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Receiver/Servo battery supplies

What’s the best system for a 1/4 scale cub?

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Percy Verance12/10/2018 10:34:41
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6993 forum posts
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There is in fact a warning in the (Multiplex) instructions regarding that Frank. Although not everyone reads the instructions do they?

Edited By Percy Verance on 12/10/2018 10:44:49

Nigel R12/10/2018 11:39:02
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1730 forum posts
350 photos

Percy - as ever "it depends" and "you answer may be more or less conservative than mine".

26 gauge standard cables are 0.04Ohm per foot (standard copper resistance).

How long is the battery cable? Maybe a foot between battery and RX. This is the worst case section, the heaviest loaded piece of cable. The round trip to that point is therefore two feet, or 0.08Ohm total.

At 5A that gives a volt drop of 0.4V. You call on whether that is significant. I believe most any RX can cope with a 1/2V drop without issue.

The cable also has to dissipate a couple of W of heat. At 5A, things are getting a bit toasty for some PVC insulation types. Silicon types can cope with 7A in this size cable, in free air (as our installations are).

Or to sum up -

Real bog standard PVC standard extension cable and switches I would say, yes, 3A tops to stay cool.

Good quality standard size silicon extension cable will deal with 5A quite happily.

If you jump to the HD cabling, 22gauge, the numbers are 8A (PVC) and 13A (silicon).

Most of the time we are operating our kit so far away from that worst case we are plenty safe. And the 'worst case' loadings are very contrived - all surfaces completely stalled, simultaneously.

sources

https://www.powerstream.com/Wire_Size.htm

https://www.multicable.com/resources/reference-data/current-carrying-capacity-of-copper-conductors/

Frank Skilbeck12/10/2018 11:53:39
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4104 forum posts
97 photos

Just for confirmation the MPX 6amp FET switch comes with a single servo lead and the 12 amp has 2, plus the 12 amp switch has a MPX 6 pin connector for the battery.

Worst case load might be a faulty servo not a stalled one, we had a model where the Rx cut out every time we connected it to the ESC BEC, turned out to be a faulty servo that was drawing a high current overloading the BEC, on a regular battery the servo appeared to work OK.

Rich too12/10/2018 12:19:32
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2791 forum posts
1020 photos

2 LiFe batteries, 2 decent switches plugged separately into receiver, and a Tech Aero IBEC - job done!

Nigel R12/10/2018 12:53:07
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1730 forum posts
350 photos

Rich, am I right in thinking that Tech Aero BEC generates a CDI power output from the RX's aux channel? Which allows the aux to be set up as a kill switch?

I guess you use some other mechanism to feed the two batteries fed into the RX (whatever that is) - seems quite a neat answer.

flight112/10/2018 14:54:39
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533 forum posts
32 photos
Posted by Nigel R on 12/10/2018 12:53:07:

Rich, am I right in thinking that Tech Aero BEC generates a CDI power output from the RX's aux channel? Which allows the aux to be set up as a kill switch?

I guess you use some other mechanism to feed the two batteries fed into the RX (whatever that is) - seems quite a neat answer.

You just plug the two batteries into the rx you need no more complications and as for the device from tec areo the one to use is the ultra ibec 4 stage filtering and voltage regulator to suit ignition type this devise also turns on or off your ignition via your RX.

Don't confuse the ultra ibec with the other items/regulator+switches they make

Edited By flight1 on 12/10/2018 14:55:37

David Rivers12/10/2018 17:55:40
33 forum posts

Nigel

When you say that NiHm are not suitable to parallel and that some circuitry is required between then. Would you consider a diode connected to each of the packs sufficient? The packs would still be in parallel but if one failed or shorted etc then the other battery would not be affected due to the blocking diode. I have looked at the power boards sold by 4max and they do just put two batteries in parallel. In the write up they say that the batteries should be the same type and voltage and equally charged etc. I think the more expensive power box systems one are a bit cleverer though.

Nigel R12/10/2018 18:53:36
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1730 forum posts
350 photos
David yes a diode is sufficient.

Nimh have a failure mode where parallel connecting can result in a positive feedback. It is not recommended practice by cell manufacturers.

Lipos are fine to connect in parallel. However that does not afford much protection against failures except for a cell going open circuit or a break in wiring.
Rich too13/10/2018 05:11:09
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1020 photos
Posted by Nigel R on 12/10/2018 12:53:07:

Rich, am I right in thinking that Tech Aero BEC generates a CDI power output from the RX's aux channel? Which allows the aux to be set up as a kill switch?

I guess you use some other mechanism to feed the two batteries fed into the RX (whatever that is) - seems quite a neat answer.

Yes, as flight1 confirmed. It is a simple and reliable system. 👍

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