David Rivers Posted October 10, 2018 Share Posted October 10, 2018 im looking for ideas and suggestions please. I am gurrently fitting out a quarter scale cub and wonder what the best system for the various power supplies is. I need a 4.8v supply for the petrol engine ignition, so thinking a 4 cell NiMh would be good. Can I use a AA size pack or should I go to sub-c. For the receiver/servo battery I am thinking that 2 packs would be good to give some security. What is the best way to connect them and what are the alternatives for battery backer? Power box are good but a bit expensive. Does anyone else make a battery backer? What type of cells do I need. The plane has 7 high torque analogue servos (Hitec HS-645MG) these can run at 4.8 or 6v so I suppose a 2 cell LiFe would be ok. Be grateful for any thoughts. Quote Link to comment Share on other sites More sharing options...
flight1 Posted October 10, 2018 Share Posted October 10, 2018 Hi I believe simplicity is best (KISS) the system i have had no problems with(the only one ) is using this ultra ibec and two batteries to suit rx and servos with two good switches, so either sub C 5 cel or life should be good. and with the nimh 5 cell two packs of same type/ age can be connected together with no problems what so ever( some use diodes), charge separately though. I am certain others will be along with their systems of choice Edited By flight1 on 10/10/2018 18:06:56 Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted October 10, 2018 Share Posted October 10, 2018 Ignition 4.8v only, if not I use 6.6v life, nimhs o.k though, sub C maybe helps with balancing. SM do battery backers, don't know if they still trade but they come up regular on BMFA Would have no probs with one battery myself on a 1/4 scale Cub. 645s will be o.k on a life. Quote Link to comment Share on other sites More sharing options...
cymaz Posted October 10, 2018 Share Posted October 10, 2018 THIS page might give you a few ideas. For safety’s sake, always have a separate battery for Rx and Ignition. Always use an opto isolation switch...Rcexl do an excellent item. Edited By cymaz on 10/10/2018 19:10:52 Quote Link to comment Share on other sites More sharing options...
Peter Christy Posted October 10, 2018 Share Posted October 10, 2018 My experience with my (older) RCexel ignition is that AA NiMhs cannot provide sufficient oomph! The engine will start OK, and seem to run, but will be well down on power. Sub-Cs work fine. (The internal resistance of AAs is too high to allow the capacitor to charge fully between firing strokes.) I believe the later RCexel will handle more than 4.8 volts, in which case a 2-cell LiFe pack would be ideal. Sadly, mine won't, so I have to use Sub-C NiMhs (model needs the ballast, anyway! ) Like Flight1, I'm a great believer in simplicity! The more you put in, the more there is to go wrong! I've known more supposed radio issues eventually tracked back to battery backers, regulators or diode protectors than almost anything else! For the receiver, my advice would be to use a single, adequately large battery pack, but perhaps using two switches in parallel. Switches, being mechanical, are more likely to fail than a good quality battery. Paralleling up battery packs only really works with LiPos, and that means adding a regulator (yuch!). Paralleling NiMhs up won't help if a cell fails low voltage. The bad pack will pull down the good one. If your servos will take 6 volts (or just over), a 2-cell LiFe pack is ideal. I fly a large scale gasser helicopter using two Sub-C packs, one for the engine, one for the radio. The only problem I've had with it is the vibration from the strimmer engine continually shaking the airframe to pieces.....! -- Pete Quote Link to comment Share on other sites More sharing options...
David Rivers Posted October 11, 2018 Author Share Posted October 11, 2018 Thanks for the info. I was looking at the power distribution boxes which look interesting. Hobby king do them as well as 4 max. I soon realised that where there have facility for connecting two batteries, there are just connected directly together in parallel with no circuitry between them. So if one battery fails or shorts etc then it would take the other with it. Has anyone used a diode on the two battery connections so that one does not backfeed the other? The other benefit of the distribution boards seems to be that you take the servo loads away from the receiver. Is this necessary on a 1/4 scale cub with one servo per surface. I wonder what the rating of the supply rails on an 8 channel Spektrum receiver are? Quote Link to comment Share on other sites More sharing options...
Bruce Collinson Posted October 11, 2018 Share Posted October 11, 2018 The Zenoah 20 and RCxl whatever in my big Mentor run on a 1s LiPo as part of a Toni Clarke package which came with the (borrowed) engine. The rx runs on a biggish NiMh. They’re totally separate and have been entirely satisfactory. The DLE 20 in my Seagull Yak has 2 2500 LiFes which are again completely separate and have been fine but with far fewer flights so far (silencer issues). Same ignition system. Isn’t it the case that most modern rxs and servos will run at over 6v, if you delve? Both have electronic kill switches which are cheap and comforting and carry monitor lamps which are helpful. BTC Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted October 11, 2018 Share Posted October 11, 2018 My 32cc petrol model with 7 high power and one regular (on throttle) servos uses a 4 cell sub C Nimh on the ignition with a mechanical and electronic Rexcel switch and a single 2s Life for the receiver and servos with a FET switch (Multiplex safety switch). All my 1/3 and 1/4 scale gliders use single 2s Life or 5 cell Sub C Nimh with FET switches (where a switch is fitted). I do use large receivers on these models with either a high current (MPX 6 pin) connector or dual regular servo plugs (the 12amp safety switches have MPX 6 pin connector to the battery and 2 outputs to the Rx) Quote Link to comment Share on other sites More sharing options...
