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Electric twin with seperate servo & RX battery.


Frank Day
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I guess this has been covered a millions times. Just getting my head around the electric powered "magic" when along comes a BT Mossie as in some of my other posts but I cant visualize the wiring with this recomended setup. I get the twin setup to the RX with one red wire removed,  but not how that all works with the additional UBEC & battery.

 

Two motors with esc's and two motor only batteries.

 

20A UBEC  and battery for servo's and RX

 

Help most welcome (diagram would be great)

 

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Dead right Frank.

 

Both ESC's red wires not connected (I normally pull the connector out of the plug and fold them back along the wire held with some insulation tape so I can reuse the ESC later in something else).

 

Red wire from UBEC left connected plugged into the receiver.

 

Power for the ESC's and the UBEC can come from the same battery if required to reduce weight.

 

With a twin I would normally connect the 2 motor batteries together before breaking the wires back out to the ESC's. That way both ESC's will see and receive the same power, so no weak battery issues.

 

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Hi Andy,

i have thought about connecting the two motor batteries but havent looked at the model yet to find a way through. Esc's and batteries are destined for the nacelles in the old fuel tank spaces, hopefully there will be a channel through the wing.

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My BT mossie has the batteries in the nacelles, hence commoning them up would require extra full rated power cabling between the nacelles so I have left them separate. On twins with the two batteries in the fuselage then I common them up. As with Hangarqueen above, if the ESCs have enough current rating I connect through Schottky diodes so I have some redundancy.

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To my mind cross connecting two batteries in a twin all comes down to how happy you are flying with the risk of a motor cut should you run one battery down to an ESC's LVC.

 

I certainly shorten the flights on my non connected twin more than I would normally to ensure the batteries are nowhere near the ESC LVCs.

This plane also uses linear BECs so the red ESC wires can safely be left connected to the rx to give some redundancy.   

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LVC cutout can be achieved a number of ways, firstly by gently discharging over a long period of time, secondly with high current draw (edf's) that can cause a short term dip into standard LVC territory on take off/WOT use. Hence is standard practice to move the LVC point lower to avoid unwanted operation.

 

Is there any practical downside of paralleling balanced lipos as opposed to running separate lipos?

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On 24/04/2022 at 21:35, PeterF said:

My BT mossie has the batteries in the nacelles, hence commoning them up would require extra full rated power cabling between the nacelles so I have left them separate. On twins with the two batteries in the fuselage then I common them up. As with Hangarqueen above, if the ESCs have enough current rating I connect through Schottky diodes so I have some redundancy.

 

I've often wondered if you really need full-rated power cabling to pair up remote batteries.  Assuming they're both equally charged, and are in reasonable condition, won't it just need a small current to keep them balanced during flight?

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20 hours ago, Allan Bennett said:

 

I've often wondered if you really need full-rated power cabling to pair up remote batteries.  Assuming they're both equally charged, and are in reasonable condition, won't it just need a small current to keep them balanced during flight?

Yes, a thin balance cable would suffice ........ until one pack goes low or the connector disconnects and then you try feeding full power to the other motor. You end up with high voltage drop and asymmetric thrust which you put the wire in to eliminate, or it gets very hot and ignites your plane. OK this is quite unlikely, but when I have commoned batteries, they are commoned with full power capability.

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I would have thought that the main reason for a connecting cable is to avoid a gradual increase in assymetry caused by one battery not holding voltage as well as the other, not to handle the full load of two motors in the unlikely event of complete failure or disconnection of one battery. 

 

So a thin wire with a fuse in it, to avoid the potential ignition scenario.  Then if the fuse blows because one pack has failed completely, you're no worse than you would be with two unconnected batteries. 

 

Having said that, on my twins I keep the ESCs in the fuselage so they can connect directly to the battery, or pair of paralleled batteries. 

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Once i get a chance to delve into the Mossie, I'll see if there is a route for a parallel connection otherwise they'll have to remain seperate although a fused balance could be an option; not sure what rate that would be on a 6S setup (this electric stuff is pretty new to me) It was built as IC and there is a cable for onboard glow so I may be able to pull some cable though the existing route.

 

Im going to go with PeterF's set up with everything in the nacelles, i did consider puting the esc's in radiator area but these are closed/werent built on mine, just a fake grill but with alittle woodwork needed i may open them up

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1 hour ago, Frank Day said:

Once i get a chance to delve into the Mossie, I'll see if there is a route for a parallel connection otherwise they'll have to remain seperate although a fused balance could be an option; not sure what rate that would be on a 6S setup (this electric stuff is pretty new to me) It was built as IC and there is a cable for onboard glow so I may be able to pull some cable though the existing route.. . . . 

