Battery backer

[Jolly Roger]

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

[FrankS]

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.

[Barry_Cole]

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

[Peter Balcombe]

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

[Barry_Cole]

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

[Jolly Roger]

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

[Barry_Cole]

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

[Jolly Roger]

[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?

[chris williams]

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

[RobbieB]

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]