Can I have two Life batteries powering my Futaba receiver?

[Former Member]

[This posting has been removed]

[Rich too]

Cheap switches asking for trouble ( I’ve had them fail), I use these in larger models **LINK**

[Martin McIntosh]

Definitely go with Percy V`s recommendation of an intelligent switch regulator. I use quite a few of the HK Failover switches which are identical and only about 12 quid. Just remember to disconnect the packs after flying or they will discharge into the switch and be destroyed (after a month or two that is).

[Allan Bennett]

Posted by Piers Bowlan on 23/09/2018 11:47:30:

Posted by Engine Doctor on 23/09/2018 11:31:09:

Posted by Allan Bennett on 22/09/2018 21:10:21:

A simple way to prevent two separate power sources interfering with each other is to simply install a Schottky diode (with a suitable amps rating) into each +ve battery lead before they go into the Y-plug or into two separate receiver inputs. Then if one fails in some way, the other will not be able to feed power into the duff one, but will continue to power the receiver. I've got such a system in several of my aircraft -- fixed wing and helis -- using BECs rather than batteries as the power sources. I've also added an LED to each 'channel' to give an indication if one of the power sources has failed.

Thats my preferred method too. I also use a separate switch for each battery fitting the diodes into a "y" lead between the switch and Rx .

OK, so to ask a silly question, what is a 'suitable amp rating"?

Suitable amp rating depends on how many amps you think your system is going to draw. I use a twin MBR1545CT Schottky rated at 15A per section. In my models it's usually fed by 10A BECs rather than by batteries. But whatever your power source, the rating depends on the power consumption of all your servos and any other accessories which might be running off the receiver supply.

[Dave Hess]

Yes, lots of urban myths about connecting batteries. Whilst there are some potential problems, there should be no risk in OP's case provided that he checks that the two batteries are the same voltage when they're put in parallel.

I've been working with electric vehicles (mainly bicycles) for the last 8 years, where we use batteries anywhere between 10S and 20S and typically with anywhere between 5 Ah and 20Ah, Working wit lipos, we'd make something like a 16S 8Ah battery from 8 individual 5000mAh 4S hard-packs - 4 in series and two in parallel or a 12S 20Ah might be a 2S4P pack of 6S soft-pack lipos to make a 12S4P pack.

Most ebike batteries these days use 18650 cells, so a typical 48v one would be 65 cells in 13S5P arrangement. If it were a problem to put cells in parallel, we'd know about it!

here are some examples, so that you can see what I mean:

Edited By Dave Hess on 24/09/2018 12:44:44

[robert chamberlain]

Hello, I am a little confused with the two batt set up. I am OK about the diodes and Y connectors and how it would isolate a bad batt. What I don't understand ( pencil and paper at the ready) is how would I charge the two batts? It would seem to me that I would need to have two charge ports on the side of the fuselage. This would require an extra wire from the charge ports on the outside of the fuselage to each batt in order to charge each one-----right?---what am I missing? Bob C

[Engine Doctor]

Posted by Piers Bowlan on 23/09/2018 11:47:30:

Posted by Engine Doctor on 23/09/2018 11:31:09:

Posted by Allan Bennett on 22/09/2018 21:10:21:

A simple way to prevent two separate power sources interfering with each other is to simply install a Schottky diode (with a suitable amps rating) into each +ve battery lead before they go into the Y-plug or into two separate receiver inputs. Then if one fails in some way, the other will not be able to feed power into the duff one, but will continue to power the receiver. I've got such a system in several of my aircraft -- fixed wing and helis -- using BECs rather than batteries as the power sources. I've also added an LED to each 'channel' to give an indication if one of the power sources has failed.

Thats my preferred method too. I also use a separate switch for each battery fitting the diodes into a "y" lead between the switch and Rx .

OK, so to ask a silly question, what is a 'suitable amp rating"?

I use schottky diodes 1000v 10Amp . You can buy them online 10 for a couple of pounds.

They will lower the voltage of a pack by about 0.25 to 0.5 v but if you use 5cell NiMh or two Life cells there isn't a problem.