 

I've never actually used a balance connection myself; I'm just throwing out ideas that I'm pretty sure would work, so it would be nice to hear from anyone who's actually done it.  The disadvantage of balance leads is, of course, extra weight.  What size fuse you use would depend on what size cable you're using for the balance lead.  With decent batteries I guess the amount of balance current needed would be quite small, maybe something like 5 amps would do.

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Hi,

 

You could just move the LVC on the ESC very low and avoid the issue that way. This would mean only activating LVC by flying around for just too long which is common or electric or IC.

 

With my BH and SG mossies achieving C of G has been a challenge, thinking about removing IC engines and replacing with electric motors and batteries might be something you look at early stages. My SG mossie (IC) has flight battery in the nose and even brass spinner nuts. The BH mossie has oversize motors (just for weight!) and again a 4S500 lipo aand 2S1000 RX lipo in the nose to achieve C of G.

 

Keep us posted with the BT mossie and it might be worth a chat with George at 4Max....he has a ASM Tigercat ...that's electric so clearly found a solution (solved it with mine 2 x Laser 100's!).

 

PS as for the balance lead idea, the smaller the CSA of the wire the greater the volt drop, so with say small wires the lipos would not balance and you would get the same issue/blow the fuse and have the same problem. Another solution is to use telemetry to warn you either or both lipo's are struggling. 

Edited by Chris Walby
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1 hour ago, Chris Walby said:

. . . PS as for the balance lead idea, the smaller the CSA of the wire the greater the volt drop, so with say small wires the lipos would not balance and you would get the same issue/blow the fuse and have the same problem. Another solution is to use telemetry to warn you either or both lipo's are struggling. 

 

Voltage drop is worth bearing in mind, but the lower the balance current the lower the voltage drop.   So I think reasonably small leads -- say 1/2 the size of what you would use for full-current capability -- would do the job.

 

These days, of course, telemetry is a boon.  But constant need to input more rudder could also tell you that one battery is struggling!

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These days, of course, telemetry is a boon.  But constant need to input more rudder could also tell you that one battery is struggling!

 

I don't think electric works like that unless you set up soft cut on the  ESC. The standard ESC setting is LVC and it can be latching so once its trigged it it will only come out of LVC with a battery recycle! And this is the problem, once you hit LVC you basically only have one motor available.

 

Not forgetting that if you have 2 ESC's off one Lipo, one will LVC before the other, just best not get to that point. A bit lie running out of fuel with IC on a twin, you take the risk, then you pay the consequence!

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7 hours ago, Chris Walby said:

These days, of course, telemetry is a boon.  But constant need to input more rudder could also tell you that one battery is struggling!

 

I don't think electric works like that unless you set up soft cut on the  ESC. The standard ESC setting is LVC and it can be latching so once its trigged it it will only come out of LVC with a battery recycle! And this is the problem, once you hit LVC you basically only have one motor available.  . . . 

 

On my electric models I often find myself having to 'nudge' the throttle open a touch as the battery voltage reduces, long before it reaches the ESC's lvc.  On a twin with separate batteries, that would result in slight assymetry.  In fact, you might also hear a 'beat' note as the motors go slightly out of sync.

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36 minutes ago, Allan Bennett said:

 

On my electric models I often find myself having to 'nudge' the throttle open a touch as the battery voltage reduces, long before it reaches the ESC's lvc.  On a twin with separate batteries, that would result in slight assymetry.  In fact, you might also hear a 'beat' note as the motors go slightly out of sync.

Wow, a better man than me who can tell a battery voltage is sagging by the sound of the prop from a model in flight.

 

PS I'll give George a call and discuss ESC settings as my understanding of ESC's was that the speed of the motor was relative to the throttle position and the IGBT's would just switched longer to achieve the same motor speed setting irrespective of relatively small changes in lipo voltage. 

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16 hours ago, Chris Walby said:

Wow, a better man than me who can tell a battery voltage is sagging by the sound of the prop from a model in flight. . . . 

 

Not really ?  With a twin, be it prop or edf, if the motors are not at exacly the same rpm you can hear a 'beat' note which pulses according to the difference in rpm.  So one motor slowing down should be noticeable by a beat note which will increase in frequency as the motor slows.

 

It will be interesting to hear what George has to say.  My understanding is the speed the motor tries to achieve is simply supply volts x motor kV.  I believe the speed controller works by switching on and off so as to achieve an average voltage which is related to the throttle position.  The motor then draws whatever amps it needs to achieve the speed that corresponds to that voltage.

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