Scale Soaring UK

Hi folks,

I'm switching MPX to Frsky but see that unlike my MPX receivers, there is no facility for dual battery inputs direct into Frsky receivers. I guess Frsky are steering us towards their redundancy buses (RB10 and RB20) which not only provide battery redundancy but also receiver redundancy. These boxes look fab, but I'm also aware that it's easy to get sucked into ever-increasing complexity in the pursuit of bullet-proof redundancy, buying shiny box after shiny box, and each time adding something else that could fail... not to mention cost and weight (oh, I DID mention them... ).

So my approach is to fit redundancy buses with 2 x rx and 2 x batteries only in my big/pricey planes, and for the rest I'll use one receiver plus a double battery power supply (the logic being that in 40 years flying I've lost 3 models to failed batteries but none due to failed rx).

So my question is... how do you guys connect 2 batteries to one receiver?

I fancy the Jeti Max Bec 2D Plus https://www.electricwingman.com/jeti-ma ... lator.aspx which has the benefit that you can use 2s Lipo and regulate down to 6v (so you can run low voltage servos). I know Ell has had great service from his, but I'm wondering if there's a cheaper option.

At the super-cheap end, I could just spend a few pence and solder a diode in the +ve lead of each battery and plug both directly into the receiver, as shown by Robbie in another thread. I'll not be able to use Lipo batteries so will need to stick with NimH, but I'm more relaxed about the battery type if I know I've got 2 to back each other up.

And then in between I've seen some Hobbyking devices that claim to do most of what the £80 Jeti does... but for only £9 you have to wonder how reliable they are, and reliability is critical, especially in the one device that has both batteries running through it.

So yes, what do YOU do? I'd love to hear your opinions/experiences.

Rog

Rog, your other option would be to use LiFe batteries with your diode system, a 2s LiFe battery is 7.2v fully charged but quickly drops to 6.6v (and then stays there), so very similar to a 5 cell Nimh. I am using the LiFe cells in a few of my models, but I use a single battery with an electronic switch, but largest model is a 1/3rd scale vintage glider.

Only Rx battery failure has been a 600 mah Nicd which i used 2 days running and it went flat........................ But I have had mechanical switches go faulty.

FrankS wrote: ↑04 Jun 2019, 13:27 Rog, your other option would be to use LiFe batteries with your diode system, a 2s LiFe battery is 7.2v fully charged but quickly drops to 6.6v (and then stays there), so very similar to a 5 cell Nimh.

That works for me too...

BC

Roger,
As I recall, the important things in maximising equipment reliability are to keep things simple, avoid potential single points of failure & use high reliability parts for each link in each potential fault chain.
Connectors (including switches) tend to be a reliability weak spot - so avoid/minimise.
Electronic components can be very reliable (if used well within limits) but sometimes are limited by connection reliability & modern stuff can have latent firmware related faults.

There are swings & roundabouts in that a dual battery into a single receiver makes the Rx a single point of failure, whereas using the RB10 unit for instance allows the use of 2 batteries & receivers, but then the RB10 becomes a potential single point of failure. It all comes down to potential failure modes and probabilities, coupled with individual component reliability figures.

The RB10/20 units give you feedback of voltages etc. and should cope with a faulty servo, whereas a dual battery/diode setup is far simpler & has less connections. The Rx telemetry can only tell you the Rx supply voltage (not individual battery voltage) & a faulty servo can still pull the batteries down.

Personally, I would tend to favour the twin diode approach (using conservatively rated diodes), but this does place more owness on you to make sure both batteries are charged at the start - otherwise you won’t know there is a problem till the good one runs out!
Hope this helps

I use one of these to monitor a pair of batteries:-

https://www.t9hobbysport.com/frsky-s.po ... -adc-ports

It obviously has other uses, but there are two analogue to Digital inputs that accept 0-3.3 volts. So with a couple of simple voltage dividers made from a couple of resistors, you can monitor both batteries in real time.

Happy days. FrSky rules..

BC

I can't thank you folk enough for the comments. Each one has helped clarify things.

So...

Big/pricey planes will get RB10/20 buses with 2 batteries and 2 receivers.

I fully agree with Peter's advice to keep it simple and avoid single components that could cause catastrophic failure, but the Frsky RB's have a great track record in demanding applications, so on balance I'm trusting them.

