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RC
Driver TESTS: |
The
Scorpion 2s
Charger and new Li-Poly Car Packs |
REVOLUTION!
Scorpion Li-poly car packs BEAT matched NiMH race packs |
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by Tom Atwood, Greg Vogel In a recent issue we announced the exciting new Scorpion Lithium Polymer (Li-Poly) RC car and truck batteries from FMA Direct as well as FMA’s new Scorpion 2s Li-Poly cell-balancing charger. In that announcement we referred to the new packs as “Kokam” packs. FMA has decided to call these new car and truck packs “Scorpion” packs, but it is nonetheless important to note that they contain the latest generation Kokam Li-Poly cells. Since that announcement, we have extensively tested the Scorpion 2s charger and the Scorpion 7.4V 3200mAh Li-Poly packs, and the results are awesome. When these “beta version” preproduction products were provided to us they were still being tweaked and improved for production. This is truly a revolution in RC electric power—we’ll tell you why! The new 7.4V Scorpion packs weigh 7.5 ounces or 5.5 ounces less than a typical 3000mAh NiMH sport pack. That’s a third of a pound less battery weight in a typical backyard-bashing buggy. All things being equal, you’ve got a lighter, more agile car. The cool thing about Li-Poly batteries is their nominal voltage is slightly higher than that of NiCd or NiMH cells, and their internal resistance is significantly lower so that you experience greater punch when you hit the throttle. A rigorous 35A discharge test on a Competition Electronics Turbo 35 GFX charger-discharger backed up what we found on the asphalt (see chart): the new Scorpion 3200mAh car pack outperforms a $100 matched NiMH racing pack. Before we get into the details of performance, let’s look at how these batteries are different from Nickel-based cells—and at the unique capabilities of the new charger. ABOUT LI-POLY BATTERIES Li-Poly cells became a mature battery technology in the computer and cell phone worlds in the ‘90s, and over the last few years have emerged as the new standard for powering many categories of RC electric airplanes. These batteries are typically made up of thin, flat plates embedded within an electrolytic gel that is wrapped in an aluminized, mylar-like plastic covering material. There are no contacts that you solder leads to; rather, the leads are already assembled and in place and fitted with Deans Ultra Plug connectors. The battery is covered with plastic shrink-wrap that covers the attachment points of the leads. Li-Poly batteries are lighter and able to deliver more current than other battery types on a per-ounce basis. The new Scorpion 2-cell (2S) 3200mAh packs are manufactured using Kokam “super high discharge” cells, and represent the 4th generation in Kokam Li-Poly batteries. These are every bit as competitive and powerful as any Li-Poly batteries we have tested to date in the RC market, and are a step ahead. They are the first 3200mAh packs we have had the chance to run and test. Li-Poly battery chemistries are rapidly improving, and the leading brands have time and again leap-frogged each other in the RC airplane market. The new Scorpion car packs combined with the Scorpion 2s charger makes FMA and Eagle Picher Kokam the clear leaders in the RC car market. Li-poly batteries have a nominal no-load voltage of 3.7V per cell, and these new packs have two Kokam cells wired in series, which is why the packs are rated at 7.4V (a rough average of voltage under load). A fully charged Li-Poly will have a no-load voltage of 4.2V per cell, or 8.4V per 2-cell pack. A 2-cell pack is called a “2S” battery, which means two cells in series. If two of these batteries are wired in parallel to produce a larger “unitized” 6400mAh battery, the pack would be called a “2S2P” pack. The 2P means two packs in parallel. Li-Poly packs cannot be discharged below a certain voltage without damaging the packs in a way that will diminish performance. When you mount a Scorpion pack in an RC car, the speed control lead plugs to a port on the battery, and a lead from the battery plugs into the receiver ESC port. When the pack sags to 6V at the end of a run, the chip in the battery terminates the throttle signal between the receiver and the speed control. On loss of signal, the speed control kills motor power. When the motor load is removed, the pack’s voltage recovers; this process leads to motor pulsing—an indication that the charge is depleted yet there is enough power to drive back to the pits at low throttle.
