Hi Duncan
I believe the 7.2v NiMh battery at 3.7amps is a good choice and will provide enough power for both main motors and all electronics via a UBEC. You connect the UBEC to the main battery (after any switch/fuse if installed) and plug the output into your Rx battery socket. This will be much lighter and smaller than another battery you were considering. Amazon do a WANGCL Adjustable UBEC DC Voltage Stabilisation Systems RC 5V/6V 3A for £6.99 with free delivery so similar cost to a separate battery and smaller and less weight. I use them on many of my models and have had no problems. You do need to keep the Aerial and servo wires away from the unit as it may cause interference if mounted too close.
dave

Thanks Doug and Dave for your input. I have done some further testing and the fault only seems to be with the BEC from one of the ESC's. The ESc's now work both motors and the rudder servo having used the BEC from the non faulty ESC. Having stripped the wire back on the ESC in order to connect the snipped wires back together again, I realised how thin the wire was so I'm now thinking of following your ideas of disconnecting the BECs and using separate batteries for the receiver/winch and the two motors - because all the testing so far has been on no load. Next thing is what type and power of batteries I should go for. Because of limited space I had thought of getting from componentshop.com the 7.2V SC 3700mAh (Compact)
Dimensions: 66.8 x 46.7 x 45.5 mm, Weight: 382g but it is currently out of stock until Nov. Not a problem because I'll still be building!! This will just fit between the winch and the rudder servo to power the two motors. Question is what capacity of battery should I choose for the receiver and the winch servo? Again size and weight will be a big consideration! Cheers everyone.

https://www.ebay.co.uk/itm/361239081408?mkevt=1&mkpid=0&emsid=e11051.m43.l1123&m

Hi Doug.

You wrote:
"9 times out of 10 you might be right. But it's a gamble.
It is always possible that there will be a current flow from one BEC to the other.
In the absolute worst case high enough to overheat the wiring and possibly even cause a fire and damage to or even loss of the model."

I didn't know, although I should have known.
Thanks for the valuable information.
I have to apologize to Duncan. Sorry Duncan.

You wrote:
BTW: same applies to connecting drive batteries in parallel; regardless of chemistry; SLA, NiMH or LiPo. (Shame on you any diehard NiCad users!)
In the case of batteries a solution is available using Schotty blocking diodes between the two batteries"

Yes I knew this.
Does this also apply to AGM lead acid batteries?
I'm asking you because, when I needed them, I put AGM-type lead-acid batteries in parallel, without protection circuits or diodes. (when they are not under load I always disconnect them).

Is the choice of the "Schotty" type diode because it has a lower voltage drop than normal diodes?
If I remember correctly the diode has a voltage drop of 0.7 volts while the "Schotty" 0.15 volts if I remember correctly.
Correct me if I'm wrong.

"Shame on you any diehard NiCad users!"
ahahahahahaha

I completely agree with the whole discussion (which I think I translated and understood well) about the amperage of the BEC integrated into the ESC.
Furthermore, as you advised him, he will be able to easily verify that the ESC BEC is not faulty.

I haven't gotten there yet, but in my case, I preferred to dedicate a 6 volt 4.5 Ah AGM battery, to power the receiver, the rudder servo and the two winches. A 6 volt 4.5 Ah AGM battery is instead connected to the ESC to power the brushed motor, but I cut the positive of the internal BEC.
Logically, I didn't cut the original cable, but that of an extension.
I must say that at the beginning, years ago, it wasn't such an obvious solution, in fact I was putting the two batteries in parallel powering everything through the ESC /BEC. Then I thought about it and changed my mind.
However, I measured much lower currents in the servos, perhaps I didn't strain them too much. I must have done an unreliable test.

Congratulations for the Italian!

Hi DuncanP
I have used these cheap ESC's on low drain motors without any problems. As Doug says the BEC rating is optimistic and with all the ESC's and servos you have attached I am not surprised something failed. The servos alone will take large currents especially the winch. The mixer may be another issue as I have no knowledge of a W tail mixer and how it may be connected internally. You need to disconnect both BEC outputs from the ESC's and install a Ubec connected to the main battery and feeding directly to the Rx battery input. You could use a BEC connection from one of the ESC's but as both Doug and I have explained the current available may be insufficient for all the equipment you have connected. The Ubec is small in size so should be able to be fitted inside your hull. Just keep it away from the aerial leads.
Good luck and I hope you manage to resolve your issues.
dave976

Hi Alessandro,
Mille grazie per la vostra approvazione😊
I like your practical tips on how to test without accidentally causing more faults👍
However (there's somehow nearly always a however😐)
I would take issue with this statement-
"You were right to disconnect a positive but if you left them both it shouldn't have caused you big problems."
9 times out of 10 you might be right. But it's a gamble🤞.
It is always possible that there will be a current flow from one BEC to the other.
In the absolute worst case high enough to overheat the wiring and possibly even cause a fire and damage to or even loss of the model.
BTW: same applies to connecting drive batteries in parallel; regardless of chemistry; SLA, NiMH or LiPo. (Shame on you any diehard NiCad users!)
In the case of batteries a solution is available using Schotty blocking diodes between the two batteries😉
I ask myself; is it worth the risk? Answer; a resounding NO.