[Nigel R]

" This would require an extra wire from the charge ports on the outside of the fuselage to each batt in order to charge each one-----right?"

Correct.

If you're using LiFe packs, you've likely got a balance connector on each pack already - just use that.

Nimhs, you'd need to arrange some other method. Easiest way I can think of it two normal switches each with a charge lead, one on each battery, with the diodes then going between the switches and the rx.

[robert chamberlain]

Won't 2 batteries plugged into 2 receiver ports fight each other if one fails? That is without diodes? Bob C

[Chris Walby]

Bob

My understanding is that RX ports have the positive and negative as common rails so in effect they are all connected together (the respective pins).

Worth asking what you are trying to achieve with dual batteries while remembering the more links in the chain then the greater chance of a failure be it human error or equipment.

What's the failure mode of a battery? Generally open or short circuit

So this is where the possible justification for dual batteries comes in as if one battery goes open circuit then the other takes over, but if one goes short circuit then it just drains the other. However if you put a couple of diodes in then if one battery goes short circuit he other battery does not discharge into it.

Worth pointing out that a short circuit/overload elsewhere (stalled/faulty servo) is not mitigated by this solution.

However there is a important consideration and that is detection by the operator that there is a fault, this can be covered by more stringent test and start up procedures although this is prone to human error.

Some servo loads carry a higher risk when it comes to failure and retracts are one of them, thus it can be prudent to put these on a separate battery or mitigate by other means. Another approach (which I use) is to have a power distribution unit and large UBEC (20A) with the theory that is a servo fails/stalls then the UBEC will just burn the servo out but not drop the RX voltage. This combined with using a large capacity RX battery (needed to get C of G) ensures both simplicity and reliability.

Hope that hepls

[Martin McIntosh]

Further to the above I still prefer to use a HK Failover switch if possible on a two battery set up but have just installed two LiFe 700 packs in a somewhat dispensable old fun fly to be used for testing gear.

I have fitted a four pole changeover toggle switch with two poles in parallel for each pack. With the centre connection to the batts., one way is independent charging and the other to the Rx via a Y lead so that they both feed at the same time. This is not for redundancy but to simply give me a pack of larger capacity since space is limited and four of the servos are digi.

I suppose I could use diodes but never tried them on a model.

[Martin Harris - Moderator]

Posted by robert chamberlain on 03/09/2020 05:30:35:

Won't 2 batteries plugged into 2 receiver ports fight each other if one fails? That is without diodes? Bob C

Rather than fighting each other, I'd describe it more as the good battery will try to give the failed one first aid!

If you have a cell fail on a model using a single 2s LiFe pack, it will give either 3.3v (nominal - maybe 3.4v in practice) or 0v in the case of an open circuit failure which may make your receiver at best unreliable, your servos slow and lacking any meaningful power or result in total failure - either situation will probably cost you the model or worse.

With a 2 pack parallel solution without diode protection, the result will be that some charge will transfer and the output voltage will drop to somewhere a little lower than 6.6v - ideally, you'll have telemetry which should alert you. You now have a good chance of recovering the model intact or discovering the fault when you do the next pre-flight battery check - which should be standard procedure with a dual battery system.

The only significant downside of a 2 pack solution is that statistically, the chance of a failure is doubled but I would rather have a failure and land the model safely!

As the voltage drop/energy transfer is more significant than with a traditional two parallel pack 4 or 5 cell NiXX solution which loses a cell (where I wouldn't bother) I would recommend using silicon diodes - simple, reliable and will reduce voltage stress on any non high voltage servos as a bonus.

Edited By Martin Harris on 04/09/2020 00:46:33

[cymaz]

Just gone down the twin Rx set up for a biplane. It’s a twin switch/ charging socket, twin batteries and the controls are split in half between both RXs

1 Rx has port aileron, starboard elevator, rudder, opts isolator. 2 Rx has stb aileron, port elevator, throttle.

Simple, gives redundancy and cheaper than a power box switch

[Chris Walby]

Cymaz,

Nice install and idea, however I still have a couple of nagging thoughts

One TX, with one battery and set of electronics transmitting

One frequency range - if someone screws around with 2.4 there is no alternative frequency the TX and RX are running on at that time.

