almost 19 pounds to only 15 1/2 pounds.

The real story is that the 36 Li-poly cells that replaced the 40 Ni-Cd cells reduce the model's weight by 36 ounces while nearly tripling the capacity; it went from 2400 to 780OmAh! The prototype Hacker Aero C-SO motor is also more than 1 pound lighter than the 90 cobalt motor, and it provides a nice efficiency boost.

Here's how Dave accomplished this:

* The power boost. Each Li-poly pack contains 12 cells and is designated 3S4P, meaning that 3 cells are wired in series, and four of these sets are connected in parallel. At full charge, the 3 cells in series provide around 12.6 volts, and that is similar to the voltage of a ??-cell Ni-Cd or NiMH pack. Because the packs are also wired in parallel, they provide four times the capacity of the individual cells (4xl950mAh=7800mAh). Dave needed more than 30 volts to run the Hacker motor, so he wired 3 Thunder Power 3S4P packs in series (3x12.6 volts) for 37.8 volts at 780OmAh.

* The weight loss plan. The original battery consisted of 40 sub-C cells; each cell weighed 2.1 ounces for a total of 84 ounces, or 5.25 pounds. A single Thunder Power 3S4P Li-poly pack weighs 16 ounces, so 3 of them weigh only 48 ounces. Do the simple math, and you can see that the model lost 36 ounces, or 2.25 pounds, in battery weight alone!

* The bottom line. Because each Thunder Power 3S4P pack costs $190 (assembled, balanced, tested and ready-to-run), the Travel Air Mystery Ship carries almost $600 worth of batteries! To make the investment more palatable, Dave configured all of his large electric models to run on these packs. That isn't a bad investment when you consider that Dave's entire hangar benefits from the weight reduction and that he has seen a 300-percent increase in flight times!

* Charging ahead. Dave charges his 3S4P packs with a prototype Li-poly-capable AstroFlight 110 Deluxe charger that charges l, 3S4P pack at 8.2 amps, 2 packs at 6.5 amps, or all 3 packs simultaneously at 5 amps. At these currents, the packs reach 90 percent of full capacity in about 1 to 2 hours (depending on whether Dave charges 1, 2, or 3 packs). At the field, Dave can fly for 20 minutes on a single charge. What about field charging? It isn't necessary. As Dave puts it, "Twenty minutes is a lot of flying when it comes to a big plane like this!"

At 5OA motor current, the load on these 3S4P packs is around six to seven times the batteries' capacity in mAh, and that is close to the maximum current limit of today's Li-poly cells. To date, Dave has charged and flown these packs about 30 times, and because these cells are still in the experimental phase, he maintains a log on the total voltage of each of the 3 packs (something the battery manufacturer, Thunder Power, also suggests).

Li-poly battery technology has come a long way this year. Progress in cell manufacturing techniques is measured month by month; by next year, we should see greater current (load) capability and reduced prices. Stay tuned!