Redundancy in larger models?

[cymaz]

It really depends on how seriously and how far you want to take the safe flying of the model and the safety of those around you. I had a couple of LMA lads come down and inspect a plane that was not over the 20kg limit....turns out it was 18.5 kg.

I adopted all their recommendations without question.....they know best when it comes to lager models. The Powerbox system I have  is brilliant......not cheap. But some don't mind spending several £100's on a a model, the same on an engine, a bit more on the radio systems then stops 99% from the end by not adding redundancy, twin elevator servos, battery back up etc.

Edited By cymaz on 10/06/2017 19:42:55

[Martin McIntosh]

jrman,

I beg to differ here because I have suffered this twice.

[Martin Harris - Moderator]

Posted by Martin McIntosh on 10/06/2017 11:00:58:

Ace,

When a cell fails it almost invariably goes s/c, not open, so you would end up with four good cells giving a high enough voltage for 2.4 to operate. If one fails on a four cell pack the voltage will drop below the critical point and a failsafe should occur.

I thought that the large manufacturer who's receivers shut down for a long reboot at relatively high voltages had corrected this problem? Are there many systems that won't cope with voltages around 3.4 - 3.6 from a well discharged 3 cell pack? In the case of my system of choice, the receiver is designed to run between 3.2V and 8.4V and I've tested down to around 3V at which points the servos were the limiting factor...the receiver never lost connection.

My own experience of losing a cell on a 4 cell pack was that there were no noticeable effects on controllability and it was the telemetry that alerted me to the problem. Admittedly, the pack was fully charged so the pack voltage was still around 3.9V if hazy memory serves me correctly.

[Nigel R]

Just caught up with this thread, some great points, but worth highlighting:

"An internally mounted switch in a soft mount, with, say a piece of fishing line through it to turn it on and off, would go a long way to improving its reliability."

Mechanical elements are (in my day job experience) the highest chance of failure. Electrical failure is usually a much lesser problem. Anything we can do to reduce the mechanical sources is of higher value than the electrical fixes.

[Frank Skilbeck]

I've used these on some models (with a single battery), at the weekend I saw a twin battery installation with these switches on each battery, because they are a FET type switch do they also act as a diode stopping reverse current to the "faulty" battery or would diodes still be needed.

[Denis Watkins]

A FET is polarised Frank, so electronically, a reverse current could not pass by it

But in an high impact event, it would not prevent busted battery pack wires causing problems.

In normal circumstances a FET switch is a good choice, but another " but"

The FET needs power to operate

But so does the rx

[Frank Skilbeck]

Dennis thanks, not too worried about what happens in a high impact event, but avoid one Seems like two FET type switches are a good option for dual battery installations.

[Gary Manuel]

Careful, because receiver batteries will discharge to the point of destruction if left connected via FETs for long periods of time.

Edited By Gary Manuel on 12/06/2017 20:24:43

[MattyB]

Posted by Martin McIntosh on 10/06/2017 23:28:35:

jrman,

I beg to differ here because I have suffered this twice.

How on earth can an unpowered RX drive aervos to the pre-set failsafe position? Sorry, that's impossible; no RX or servos I have ever seen work without power!

In a power failure the servos will freeze in their last known position; the only variant on this is a "graceful" battery failure where the pack is nearly but not quite fully flat. In that situation the moving servos depress the batt voltage, the RX turns off, once off load the batt recovers slightly and the RX reboots and control may be regained til the next major servo movement at which point the cycle restarts. You still won't see an RX failsafe in this situation though.

Edited By MattyB on 12/06/2017 22:46:44

[Martin Harris - Moderator]

Perhaps Martin was using a Futaba PCM rx with low voltage fail safe - IIRC that closed the throttle when the pack voltage dropped below a preset level...

[Martin McIntosh]

No, I use JR DSM2 which does not like less than 3.7V for the rf link but certainly failsafes. It is true that the servos operating drag it in and out of the failsafe mode, hence I now set it with all surfaces neutral and u/c up so that generally only the throttle servo will operate.

[jrman]

Well said MattyB. I've never seen a Rx or servos that operate when the battery fails. No electricity means nothing works!

[The Wright Stuff]

For the sake of sanity, I hope some of these posts were crossed. Surely it depends on the nature of the battery failure? A fail safe can kick in if a critical voltage is set, but not if a total power failure occurs (broken wire, switch failure, open circuit cell failure, etc.)

[Former Member]

[This posting has been removed]

[Rich too]

Funnily enough I have just had a switch failure on a new model. I just use two batteries and two hd slide switches, plugged directly into the receiver. After a recent couple of hours flying, I went to recharge the batteries today, and discovered one of the batteries still fully charged! I tried the switch and the ignition light came on and went immediately off. Second time it stayed on. Now, when I use the switch it will do the same thing, and the ignition light only stays on after the second time - very odd.

Anyway, this at least proves that at even simple redundancy works well. I now cannot trust the switches in the model and think I will buy a Boomarc pin flag switch.

[Martin McIntosh]

Posted this elsewhere but it may be relevant. On a JR RD921 Rx which has two power inputs I have found that one battery needs to be disconnected from the system when recharging otherwise the chargers show one pack full and the other not, despite them having been discharged equally.

[Martin Harris - Moderator]

It's always been the case that you shouldn't charge NiXX cells in parallel - one of the pre-requisites for dual battery working is separate charging arrangements...

[Martin McIntosh]

Not intentionally charging in parallel, the point is that with the switches off there should be no path to both packs other than maybe the negative wires. Beats me.

[Martin Harris - Moderator]

Delta peak chargers? I've never been able to fathom quite why but I've seen advice that you can't use them with the negative wires commoned and have experienced problems myself with false peak detection when I've tried charging two packs simultaneously. Possibly might be related to using a common power supply?

[Nigel R]

The battery negative wire on the charger is not necessarily a ground. There's a fair chance that some of the peak detect circuit is snuck in between that negative connection and a nominal ground.

Hence, a bad idea to have a common connection between two batteries, when connecting them to two separate chargers. At the very least it will affect the sensitive peak detect circuitry. At worst, it might confuse the charger enough to overcook a battery (or conceivably the charger).