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[Captain Flack]

I have  read, and subsequently done, about removing the red wire from one ESC, when using two ESCs together .  However, there seems to be no problem with connecting two power supplies to an RX to provide a backup in the event of one battery failing.  Is the removal of one red wire to prevent the ESCs from having a problem or is it, as I originally thought, to prevent two supplies to one unit?If the latter, how come it doesn't apply if you connect two supplies to one RX?

[DaveM]

I've never seen two batteries connected to the same receiver for the same reasons that it's not sensible to connect two BEC-equipped speed controllers without disabling the BEC on one. You don't need two power supplies to the same device, especially if there's a chance they are at different voltages. If there's even a slight possibility that things might go wrong then I try to avoid provoking the situation.

However, as I've often written, suit yourself...

DaveM

[Tug Fanatic]

The reason for removing one wire is so that the BEC is supplied from one Esc. If you try to supply BEC from both Esc at the same time they do not play well together.
I agree with DaveM that one battery is fine but that 2 in parallel does not make for an effective back up although it can obviously increase the capacity available. For it to act as a back up I believe that you would need a changeover switch of some type so that either battery could be used but only one at once.

[JimG]

In the aircraft world is is not uncommon to have two batteries connected to one receiver. In the best setups each battery can have its own switch as faulty switches is one of the biggest causes of failure. It has been shown that two batteries in parallel like this will still work OK even if one of the batteries fails.In the case of escs it is the BEC circuits that cause the problem, if they both give out identical voltages no problem but tolerances in components can lead to them producing slightly different voltages. With some circuits this can lead to one of the BECs becoming unstable which can also cause problems with the other lweadingf to failures. It is therefore recommended that the BEC on one ESC is disabled, normally by cutting or removing the red wire from the plug.
Jim

[DaveM]

Two packs connected first in parallel and then via one single connection to the receiver would be OK, but plugging two packs seperately into two different receiver headers would risk one pack discharging into the other via the very thin copper lands on the circuit board. Result would be one very unhappy receiver.

As Tug says, if you wish to use one as a backup for the other then you'd need a changeover switch........but you'd have a problem to operate it with the boat out on the lake and the receiver battery not working 
DaveM

[petermun]

Dave,  I am not sure of its identification number but I use one of your old safe parallel battery connector circuits.   Pete

[Tug Fanatic]

................................As Tug says, if you wish to use one as a backup for the other then you'd need a changeover switch........but you'd have a problem to operate it with the boat out on the lake and the receiver battery not working 
DaveM

Automatic changeover powered by spare battery as a failsafe - it can be done but is the complication necessary. I doubt it.

[DaveM]

Dave,  I am not sure of its identification number but I use one of your old safe parallel battery connector circuits.   Pete

It's P103, Pete, but it wasn't intended for this purpose (as I read the original posting).
Tug
My take is that if I have any suspicion that a pack might fail then I'd rather bin the thing and fit another one.  Don't forget the 737 Max 8 paradox - try to prevent one potential disaster and you can unwittingly create another one.
DaveM

[JimG]

Two packs connected first in parallel and then via one single connection to the receiver would be OK, but plugging two packs seperately into two different receiver headers would risk one pack discharging into the other via the very thin copper lands on the circuit board. Result would be one very unhappy receiver.

As Tug says, if you wish to use one as a backup for the other then you'd need a changeover switch........but you'd have a problem to operate it with the boat out on the lake and the receiver battery not working 
DaveM

It was shown many years ago on RCUniverse by a battery expert Red Scholfield that the myth of a high current due to one battery discharging into the other was wrong. The copper on the circuit board can take a higher current than many think, aerobatic models with high power digital servos can take a lot of power when several are working together. These servos can take over 1 amp each when working hard. This caused the original problem with Spectrum receivers as the high current caused the battery voltage to drop and the receiver processor stopped working.
Forget any changeover switches, can't be used on the water or if automatic gives another point of failure. If you are really worried then use 5cell 6V packs and put a diode in one of the battery wires to the receiver. Then you can't get cross charging although again adding another potential failure point.
Jim