For my other planes I'll entrust the RF link to one Frsky (X8R) receiver, supplied by 2 batteries, each with a diode fitted so they can back-up each other. The logic here is that it avoids the single-point failure risk of a battery-backer (plus extra connections). Also, I think the risk of 2 simple diodes failing at the same time is tiny compared with the risk of a fault in a relatively complex battery-backer.

Finally, I'll buy some LiFe 2s packs as they're reported to be more robust than LiPos, more reliable than NimH, and best of all, I can run them straight into my receiver (through their diodes) without regulators - saving more connections and components.

Hope this makes sense but please shout if you can see an obvious flaw.

Rog

Roger,
You need to use these:- https://www.t9hobbysport.com/frsky-rx8r-pro-receiver
With the RB-20 to get full functionality.

For the little extra cost, I would standardize on them.



BC

[quote=Barry_Cole post_id=23194 time=1559675595 user_id=51]
Roger,
You need to use these:- https://www.t9hobbysport.com/frsky-rx8r-pro-receiver
With the RB-20 to get full functionality.

For the little extra cost, I would standardize on them.



BC
[/quote]

Thanks Barry. Yes I looked at the RX8R but couldn't see what advantages it offers in terms of redundancy. Could you explain please, speaking really slowly?

Don't worry, Rog, he does everything slowly...

A couple of other points Rog,

Where you intend using the simple diode system (by far the simplest and the best in my view), you will need to consider how you are going to charge the two battery packs - in the model or out as that will determine where you place the diodes in the circuit and what type of switches you need. The attached diagrams should explain it all if you haven't seen them already. If you intend taking them out to charge it doesn't really matter.

Also, as Peter mentioned, select good high power diodes, at least 5 ampers and that would be for standard servos. If you have a flock of digies then 8 would probably be better - they will warm up under load.

.................and finally, be aware that there will be a voltage drop across them, up to .8/.9V under full load but with five cell nimh, life or pollies that won't matter.

Good luck.

[attachment=1]battery_backer.pdf[/attachment]
[attachment=0]switch.pdf[/attachment]

I agree with all he suggests but Robbie's two .pdf files are in conflict:
"switch.pdf" shows both the Pos and Neg lines being switched. In reliability terms this doubles the probability of a failure in that supply line - power loss occurs if either one of the switch contacts fail.
In "battery backer.pdf" file, the double pole switch contacts are shown in parallel in the Pos line (GOOD). This hugely reduces the probability of failure as both sets of contacts have to fail simultaneously (ProbFail therefore classified as "extremely remote") to cause a loss of power in that line.

Roger,
Two reasons for using the RX8R, it is fitted with the latest series of RF chips and is stated as giving up to 40% more range than the X8R (Although this seems to have more than enough range). Also if you are going to use the RB-20, the RX8R supports S-Port switching which means that whichever Rx is being used, all the telemetry will be sent back. If you use the X8R you have to bind one of them with telemetry off.

Also if you are using a single Rx you can plug a second receiver like the RX4R into the S-Bus IN port as a backup.

Almost all of the receivers have all 16 channels, available. If you need a few more, you can plug a 4 channel decoder into the S-Bus out port.

Hope that helps. I sent you an e-mail with my phone number if you want a chat.

BC

[quote="spike spencer" post_id=23199 time=1559717232 user_id=74]
I agree with all he suggests but Robbie's two .pdf files are in conflict:..........'
[/quote]

Yes I suppose so but that configuration was for a flying buddy that never removed his batteries but suffered persistent black wire blues so isolating the negative feed at the switch helped limit the damage - a little. I personally do it as shown in the first diagram.

You could always re-introduce that higher level of redundancy if you wanted to switch both poles by using a 4 pole double throw switch.

It's also worth mentioning that switches well within the required current rating are chosen and they are the very best quality too - silver flashed/make and wipe contacts preferably.

More great ideas - thanks again.

As for diodes, this looks like it would be well within itself?...

https://uk.farnell.com/multicomp/p600d/ ... KU-9564985#

And switches...a confession: I stopped using them 10 years ago. I just carefully plug the battery directly into the rx when I'm ready to fly. Yes it's not as convenient as a switch, but it cuts out 2 connectors and one component so reduces risk. However, if I'm fitting diodes I'll need the extra connectors anyway between battery and rx so may go back to switches.

Can anyone suggest some decent quality switches at a reasonable price?

Barry - thanks for email - yes RX8R look good on balance.

