Mar 27, 2011, 09:55 AM Registered User United States, NJ, Roseland Joined Apr 2009 2,216 Posts Discussion How do you read these servo numbers? Current Drain (4.8V) 6.0mA / 170mA Current Drain (6.0V) 6.0mA / 200mA Is the 6.0mA at idle and say 200mA at full power? Also, is the mA mean milliamps per hour? Thanks, Thomas Edison
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 Mar 27, 2011, 10:27 AM Wake up, feel pulse, be happy! United States, MN, Minneapolis Joined Aug 2009 12,884 Posts 6.0mA is how much current the servo draws just sitting there (idle). 200mA is most likely how much it draws when it's moving at full speed freely through its range of motion. It'll draw a lot more if it's stalled (trying to move but unable because something's holding it). 200mA would be 1/5th of an amp. That's important to know for picking an ESC; most with built-in BEC units will only be able to supply 2-3A to the servos before they fry. "mA" means milli-amps. One mA is equal to 1/1000th of an amp. It's the same as the current (amps) that your motor draws from your battery and ESC, but it's on a smaller scale. For our purposes, it has nothing to do with time. "mAh" is milli-amp hours. That's a measure of how long something can supply a given current. A 1000mAh battery will supply 1000mA of current (1A) for an hour.
 Mar 27, 2011, 11:47 AM Registered User United States, NJ, Roseland Joined Apr 2009 2,216 Posts TP, have a sailplane using 6 servos with the above specs. I want to use a 850mah LiFe rx pack. Will that be enough and what kind of runtimes can I expect? If the servos use 6.0 mA amps just sitting there, what does that equal per hour? I figure the 2 flap servos will idle till landing.
Mar 27, 2011, 12:02 PM
Wake up, feel pulse, be happy!
United States, MN, Minneapolis
Joined Aug 2009
12,884 Posts
Quote:
 If the servos use 6.0 mA amps just sitting there, what does that equal per hour? I figure the 2 flap servos will idle till landing.
It doesn't equal anything per hour. 6.0mA is 6.0mA. The time gets involved when you have a limited supply of current. If you have a 6.0mAh battery, it can supply 6.0mA of current to the servos for one hour before dying.

If you have a 600mAh battery, it can supply those same 6mA for 100 hours before it's empty.

6 servos x 6mA = 36mA of idle draw (this will probably be a little more, though, because your control surfaces probably aren't mass-balanced so the servos will have to work to hold them in position against gravity).

So, ignoring any servo stresses, you're drawing 36mA off an 850mAh pack. That pack can supply 0.85A for an hour, and you're drawing 0.036A. Divide and you get about 23.6 hours of just sitting there.

Now let's assume all your servos are maxed out at 200mA. This means you've got a total draw of 1.2A..... 0.85/1.2 = 0.7 hours, or 42 minutes of full-draw runtime.
Mar 27, 2011, 12:03 PM
Why didn't they run? A pass???
United States, WA, Woodinville
Joined Mar 2007
8,206 Posts
Quote:
 Originally Posted by hidaven TP, have a sailplane using 6 servos with the above specs. I want to use a 850mah LiFe rx pack. Will that be enough and what kind of runtimes can I expect? If the servos use 6.0 mA amps just sitting there, what does that equal per hour? I figure the 2 flap servos will idle till landing.

You will want to have another pack on hand for a day's worth of flying, or at least your charger to top off the pack after a couple flights.
Mar 27, 2011, 12:05 PM
Wake up, feel pulse, be happy!
United States, MN, Minneapolis
Joined Aug 2009
12,884 Posts
Quote:
 the mA = milliamp Whats missing is the H = hour What you originally typed..6mA actually is 6mAh so 6mAh is 6 milliamps drawn per hour...at idle.
Wrong. Read my post
 Mar 27, 2011, 12:22 PM An itch?. Scratch build. South Wales U.K. Joined Mar 2003 15,029 Posts Can't give a definitive answer as it depends on just how often the servos move and the load on them. I also assume you do mean just a glider and not an electric powered glider. But considering we used to fly IC powered models for quite a long time, (off and on for most of the day), on just a 500mAh 4.8v nicad, (, I don't think you would have any problems, unless you are into continuous aerobatics for a couple of hours. Actually the transmitter battery would probably give up before the receiver battery. At 6ma standing current, for 6 servos, that's 6 x 6mA per hour. 36mAh. But I bet you can't fly for an hour without waggling one of the sticks So my answer would be, 'no problem', as long as you remember to charge the pack before going flying.
 Mar 27, 2011, 12:25 PM Registered User United States, NJ, Roseland Joined Apr 2009 2,216 Posts Actually, if you can only run the LiFe's down to 25% or so you only have 75% of 850mAh Cutting it a bit close. As Cry stated, better get another pack just to be on the safe side. Actually, I can charge at 4c so a recharge should be what 10 min? : ) Eflighttray, it is an electric glider but im running a seperate LiFe rx pack for just the servos. Last edited by hidaven; Mar 27, 2011 at 12:36 PM.
Mar 27, 2011, 12:31 PM
Suspended Account
Joined Jul 2006
22,991 Posts
Quote:
 Originally Posted by hidaven Also, is the mA mean milliamps per hour?
TP16 gave a very good, and accurate explanation.

I'll just throw in that the unit of time in this system of measuring is the 'second'.

A servo using 6 mA of current is using .006 coulombs of electrons every second.

"Current' is a 'rate'.

Coulombs is the actual physical quantity.

It'll all become clear as soon as you can say out loud the following sentence, 10 times in a row, quickly....

"A box of biscuits, a box of mixed biscuits, and a bisquit mixer."
Last edited by NoFlyZone; Mar 27, 2011 at 12:54 PM. Reason: Coulombs added to make it precise.

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