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Old 08-11-2004, 11:05 AM
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Matt Kirsch
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Default RE: need some confidence(help)

Okay, you don't have nearly the overhead I was presuming before. 4200mAh, or 4.2Ah, at 7C is only about 29 Amps. That means you can only draw 29 Amps continuously. Check the motor specs at www.modelmotors.cz to see how many Amps the motor will draw with an 11x6 (or as close as you can get) prop on 10 cells (comparable to 3S LiPoly). If it's more than 29 Amps, you either need to go with a smaller prop, add a third pack in parallel, or limit your full throttle bursts.

For your much larger Mustang plans, you will need to do more than just add two more packs in parallel. VOLTS are the electric equivalent of cubic inches. Your trainer normally uses a .40 engine, while the Mustang needs a .60. You need to increase the voltage, that is add more cells in SERIES, to fly the Mustang. You'll want at least 6S3P using the cells you currently have (electric "cubic inches" don't scale the same as engine cubic inches, which is why you need twice as many Volts).

I'm not sure how I can explain the BEC thing any better, but I'll try: The BEC is a voltage regulator that burns off the excess volts as heat to reduce the main battery voltage from whatever it is to 5 Volts to run the receiver and servos. As the difference between the battery voltage and 5 Volts gets larger and larger, the more heat it has to generate. This is just a tiny chip we're talking about, and it can easily overheat and burn up. The more work the BEC has to do to reduce the voltage, the less capacity it has left over to run the servos. When you hit 10 cells (12 Volts), you've hit the practical limit on the design of these BEC chips. It could regulate from a higher voltage, but it wouldn't have any excess capacity left over to run the servos, so it's useless with more than 10 cells.

You have a 3S2P LiPoly. When we talk about "cells" in general we mean NiCd or NiMh cells with 1.2 Volts per cell. LiPolys have 3.7 Volts per cell, but are equivalent to about 3.5 NiCd or NiMH cells. So, a 2S*P LiPoly is equivalent to a 7-cell NiCd, a 3S*P LiPoly is roughly equivalent to a 10-cell NiCd, and a 4S*P LiPoly is equivalent to a 14-cell NiCd. With your 3S2P LiPoly pack, you need to follow the instructions pertaining to 10 cells. Any limits in the instructions for 10-cell packs must be followed.

If your BEC can only run 3 servos, and you need to run 4, there are a few options:
1. Do it anyway and hope for the best. Not recommended
2. Try to find a speed control with a BEC that can handle more servos.
3. Disable the BEC and use a regular receiver pack like what's used on glow planes.
4. Disable the BEC and use a UBEC. A UBEC, or Ultimate BEC, is a regulator of a different design that can handle practically any number of cells and any number of servos. It's a separate product that costs about $60.