But by the far the bigger issue for me is if you fail one RX I personally don't think I could fly a model with the remaining controls. If the RX fails with controls neutral perhaps then just direct it to a crash site, but say a RX battery fails and you have a fair bit of control stick in then I really am not sure.

Anyone flow a model and "landed it" with port aileron, starboard elevator, rudder, or stb aileron, port elevator, throttle?

Still not convinced about duplicating complexity and maintaining reliability. I can see the logic with isolating unreliable components (UC stalling) from critical flight controls, but having two very reliable devices (RX's) sharing critical tasks just increases (IMHO) the chance of failure?

What do the LMA guys do?

[Tim Flyer]

Here is my simple twin battery installation in my Chipmunk. I just use two Futaba HD electronic switches (about £25 each!) which are plugged into separate receiver slots I do not use diodes or any separate electronics . Futaba say this is fine .

Edited By Tim Flyer on 04/09/2020 09:28:12

[Martin McIntosh]

I have seen an F3A model on Frsky dualled with an 833mHz Rx via another gizmo which checks the signals and apparently only lets through good ones, cheap too.

MH, two LiFe packs in parallel are surely little different to a 2P Lipo?

[Andy J]

interested in this thread as tried to establish in the past what is the ideal set up within a larger model. Regarding the need to have dual batteries doubt this is actually required. Yes a battery could fail in flight but the most likely cause of brown out is incorrect capacity of a battery being selected or failure to charge the battery prior to use.

Therefore came to the conclusion that the ideal set up was a single large mah rated battery, two switches wired in parallel with the switch output wiring split to feed individually 2 separate Rxs.

[Engine Doctor]

Posted by Andy Joyce on 04/09/2020 11:03:00:

interested in this thread as tried to establish in the past what is the ideal set up within a larger model. Regarding the need to have dual batteries doubt this is actually required. Yes a battery could fail in flight but the most likely cause of brown out is incorrect capacity of a battery being selected or failure to charge the battery prior to use.

Therefore came to the conclusion that the ideal set up was a single large mah rated battery, two switches wired in parallel with the switch output wiring split to feed individually 2 separate Rxs.

Everyone to their own ;however, When you have experienced an expensive or large model suddenly not responding and subsequently find out when sifting through the bits that its a cell that has failed in effect totally switching off the power , not even allowing a fail-safe to work, you suddenly realise that a second battery and dual switches is a good idea . The diodes give added protection should one of these batteries develop a defective cell from discharging the good battery . Similarly dual switch harnesses give redundancy should one fail

We are not talking about brown out that's a totally different thing but battery / cell failure . Dual batteries with the increased capacity will also protect from brown out .

Also testing the flight batteries under load is a good habit to get into as batteries can show fully charged only to completely collapse when put under load .

Cell failure is comparatively rare however it does happen so why go to the trouble of fitting dual Rxs and switches only to use one battery?.

 

Edited By Engine Doctor on 04/09/2020 12:31:23

[stu knowles]

I am another long time user of two batteries into two switches into both single and twin RX equipped models.

I accept that in the case of a dead short both batteries will be discharged but in all the time that I have been flying I have never come across this situation. I don't use diodes to minimise the number of connections/ joints

I have encountered batteries running out of amps when they were thought to be good / charged many times.

My belief, and your mileage may vary on this, is that if one battery was fully charged and the other flat, then the flat battery does NOT leach power out of the good one. I believe this to be an internet myth but each must decide for themselves. I use both NmH 'C' cells and LiFe batteries.

I won't use AA size RX batteries in anything with more than three channels

I am reliably informed that people in LaMAland did trials on several models and found that they were perfectly control-able using just one half of an elevator and just one aileron.

My most complex model has two batteries, each with two output leads, four switches with one switched lead from each battery going to each of the two RXs.