The other consideration with BECs is; how much current can it supply?
Can it cope with all the servos and gizmos I want to build into a scale model?
In most cases definitely not! So what happens if it is overloaded?
So my reasoning is; if the boat/ship has sufficient load capacity I prefer to use a separate higher capacity NiMh battery to supply the RX power, and consequently all the servos and gizmos I want to use. I then disconnect/switch off ALL BECs.

Bear in mind that standard analogue servos gobble about 500mA, digital servos or high torque sail winches etc maybe up to an Amp or more.
So the typical 1 or 2 Amp capability of most ESC/BEC is soon overloaded. What the consequences would be I don't rightly know but I don't propose to risk testing it!
(Unless someone is willing to sponsor a research project😁😂)
OK I have seen some ESCs (from oriental suppliers) advertised with BEC 5A. But looking at the package size of the thing (about 2 thumbnails, an' I've got big thumbs😁) I seriously doubt such claims.

OK, I understand that most of the above is not relevant to the 2 channel speed merchants and maybe Puffer drivers (Authentic steam enthusiasts excluded. They are an Elite breed unto themselves to which I doff my Admiral's cap).
But to us multi-function scalers I think it's highly relevant.

I herewith open this topic to discussion and look forward to other members opinons and contributions.
Cheers, Doug😎
AKA FLEET😉

Hi Duncan, Thank you, Mon plaisir😉
I'm just one of many "bin there done that, tried that don't recommend it because ..." guys on this site. Very glad I could help. Look forward to the results of the further tests.
And seeing the project (whatever that is ????) in action on the wet stuff.
Best of luck, remember: we're all behind you whenever you need a shove in the right direction😁
Cheers, Doug 😎

Hi Duncan.

I was translating my answer, but Doug beat me to it.
I read his post and realized that my post would have been useless. He already told you everything.

I think there is a lot to learn from Doug.
Great Doug!

The first thing I would have written to you is, in fact, that you can never have 12 volts in parallel. For the twelve volts a series connection is needed. Furthermore, with 12 volts you would have damaged the receiver and something else for sure.
In parallel, negatives are united with negatives, positives with positives.
If you want to test with the multimeter tester, pay attention to short circuits and do not go directly to the pins of the receiver. It's risky.
You should take a connector and strip the two cables, positive and negative.
The trick of cutting them to different sizes (one shorter and one longer) will prevent them from accidentally touching each other and shorting.

Basically, I would have recommended the same steps that Doug suggested.
Check each item separately, just as you suffer him.
I must say that your problem is very strange, but it can happen.

Honestly, I don't know and have never used the W-tail mixer.
From the photo you attached, you can be sure that the power they receive: the servomechanism, the winch, the receiver comes from the BEC of the ESC.
You were right to disconnect a positive but if you left them both it shouldn't have caused you big problems.

If it's any consolation, I don't think you made any mistakes. At least I think so.

I invite you to follow Doug's suggestions and let us know the results, I'm curious to know which device failed.

Thanks Doug for all you advice and how to do the further tests. I will try the tests in the next day or two and let you know. I'm sure your advice will also be useful to other members. Cheers Duncan 😊

So far so good Duncan👍
Now to complete the tests and fault finding (my Test Engineer's instincts coming to the fore😉)
First test the other ESC alone.
Power it up and check for BEC voltage on the red wire to the RX and if the BEC switch is working reliably. Those little slide switches are notorious for failing.
Second If all OK with the BEC swap the ESC in your last test with this one and check if all behaves as before. If so the ESC is OK.😉
Even if not OK (i.e. no BEC volts) remove the RED wire and put both ESCs back in circuit.
The ESC itself may be OK even if it's BEC isn't.
With the RX getting power from the known good ESC all should work.
If not you gotta DOA ESC😭
Hope that makes sense.
Cheers, Doug😎
BTW re "so it didn't get 12volts to the receiver". That can't happen.Voltages in parallel do not add together. They have to be in series to do that.
What can happen is that the stronger of the two voltage sources will try to back charge the weaker one. Possibly damaging one or both of the source circuits. In this case the BECs.

Hi Duncan,
Do not put Web links into the text.
ALL text which comes after it gets chopped off😠
As has happened here with your post🤔
Always put links right at the end and each one on a separate line.
I also found this out the hard way!
Try to edit your post. You may find the rest of the text is still in the Edit box.🤞
Then just shift the link right to the end.
Curiously some of the text is visible in the notifications, which is how I twigged what has happened.
Cheers, Doug😎