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Old Nov 19, 2014, 10:54 AM
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United States, VA, Forest
Joined Nov 2009
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Separate battery for receiver

I have searched and could not find an answer to this. New to electric so please bear with me. If I have say a 2 meter sailplane and the motor is about to cut off due to low voltage and I was in a brick lifting thermal, how long could I continue to fly safely. How much power is left in the battery? Everything I have read says there is enough power to get you home, but that is assuming you are coming straight home after the motor cuts off. On a sailplane, that could be 30 to 45 minutes later. What I was thinking of doing is putting in a separate receiver battery. Could a 4.8 volt pack just be plugged into the battery port of the receiver? I have used 2 batteries (plug one into battery port and the other in any other port with separate switches) numerous times on glow powered planes with no problems at all.

Thanks,

John
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Old Nov 19, 2014, 05:35 PM
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USA, IL, Wheeling
Joined Jan 2003
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If you go separate receiver pack, you need to pull the positive lead from the ESC so the BEC is disabled.

How long you fly after LVC is hard to say ... LVC cuts in UNDER LOAD, once the motor load is gone, the radio load is MUCH lighter. You need to experiment with a good multi-meter to see what kind of load the radio puts on the battery, Unless you are running very small motor packs, you can probably fly a long time after LVC.
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Old Nov 19, 2014, 05:41 PM
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Hi, john.

If you use a separate battery you need to disconnect the red power wire running from the ESC to the receiver. If you use a pin or the tip of a hobby knife you can actually pull the little brass pin out of the connector and then just shove it back in if/when you want. Or? You can simple cut the wire.

I don't make a habit of getting anywhere near the cutoff point. It's such a simple matter to switch a battery or top off the charge between flights and, from what I understand, LiPos are quite happy being recharged even though they aren't down all that far.

One day last year I ran up against the limit with my foam Yak and planted it in the top of a tree. This was around 7:00PM. I sat bolt upright in bed around 2:00AM and realized that I hadn't tried waggling the controls to see if they would knock the plane loose. It didn't work but there was still plenty of juice left. Unless you have a servo that is stalled you should be safe for hours after the shutoff; especially if you can set a higher voltage on your ESC.

Cheers!
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Old Nov 19, 2014, 05:43 PM
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I'm sure that you'll get a number of replies to your post.

I have e-sailplanes that I do run a separate receiver battery, and some that I don't.

In general, if you are running your motor battery down to the voltage cutoff point, and this battery also powers your receiver, you potentially are running a risk.

If you've got the room in the plane, I strongly suggest running a separate RX battery. Specifically, I would recommend a LiFe battery if your servos are rated for 6V. Just remove the red lead from your servo plug on your ESC, and you can plug the battery into the receiver directly (avoiding the potential of a switch failure).

If you are running a separate RX battery, it is imperative to remove the red lead from the ESC servo plug so that the ESC isn't also powering the receiver.

LiFe batteries are easy to maintain, are small, can work without a voltage regulator for many many servos, and allow you to fly all day long!

YMMV

-- the markster
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Old Nov 20, 2014, 06:07 PM
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United States, NJ, North Brunswick Township
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Quote:
Originally Posted by peterlngh View Post
If you use a separate battery you need to disconnect the red power wire running from the ESC to the battery.
Peter,

Did you mean to say "remove the red wire running from the ESC to the receiver"? If you remove the red wire to the battery, there's no power for the motor.

Kevin
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Old Nov 20, 2014, 06:22 PM
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Quote:
Originally Posted by JerseySailor View Post
Peter,

Did you mean to say "remove the red wire running from the ESC to the receiver"? If you remove the red wire to the battery, there's no power for the motor.

Kevin
You are right. Thanks for the catch. I have fixed the original post.
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Old Nov 20, 2014, 11:58 PM
WAA-08 THANK FRANK!
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Las Cruces, New Mexico, United States
Joined Jun 2002
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My best answer - buy another battery. The cost of batteries is minimal vs. the risk of crashing your glider. A 2200 mah battery will run 30 amps for 4 minutes. That is a lot of climb time for most gliders. I would set a timer for 3 minutes of motor time and never have to worry about losing the glider due to battery capacity.

