Information Articles February 22, 2008
There are several common questions that are regularly asked regarding the operation and maintenance of electric RC systems. This article lists the most common questions and their answers.
Which Lithium Polymer Batteries Should Be Balanced?
Answer: Any Lithium Polymer (LiPo) battery with more than two cells should be balanced. A LiPo battery is composed of individual cells, and each cell can charge and discharge at different rates. Eventually, the difference will increase until it can harm the entire pack. When charging the pack, a balancer will set all the cells to the same voltage. By not having certain cells charged and discharged beyond their limits, the life of the entire LiPo pack is increased.
What Does the KV Number Mean?
Answer: The KV number on a brushless electric motor shows what speed it will rotate at per volt of input. This will allow you to determine the unloaded speed you can expect from the motor at different battery levels. This number is measured under the assumption that there is no resistance keeping the motor from turning. In reality, when using a motor you would have a load attached, for example a propeller. This load will cause the motor to slow down. The KV rating will not show exactly what the performance of a motor is when a propeller attached, but it will give you an approximation. A higher KV rating is given to faster motors, and a lower one to slower motors.
What’s a BEC?
Answer: A BEC (battery eliminator circuit) eliminates the need for a separate battery to power the radio system. It does this by drawing power directly from the flight battery, and uses this to power the radio. A BEC can also act as a backup system, continuing to draw the last bit of power from the flight battery for the receiver and servos to use even after their is not enough to power the main motor. This will enable the pilot to maintain control of the aircraft even after the engine stops. Most of the time these devices are part of the electronic speed controller, but they are also available as a separate unit.
What Size ESC Do I need?
Answer: There are two factors to consider when choosing an ESC, the voltage and the amperage. The rated voltage and amperage should always be above the maximums that will be used in the aircraft. Select an ESC that is rated for the same or more cells than you plan to use, and with a current rating at least as high as what your motor is rated for.
What’s the C Rating On a battery?
Answer: The C rating on a battery relates to the maximum current that can be drawn from that battery. A 15C battery can have current drawn from it at up to 15 times the rate that would discharge it in one hour. A 1320mAh (milliamp hour) battery would be discharged in one hour if you were to draw 1320mA from it for one hour), and would be discharged in 1/15th of an hour if you were to draw 15 x 1320mA (19800mA or 19.8 Amps) from it. There are three types of C ratings. Continuous is the amount of current that can be drawn constantly until the battery is depleted. Sustained shows the amount of current that can be drawn for a short period of time. Burst is the amount of current that can be drawn only for an instant. Most lithium batteries will have all three of these ratings printed on them.
Should I use a Watt Meter?
Answer: Yes, the watt meter will show how much electricity your plane is using. You will need a watt meter to determine if your aircraft is exceeding it rated limits.
What Size Prop Should I Use?
Answer: The best size propeller to use is the one the manufacturer of your engine recommends. It is possible to experiment with different propeller sizes, but use a watt meter to determine if your aircraft is exceeding the limits of its electric system. The motor specification sheet will show the voltage, amperage, and RPM of the motor with different propellers.
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