Rog

Roger,
If you prefer not to have switches you could do this:-

That would do it nicely Barry. Although I've got to admit I was expecting (a) someone to persuade me switches are a good thing, and (b) where to buy reliable switches cheaply.

Roger,
Unfortunately, switches are effectively just a moveable connector, where the connection is operated by a lever or slider - so suffer from similar reliability issues to connectors.

As has already been mentioned, it’s best to:
a) use contacts rated conservatively for the expected maximum current,
b) parallel up multiple sets of contacts (to reduce impact of one making a poor contact),
c) use wiping contacts if possible (to self clean as contact is made),
d) use good quality contacts (gold plated are best to reduce tarnishing impact).

[quote="Peter Balcombe" post_id=23214 time=1559771448 user_id=165]
Roger,
Unfortunately, switches are effectively just a moveable connector, where the connection is operated by a lever or slider - so suffer from similar reliability issues to connectors.

As has already been mentioned, it’s best to:
a) use contacts rated conservatively for the expected maximum current,
b) parallel up multiple sets of contacts (to reduce impact of one making a poor contact),
c) use wiping contacts if possible (to self clean as contact is made),
d) use good quality contacts (gold plated are best to reduce tarnishing impact).
[/quote]

Thanks Peter. Could you possibly give a link to a suggested type?

Hi Rog,
Have you ever thought about magnetic switches? Just to throw another idea out there....
Mel and I have used Jeti - https://www.esoaringgadgets.co.uk/index ... ath=46_108
and
T9's Hall switches - https://www.t9hobbysport.com/search?s=Switch
to good effect, on our larger models, and they have no moving contacts to wear and fail, and with the Jeti main switches you can control both high voltage motor and receiver/servo circuits at the same time with a ESC with a energising switch function, expensive yes, but they work well for me, but just call me "a Jeti fan girl!..." sorry for crashing the party ...
Antonia

Thanks Antonia - a good alternative, albeit at a price as you say.

Roger, unfortunately I don’t have any specific component in mind as I have not used anything heavier than the heavy duty charge point slide switches in models I have built so far.

It depends how many servos you are expecting to cope with, but you are probably looking at reliably switching up to 8-10A to be safe with a large model with lots of high power servos.
There are some heavy duty double pole slide switches out there, but I think most tend to be rated at <5A, so you need to check the specifications for each carefully.
Miniature toggle switches are also usually limited to 6A @ 28V, so you are probably looking at standard size toggle switches to get 8A or more. Toggle switches also tend to have silver or gold plated contacts which is better than plain brass.

There are so many different switches out there, so you have to be able to look at the specifications to see what each are capable of. Unfortunately, many modelling suppliers just sell “switches” or “heavy duty switches” without identifying actual current ratings. You often tend to have to go to an electronic component supplier to get specifications. Note that you need to look at the low volts DC rating, not the 240v AC rating.

Thanks Peter and Antonia.

Those switches look nice but as my main aim is reliability I think I'll stick with directly plugging the batteries into the receiver (via diodes). This minimises connections and component count.

It is a bit fiddlier than switches every time I want to fly, but the first models I'm converting to 2.4 are all large with accessible receivers - even my big hands fit.

When I get round to my smaller models where space is tight, I may try the T9 switches Antonia mentioned - they have a nice battery backer function too.

Thanks again.
Rog

[quote="Jolly Roger" post_id=23245 time=1560001766 user_id=497]
Thanks Peter and Antonia.

Those switches look nice but as my main aim is reliability I think I'll stick with directly plugging the batteries into the receiver (via diodes). This minimises connections and component count.

It is a bit fiddlier than switches every time I want to fly, but the first models I'm converting to 2.4 are all large with accessible receivers - even my big hands fit.

When I get round to my smaller models where space is tight, I may try the T9 switches Antonia mentioned - they have a nice battery backer function too.

Thanks again.
Rog
[/quote]

On bigger models i do away with switches now, and just solder a multiplex greenie to the battery. Remove the greenie to disconnect and no wear on the RX or battery connections. If you then use diodes you can also charge by building a standard charge lead you make up to plug into you greenie so bypassing the diodes. With a two batt system has the advantage the battery and rx connection is fully redundant to the RX. This is not the case with the jeti bec 2, with 1 lead into RX. Obviosuly you need a spare input into the RX. I use two LiIons that a wired using a standard 4S balance lead and i can charge them and balance as a 4s pack from one input to the battery charger.