Paul Marsh Posted October 11, 2018 Share Posted October 11, 2018 I use the HK Failover switch, even on 15cc models, now. Quote Link to comment Share on other sites More sharing options...
Former Member Posted October 11, 2018 Share Posted October 11, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted October 11, 2018 Share Posted October 11, 2018 Power distribution boards with 2x Rx/ servo battery inputs are (usually) designed so that if one battery fails the other takes over, that is certainly the case with the 4Max ones I use. I’m a bit wary of the HK ones as the one that I did buy didn’t seem to work correctly, may just have been a duff one but it’s no longer used. On my larger gassers I use 2x LiPos for the Rx/ servos and NiMh for the ignition on smaller ones I use LiFe with Optipower Ultraguards to give me Rx backup. Quote Link to comment Share on other sites More sharing options...
Fatscoleymo Posted October 11, 2018 Share Posted October 11, 2018 I've been flying a 1/4 scale cub for 6 years, 120 four stroke, single 4.8v nimh 2000ma rx pack. 4 slightly beefy multiplex servos. Single standard switch. Only extra is an onboard glow and battery. My favourite all time model. I know you have the added complication of a petrol ignition but why all the other complications...… as long as you make sure it's charged up...just go fly. It's a Cub, just needs to be helped around the sky.... Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted October 11, 2018 Share Posted October 11, 2018 No complication adding an Ultraguard, but oh boy, what peace of mind! Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 12, 2018 Share Posted October 12, 2018 My Cub is traditionally built (from a Premier kit) powered by an ASP 160 twin with onboard glow and certainly no lightweight at (IIRC) around 18.5 lbs before adding 14 ozs of fuel - and on occasions payloads of 10 - 15 ozs. I know it's "only a Cub" but as this puts it firmly into the >7kg category with considerable kinetic energy even with relatively modest airspeeds, I consider it important to take precautions against easily avoidable loss of control, so have adopted the simple solution (to my mind, potentially the most reliable) of using twin receiver batteries with two separate switches and connections to the receiver. Preflight checking of the individual batteries by the "mag check" method and telemetry monitoring to alert me of any unusual conditions give me confidence that I have a reasonable chance of surviving a power supply problem. Quote Link to comment Share on other sites More sharing options...
Former Member Posted October 12, 2018 Share Posted October 12, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Piers Bowlan Posted October 12, 2018 Share Posted October 12, 2018 You remember correctly Percy regarding SM Services - gone. I haven't heard of Mainlink for years but there website seems active. Aren't they a rail network? Edited By Piers Bowlan on 12/10/2018 08:38:33 Quote Link to comment Share on other sites More sharing options...
Nigel R Posted October 12, 2018 Share Posted October 12, 2018 There is much wisdom in Pete C's approach of parallel switches on a single big pack. You've doubled up on the least reliable link in the chain by doing that. Nimhs are not suitable for directly connecting in parallel, you need a widget to isolate / monitor / switch between the two. Connecting directly in parallel reduces the reliability. "The other benefit of the distribution boards seems to be that you take the servo loads away from the receiver. Is this necessary on a 1/4 scale cub with one servo per surface" The HS645, if one stalls, the current draw is 2 1/2 Amp (stated on hitec website). Outright worst case is thus 7 x 2.5 = 22.5A. Clearly this is a highly unlikely scenario, but there it is. I would consider going with a basic power distribution unit. Have a think about RX lipo capacity and the cabling as well, standard servo cable is 5A (ish) capacity (from memory, might be a bit more or less). Switch capacity also worth thinking about. Edited By Nigel R on 12/10/2018 09:04:45 Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted October 12, 2018 Share Posted October 12, 2018 Bear in mind if you use any form of safety switch that these continue to pull a very small current when in the off position, so if you leave the battery plugged in for any appreciable period it will flatten it. Quote Link to comment Share on other sites More sharing options...
Former Member Posted October 12, 2018 Share Posted October 12, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 12, 2018 Share Posted October 12, 2018 Posted by Frank Skilbeck on 12/10/2018 09:27:46: Bear in mind if you use any form of safety switch that these continue to pull a very small current when in the off position, so if you leave the battery plugged in for any appreciable period it will flatten it. And another reason to heed this advice! Quote Link to comment Share on other sites More sharing options...
Former Member Posted October 12, 2018 Share Posted October 12, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Nigel R Posted October 12, 2018 Share Posted October 12, 2018 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/ Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted October 12, 2018 Share Posted October 12, 2018 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. Quote Link to comment Share on other sites More sharing options...
Rich too Posted October 12, 2018 Share Posted October 12, 2018 2 LiFe batteries, 2 decent switches plugged separately into receiver, and a Tech Aero IBEC - job done! Quote Link to comment Share on other sites More sharing options...
Nigel R Posted October 12, 2018 Share Posted October 12, 2018 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. Quote Link to comment Share on other sites More sharing options...
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