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Old Oct 16, 2012, 10:37 PM
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I think I have it figured out...

Sorry for the dumb question.. but after years of not being in the hobby I am trying to get a grasp of this new Lipo situation. Anyhow I am trying to get a battery for my new glider and just want to get something straight.

So if all of my servos draw around 15 amps and I have a 150mah battery... I can run it for 10 hours before it dies... Right?

So I also noticed that different batteries will allow a different amount of amps to be pulled from the servo... how does this factor in?
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Old Oct 16, 2012, 10:45 PM
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So. Cal.
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Not quite.

15A * 10h = 150Ah

1Ah = 1000mAh

Therefore you'll need 150,000mAh battery if your servos draw 15A continuous and you want to run for 10 hours.

You'll also need to ensure that the pack that you choose can easily deliver the current demands of your application. Note too that manufacturers generally inflate the current delivery capabilities of their batteries, and some quite dramatically so.

Mark
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Old Oct 17, 2012, 04:19 AM
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Staffs, UK
Joined Nov 2003
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Lots of confusion there, I'm not surprised your servo is dead .

Servos don't have current pulled from them, they take current from the battery.

At different battery VOLTAGES they will take (pull) different currents.

In current measurements the little "m" is important, it means 1/1000th. 15A is a thousand times more than 15mA.

Unless you have really huge servos or dozens of them they won't take anywhere near 15A continuous.

Do you want to start again, perhaps this time saying what servos and how many you're talking about ?

Steve
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Old Oct 17, 2012, 01:01 PM
The 6 P principle works for me
elecfryer's Avatar
Ventura CA
Joined Aug 2007
980 Posts
Quote:
Originally Posted by DeadServo View Post
Sorry for the dumb question.. but after years of not being in the hobby I am trying to get a grasp of this new Lipo situation. Anyhow I am trying to get a battery for my new glider and just want to get something straight.

So if all of my servos draw around 15 amps and I have a 150mah battery... I can run it for 10 hours before it dies... Right?

As stated above, 15 amps = 15,000 mamps (1 amp = 1,000 mamps or 1,000 mili amps (an amp divided into 1,000 parts). 150 / 15,000 = .01 hours or less than 1 minute! HOWEVER as also stated above, the chances of your model airplane servos drawing in 15 amps is 0! To just move the surfaces servos do not draw much current. When there is resistance (as in air flow across the surface) then the load on the servo will increase as will the current!

So I also noticed that different batteries will allow a different amount of amps to be pulled from the servo... how does this factor in?

Servos "pull in power" from the battery, not the opposite!
Additionally, a 4 or 5 cell NiMH battery will be fine for many R/C plane applications. There are load meters that you can insert between the battery and receiver to get an idea of how much current is being used.

Michael (if your not fryin, your not tryin!)
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Old Oct 17, 2012, 04:22 PM
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Originally Posted by slipstick View Post
Lots of confusion there, I'm not surprised your servo is dead .

Servos don't have current pulled from them, they take current from the battery.

At different battery VOLTAGES they will take (pull) different currents.

In current measurements the little "m" is important, it means 1/1000th. 15A is a thousand times more than 15mA.

Unless you have really huge servos or dozens of them they won't take anywhere near 15A continuous.

Do you want to start again, perhaps this time saying what servos and how many you're talking about ?

Steve
Well your right.. my servos are "Dead" at the moment.. just sitting in the box at the moment.

But to answer you question... I will be running 3 x HS35HD servos from Hitec. The link is here if this helps:
http://www.hitecrcd.com/products/ser...hs-5035hd.html

As for what battery I will use.. well at the moment all I have is a bunch of Nimh from 6 or so years ago... but I was hoping to make the switch to Lipo.

The only stipulation is that the battery would have to be pretty darn small as it will be going into a small DLG glider... Binary 900 to be exact.

So yeah.. in summary all I have so far is the 3 servos. The receiver I will use will most likely be my AR600 if that makes any difference. I am trying to keep it to 2 cells so from what I understand I wont have to use a regulator? Ofcourse I was wrong before so I am probably wrong again here too. Ugh
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Old Oct 17, 2012, 05:34 PM
The 6 P principle works for me
elecfryer's Avatar
Ventura CA
Joined Aug 2007
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Lets try this, I have (and Love) a Spin DLG. 10 oz ready to fly, 4 servos (HS-45s) and the servos along with the receiver are all powered by one 350mAh 4 cell NiMH battery. It is good for 45 - 60 minutes of flight time.

Michael
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Old Oct 17, 2012, 09:42 PM
IT'S NOSE HEAVY!!!!!!!
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United States, CA, San Jose
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Ok, all semantics aside, and not accounting for "silly" errors in the OP...I have a somewhat mirror question.
Can one take the mAh rating and accurately use that to judge useable time in the air if servo draw is known? (I know, I know. Draw is extraordinarily dependent on conditions)

So if, say, loaded servos draw 15mA (let's assume for the sake of the math that this is an actual real-world draw while in use) and it is used for 10 hours straight. Under this specific condition, would the 150mAh batterie be drained to "minimal safe voltage" or will they be completely drained to "zero" volts.
To ask it another way, is the mAh rating taking into account that it isn't taken below it's threshhold for damage? Or is it like Scotty giving it all she's got... until she's done for good?
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Old Oct 18, 2012, 12:06 AM
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Australia, VIC, Melbourne
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Originally Posted by cityevader View Post
To ask it another way, is the mAh rating taking into account that it isn't taken below it's threshhold for damage? Or is it like Scotty giving it all she's got... until she's done for good?
The latter, the usable capacity of a LiPo is ~80% of the sticker value for a good lifespan.

If you want to estimate how long you can fly on the RX pack, fly about for a bit (say 10mins), then charge the pack. Your charger should tell you how much you put back in, divide by the flight time for your mAh/min...
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Old Oct 18, 2012, 12:25 AM
IT'S NOSE HEAVY!!!!!!!
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ahh, a very useful piece of advice regarding the charger and timing. Not so critical in my 2000mA pack in the Passer. Quite more important for the 250mA pack in the Speedo. Better to only take it down partway and then swap packs and charge the other one later and multiply/divide, rather than lose a plane with insufficient capacity remains when static voltage with a meter may seem fine.
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