For my ALES ships, I can get easily get at least 4 motor climbs on a full pack. With nominal lift, that is 40+ minutes of flying time on a battery. Then land, swap batteries and go fly. Note - ALES uses an electronic switch that limits motor runs to a set altitude or 30 seconds - whichever comes first.

Know your amp draw
Know your battery
Fly within your capabilities
Don't fly to LVC -
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Old Nov 21, 2014, 04:59 AM
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Thanks guys!

John
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Old Nov 21, 2014, 11:24 AM
Flys Like A Brick
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Washington, MI
Joined Nov 2010
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My bec can be set for LVC. Mine is 3.3volts per cell and then goes to a soft shutdown of 50%. The voltage will rAise some even with the motor running. I am getting at least 9.9 V to the BEC. More than enough to run the Rx and servos. If you can't get home that way having the Rx on a Separate battery is not going to do you any good. With a 2cell pack it's 6.6v still enough. The mah draw of Rx and servos is very low so even a pack that won't let the motor run will get you home if you have the height.

Going to LVC is just not a good practice. We all have done it, but it is just asking for trouble and it is hard on the battery pack

What is more important a good thermal or loose a plane? How do you know how much is left in a separate Rx battery?
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Old Nov 21, 2014, 12:48 PM
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United States, AZ, Mesa
Joined Feb 2008
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Telemetry ...

All of the posts above are spot on. My top priority is to get the plane back, followed by being kind to my batteries.

Using LVCs, timers, etc, are methods that we have all used at some time or another, mostly because those were the only tools we had available to us, and they worked most of the time.

Thanks to cheap electronics we now have telemetry available at prices that most of us can afford. This LIPO sensor can monitor each cell of your pack individually, as you fly, so you can detect when a single cell starts to fail - before the "walk of shame". Only $15 !

http://www.alofthobbies.com/frsky-sp-flvs.html

ct
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Old Nov 21, 2014, 01:13 PM
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I haven't started to play with telemetry, yet, but modern electronics are nearly miraculous to old timers like me. My first 3 channel radio weighed well over a pound and would lose signal if someone started a boat motor on the lake where I flew. This was especially problematic when I had the brilliant idea of launching my underpowered seaplane from the bow of my boat!
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Old Nov 21, 2014, 01:42 PM
Flys Like A Brick
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Washington, MI
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Quote:
Originally Posted by peterlngh View Post
This was especially problematic when I had the brilliant idea of launching my underpowered seaplane from the bow of my boat!
LOL

I have the picture in my mind
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Old Nov 21, 2014, 02:40 PM
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I really wish there had been video cameras in those ancient days. The radio in question, a Rand Galloping Ghost, would default to idle on the throttle when it lost signal. Since I was standing in the bow there was no way for my better half to realize that the plane was gently settling to the water about ten feet in front of us. Thankfully, the bow wave pushed the plane out of the way so only one wing tip got mangled and it never went near the prop but I quickly decided that a bigger engine was a better option than launching from the boat.

Cheers!
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Old Nov 21, 2014, 03:10 PM
I'd rather be Flying
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Nashville, NC, USA
Joined Mar 1999
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I'm comfortable using the BEC in the ESC for thermal flying after the motor run, but not if I've run the pack down to LVC. So, I don't routinely fly to LVC, and if I inadvertently do so, I land soon after.
Here's why. This chart is at resting voltage.
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Old Nov 21, 2014, 06:43 PM
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Northumberland, England
Joined Jan 2007
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Davecee,
without knowing the battery internal resistance & current draw it's impossible to say at what point on the chart is reached at LVC.

I only use BEC in EP models but mainly because it eliminates the need to remember to charge a seperate battery.
Like you I avoid flying to LVC but my main concern is battery longevity. With my E gliders I set a timer in the Tx to count down whilst the power is on to beep when the accumulated time, that I reckon is OK, is up.
A point not mentioned previously is that a switch mode BEC provides the Rx & servo power more efficiently than a linear BEC. I make a point in using an ESC with a switch mode BEC in any model with more than 3s lipos &/or where the servo load is likely to be high.
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