[malcolmfrary]

What the esteemed contributor to RCUniverse neglected to mention was that after a very short time, you had two partially discharged batteries.  From what I recall of the posting (age, memory etc) it was implied, but not stated, that one of the batteries was not as good as it should have been.  The internal resistance of the charging battery would resrict current.  My experience with large lead acid batteries was that when one bank had been charged, it was a good idea to let it settle a bit before linking it back into the system, else the battery room got rather smelly with all of the little bubbles coming off the plates.  Never blew a main feed fuse, though, so the current was probably within 0.5C.
The two batteries connected together can never be regarded as a "reserve".  They will always, once things have settled, be at the same state of discharge (the glass will be half empty).  Even if connected via diodes such that neither can receive a charge from the other, they will remain separate - the higher voltage one will supply all of the current until its voltage drops to that of the lower one, when that one joins in.
Just fit a larger battery, and/or get a warning circuit either by buying or building or get one of those moden radios that does telemetry and tells you to end your session. 
Never plug a battery into a BEC output, it will not end well.  The BEC is not designed as a charger, and the battery probably wants to settle at a different voltage.

[chas]

I'm all for increasing our collective understanding of things electrical / electronic in our models, but is some of this thread heading towards needless complication of simple installation. The o p was seeking to understand something, and got the answer.
    Having seen models where the K I S S principle was ignored I'll be sticking to following the instructions and just enjoying mostly trouble free sailing.
  Is isn't a criticism, just an observation.
Chas

[Captain Flack]

"Just use 1 reciever and run two batteries into the one reciever.  Put each battery on a separate switch.  It will not hurt the reciever running both batteries into one.  They can take a lot more than just two batteries. "This one of many, many  posts regarding this subject which prompted my original question.

[DaveM]

The thing about asking for advice on forums like this is that you then have to decide which to ignore and which to accept - if any. Only you can make that choice. I've always operated on the maxim which states "If in doubt, don't." Works for me but hey - how much do I know? Suit yourself.

DM

[chas]

I've never seen a properly charged and maintained receiver battery fail in the last 50 years, but doubtless someone will have done.
  The more complicated the installation, the more likely it is to go wrong.
On choosing advice from a forum, perhaps from a retired manufacturer..............   Carefully from anyone who lacks first hand experience.
   I've probably put my foot in it again, so sorry in advance if I've upset someone.
Chas

[RST]

I wouldn't put 2 ESC's into a RX with BEC enabled but ssssssshhhhhhhh.  I've been paralelling up Ni-Mh and SLA for decades and nothing's ever blown-up (whispers) I'm still doing it now -EEK!

[Captain Flack]

Thanks for all the replies.  Seems to be no hard and fast rule aboput this, but i think I'll stick with the one battery.

[mrlownotes]

I have researched this. I have often wondered why we have been warned about supplying a receiver from more than one ESC at the same time. I have extracted the following statement from another 'aero modelling' forum. I believe it provides an understandable explanation.
Here we go:-

There is a feedback loop in the circuit of an ESC's internal BEC. It reads it's own output voltage and when it see's this voltage get out of tolerance, it adjusts to compensate. When two ESC's are connected in parallel, if there is even the slightest difference in output voltage, it can trigger the feedback loop in the other unit to adjust it's output and then the conflict begins. One goes up, the other goes down etc. Basically they fight each other. If the two units were assembled with the exact same production run of components and the output voltage was very closely matched, then I think you might get away with it.

Similarly here's another extract from the same thread :-

It's a NONO with switching BECs. The reason is simple, they are always outputting full power but switch it ON and OFF.
 It's this switching ON & OFF that won't work well with another switching BEC.
 The switching ON & OFF will most of the time not be in sync. Will damage one or more of the BEC systems.

So, we pays our money and we takes our choice. I'll stick to the disconnect, I wouldn't want my ESCs to fail for any reason.
I hope that helps to make a better informed opinion.