I am posting this for Ed. It is a PM I sent to him earlier today. There is some good info here for others like me who are learning electrics.

Clint

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Ed,

I just read your "Everything you want to know about electric flight" write up. It is fantastic and helped me out a lot. Thank you for writing it.

I am still struggling a little bit on battery theory.

I have an Electristar that is built and ready to go. I have two LiPo batteries for it. They are 14.8 Volt 30C, one is 3200mah and the other is 3700mah.

Because it is a balsa plane I wanted to learn on something else first so I bought a Multiplex Mentor and installed the Multiplex pack consisting of a Himax 3528-1000 motor and BL-55 ESC. This setup calls for a 11.1Volt LiPo, which I bought one 32oomah of.
Can I use the 14.8 Volt batteries that I already have for the Electristar in the Mentor if I don't run it at 100%? If not, why? If I buy another 11.1 Volt LiPo what is the highest mah I can use for longest flight time?

Thanks in advance for any help with this. I would love to be able to take three packs with me and have a nice long flight time at the field between recharges.

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Good questions,

The answer is not based on the plane but on the motor, ESC and prop. If you go to the chapter on selecting power systems you see that the combination of battery voltage and the prop will determine how much current, the amps, that will be drawn by the motor. So the first question is whether your motor is rated for 14.8 volts. You have to check the specs, or RTFM, read the friendly manual.

Second thing is, is your ESC rated for 14.8 volts. RTFM

Third, if you are using the BEC on your ESC, what is that max voltage that can tolerate? Most can not go over 12V. This is covered in the BEC chapter.

Now, assuming the motor and ESC can handle it, can they handle it with that propeller? Only two ways to know. Run a wattmeter to measure the current, or go to the motor manufacturer's documentation that shows amp draw at voltage with selected props. That is a Himax motor and they provide EXCELLENT reference material. Go to your motor, on this page, and click the "learn more" button:
http://www.maxxprod.com/mpi/mpi-262.html

The first thing we see is that this motor is not rated for 14.8V. Max recommended is 12 NiXX cells or 3 cell lipo. So you can't use the 4 cell lipo without risk of burning up the motor. So I am not even going to check the ESC. And you did not provide the specs of the prop so I can't comment on that.


Now, can you run it at partial throttle and be safe? No!

Your ESC delivers the full pack voltage to the motor all the time, full throttle or partial. As described in the article on the ESC, it controls the speed of the motor by pulsing that full voltage. How fast it pulses it determines the speed of the motor. You can go ahead and put the 14.8V pack in the plane, just don't be surprised if the motor fails.


As for max MAH, as discussed in the articles, you can put in the largest pack you feel the plane can carry as long as it can handle the amperage draw that the motor will demand.

Go back and read the articles and this will make more sense.

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Clear Skies and Safe Flying!
Ed Anderson
Moderator
Park Pilot Program Partner