JiggerInc
Senior Heliman
Location: Arlington, VA
| Thanks jcrack, well said. I also think Deltaman's description suggests volts as something that is consumed from the battery, while a motor draws a constant current. It's a more accurate model to think of 1) a motor requiring a certain number of watts to generate a given headspeed... 2) amps being consumed based on the need of the load.. while 3) voltage is constantly applied to the system. A water analogy usually serves well...
Voltage is like the pressure in a pipe. Amperage is how much "flow" of water is moving through the pipe. Even when there's no flow of water, there's still a constant pressure. "A given voltage, upon encountering infinite resistance will produce 0 amps of current." ... as the resistance lowers (connecting a light bulb to the terminals of the battery, as opposed to a block of rubber), the voltage pushes more current though the system.
A battery's rated voltage is how much pressure it puts on a line. As you open the valve (throttle up) water begins to flow. How much water flow is required to do a certain amount of work is dependent on the pressure. At higher pressure, less water can get more work done. At a lower pressure it requires a greater volume of water to do that work. In the analogy, "work" might be something like pushing back an angry crowd with a firehose. At a high enough pressure, an almost microscopic flow of water can cut through steel!! At regular sea-level pressure, the amount of water flow required to errode steel would be unimaginable.
To continue the analogy, a battery's ability to provide current is rated in terms of a multiple of its amp-hour rating. A 10C 1500mAH battery can flow up to 15A of current before the amount of "water" flowing in the "pipes" begins to drop the pressure... That water flow also causes friction against the walls of the pipes... hence, heat. If you put a low enough resistance between the terminals of a battery (short them out) the current that flows between them goes towards infinitity and you have a nice fire/explosion.
It's not a perfect analogy, but hopefully it illustrates the relationship.
Finally, if it helps in rationalizing the flight times, convert the battery's amp-hour rating to a watt-hour rating by multiplying by the voltage. In your case 1.5Ah * 7.4V = 11.1watt-hours. If your blade requires 111watts of power to fight gravity and get in the air, you'll get 0.1 hours or 6 minutes of flight. A 1.5Ah * 11.1V lipo will be 16.65watt-hours.. but that extra weight might mean you need 150watts of power to fly.. so you'll get slightly more than 6 minutes of flight. It's diminishing returns.. each cell you add also adds weight, so you get less and less additional flight time for each new cell. |
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