SCORPION 2S CHARGER AND BATTERY SYSTEM The new Kokam 2S car batteries cannot be charged using a NiCd or NiMH charger. NiCd and NiMH chargers use an entirely different charging algorithm. (You should never charge a lithium battery with anything but a lithium charger or you will overcharge the battery, which could lead to a fire.) The new Kokam 2S 3200mAh batteries have unique 3-prong charging plugs that are incompatible with other chargers. A 3-prong charging port on the side of the Scorpion 2s charger will accept only this plug. Scorpion packs are warranted only if they are used exclusively with the Scorpion 2s charger. One of the most important issues with Li-Poly batteries has been the need to ensure that the individual cells within a pack are properly balanced. The new Scorpion 2s charger is unique in the RC world in that it employs an extremely reliable and safe “cell-balancing” charging system that checks the voltage and charge current being delivered to each cell multiple times a second. This guarantees that the cells are matched to within hundredths of a volt when charging is completed. The ability of a charger to monitor all cells in a series pack individually is a major breakthrough in improving charging reliability and safety.
GETTING STARTED The Scorpion 2s charger can fast-charge Kokam Li-Poly car packs to 90% capacity in only 20 minutes, and it is very easy to use. When you plug the charger into an 11 to 15V DC power source, the red light on the left side of the charger’s top panel illuminates to indicate power is on. When you plug the battery’s charging lead into the special port on the charger, the yellow “Fast Charge” light on the charger front panel illuminates. A dial lets you select a fast charge rate of from 500mA to 10A. If you are fast-charging a Kokam 3200mAh pack, you would set the charger to 9.6A (a “3C” rate—see the Li-Poly Terms sidebar). When the charge is midstream, the yellow light illuminates steadily and the green light slowly flashes. When you have reached 90% of charge, the yellow light goes out and the green light rapidly flashes. You can now run your pack in an RC car—it is ready to go, and at 90% will probably outperform any 6-cell nickel pack you have previously run—or you can wait if you want that last 10% of capacity. In Top Off mode, the Scorpion 2s will charge to 100% capacity in one to three hours, depending on battery capacity and condition. We’ve seen the steady green light indicate a full charge on the 3200mAh packs within 30 minutes of reaching the Top Off point. Never charge a Scorpion car pack using a 3rd party charger hooked to the Deans Ultra Plug connector that is intended to be connected to your car’s ESC. This will risk imbalancing the cells and will cancel the product warranty. FORMING A PACK The Scorpion 2s charger has a unique capability—it can condition the Kokam batteries for best performance. FMA notes this could amount to a long-term gain in pack capacity and performance (we have not yet rigorously tested the forming process). It does this by cycling the batteries in “Form Pack” mode, which can be selected with the current adjust dial. When you select Form Pack, the charger cycles the pack eight times. It charges at 500mA and discharges at 750mA. This regimen is applied to any of the three new Kokam car pack sizes (1250mAh, 2000mAh and 3200mAh). In Form Pack mode, the yellow and green lights alternately flash.
We took a brand-new 3200mAh Scorpion pack and ran it through four forming cycles on the Scorpion 2s and then fast-charged it. I took another 3200mAh pack and fast-charged it without forming it. When I discharged both packs using a West Mountain Radio Computerized Battery Analyzer at a 1C rate (3.2A) and overlaid the plotted curves on my laptop, the discharge curves were identical. Our sample packs were impressively consistent. FMA has noted that if you fast-charge any of the new packs repeatedly, the ability to best condition the pack via forming will be lost. You’ll see from our performance evaluation, however, that the new Kokam 3200mAh packs hold their own extremely well right out of the box, even in the absence of forming.
FMA and Eagle Picher Kokam have produced a break-through electric power system for RC cars and trucks whose performance is superior to the best technologies previously developed. By putting the Li-Poly cell-balancing function in the charger while at the same time controlling cutoff of the battery in the car, FMA and Eagle Picher Kokam have brought Li-Poly power to a new standard that meets the needs of both backyard bashers and racers. Because internal cell resistance is lower in the Li-Poly batteries than in NiMH packs, the new packs outperform top racing packs with unbelievable throttle punch and consistently maintained higher voltage.
A revolution in electrics is before us. RC Driver will continue to give you the latest news on the emerging world of Li-Poly power now offered by FMA and Eagle Picher Kokam—stay tuned for the latest updates and new developments as we continue to wring out these new systems. Links Astro Flight
Inc. Competition
Electronics EaglePicher
Kokam FMA Direct RC Dyno
Systems Team Associated Team Losi,
distributed by Horizon Hobby Inc. West Mountain
Radio W.S. Deans For more information, please see the source guide in the magazine. |
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Driving
the Kokam 3.2’s