(stands back and waits for his B certificate to be publicly shredded as he is excommunicated from the BMFA)

[Martin Harris - Moderator]

Posted by Martin McIntosh on 04/09/2020 09:55:58:

MH, two LiFe packs in parallel are surely little different to a 2P Lipo?

Not much but who mentioned LiPos? Certainly not me as I would not use them for receiver packs as the voltage is too high IMHO for standard servos and there are issues in charging in situ. I dislike the idea of adding further potential failure points by using electronic voltage regulators.

Chris - not exactly the same situation as a failed split receiver system but I once had an aileron servo unplug itself during a roll - after applying the corrective aileron to stop the roll my first thought was that I'd lost radio as the model was way out of trim in pitch as well as roll due to the aileron deflection and I struggled to attain level or controlled flight for a few moments until I found that with opposite aileron held in plus appropriate elevator the model was controllable enough to land without damage - which is how I was able to confirm the cause.

[cymaz]

Posted by Chris Walby on 04/09/2020 08:23:24:

Cymaz,

Nice install and idea, however I still have a couple of nagging thoughts

One TX, with one battery and set of electronics transmitting

One frequency range - if someone screws around with 2.4 there is no alternative frequency the TX and RX are running on at that time.

But by the far the bigger issue for me is if you fail one RX I personally don't think I could fly a model with the remaining controls. If the RX fails with controls neutral perhaps then just direct it to a crash site, but say a RX battery fails and you have a fair bit of control stick in then I really am not sure.

Anyone flow a model and "landed it" with port aileron, starboard elevator, rudder, or stb aileron, port elevator, throttle?

Still not convinced about duplicating complexity and maintaining reliability. I can see the logic with isolating unreliable components (UC stalling) from critical flight controls, but having two very reliable devices (RX's) sharing critical tasks just increases (IMHO) the chance of failure?

What do the LMA guys do?

That’s exactly what the LMA guys do

[Martin McIntosh]

Martin, the 2P Lipo was just as a comparison since many are sold as such. I would not dream of using a Lipo on a Rx or Tx, electric only, because I have had a large number of failures with those.

I said above that a 2.4 coupled with an 833 via a logic decision device would seem the best bet but I should have said 868mHz which has far greater range than 2.4 but is not well publicised.

Test flew my simple set up today using two LiFe`s via a four pole switch and so far it works fine. 6ch Orange Rx and digi servos.

[Martin Harris - Moderator]

You've lost me a bit with the 2P LiPo - I'm not aware that they are generally available but I wouldn't have looked into their availibility anyway! Are they sold as an off the shelf dual pack for redundancy?

Jeti offer exactly such a solution as your suggestion with the DS24 which has a third transmitter module working on the nominal 900 MHz band (according to country) which kicks in if both 2.4 GHz links drop out. No doubt FRSky either has a similar solution or will soon offer it.

Edited By Martin Harris on 06/09/2020 17:15:29

[Martin McIntosh]

Martin, a battery can have say 2 cells in series, i.e. (2S) 7.4V with another similar in parallel (2P) to double the capacity and the available current. The cells will immediately balance the voltage between each other if there is a small difference.

[Martin Harris - Moderator]

I'm fully aware of how batteries can be configured - my most used LiPo packs are quite venerable 4000 mAh 4S2P Rhinos - but now that cells are generally available in larger capacities they are something of an oddity. What I was struggling with was your observation " MH, two LiFe packs in parallel are surely little different to a 2P Lipo? " and how it applied to any comment I'd made.

Perhaps there's some confusion. I think we hold similar views on the use of parallelled packs?

I've always understood that the measured voltage would be somewhere between the high/low packs but unless there was a reasonable difference in voltage (charge level) little or no energy was transferred between packs.  However, in use the higher voltage pack will supply the energy drawn until the voltages equalise when both packs will contribute equally.  This is the basis on which unequal capacity packs can be used to their combined capacities.  Connecting balance connections (such as in my Rhino packs) will allow balancing to extend to parallelled cells within the packs during charging.

Edited By Martin Harris on 06/09/2020 23:46:02