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Electric flight - the best and worst of Li-Po

by Bruce Bird

What happened to that mosquito snarl that always seemed to accompany the sight of model airplanes? The answer is they're going electric and silent. Li-Po (Lithium Polymer or Li-Poly) batteries are rapidly becoming the standard power source for electric model airplanes. Li-Po batteries are smaller, lighter and hold more power than their predecessors (Nickel Cadmium and Nickel Metal Hydride). There are three specifications that need to be taken into consideration when making a Li-Po battery purchase for your electric model airplane.

The voltage of the battery will tell you how powerful it is, the amperage rating will give you an idea how long it will last, and the burst/constant power rating will tell you how quickly the power will discharge. A major difference between Li-Po batteries and others available for RC model airplane use is that it is much easier to mistreat the battery thereby creating unsafe situations the consequences oc which can be dramatic. Li-Po batteries can be very dangerous if you don't follow all of the safe handling procedures set out by the battery manufacturers.

In most cases, Li-Pos used for model airplanes come in two voltage ratings ratings; 11.1v and 7.4v. The rating dictates how many volts the battery will discharge to your brushless RC motor. Electric RC model aircraft motors themselves have ratings that relate to this voltage discharge specification. The voltage of a Li-Po battery is directly tied to the RPM output of a brushless motor in that a motor rated for 1000kvm will rotate 1000 times per minute for every volt it receives from the battery. In other words, a 1000kvm motor is capable of 7400 RPM when using a 7.4v Li-Po, and 11,100 RPM when using an 11.1v Li-Po. This information, combined with the prop type and weight of the plane gives you all the data needed to calculate the final thrust output of a given motor.

The amperage rating of Li-Po batteries is rated in milliamp hours (mAh). This technical term describes how long the Li-Po will provide electrical power in much the same way that the size of the fuel tank on a glow powered airplane will dictate how long the engine can keep running. Typical electric RC model airplane flights can last 20%-50% longer when using Li-Po compared to other batteries. The larger the mAh rating, the bigger and heavier the battery will be. This is a major factor to take into consideration when putting together any electric RC model aircraft.

Li-Po battery burst rates describe the output characteristics of the battery. Burst rates tell you how much power the battery is safely capable of discharging in one big burst. This number is often higher than the constant power rating. Once the burst of energy has been discharged, the constant power rating tells you where the power output will settle down to.

Li-Pos come with very strict safety guidelines that tell you how to care for them. Charging rates, charging times, and damaged battery disposal procedures just to name a few. Damage to your RC electric model airplane can be avoided by following these guidelines. Properly matched brushless motors and Li-Po batteries can provide hours of clean and quiet flying enjoyment. Take the time to understand everything that's involved with a Li-Po setup and you will have more fun and more time in the sky with your electric model airplane.

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