trebor Posted July 30, 2016 Share Posted July 30, 2016 I bought another second hand plane and on checking it over found 2 - 6v 2000mah batteries plugged into the same Rx. Is this normal as I've never seen it done before ? Edited By trebor on 30/07/2016 15:02:51 Quote Link to comment Share on other sites More sharing options...
Former Member Posted July 30, 2016 Share Posted July 30, 2016 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
PatMc Posted July 30, 2016 Share Posted July 30, 2016 Why would it need any diodes, Percy ? It's a simple parallel battery situation. However, what would be a good idea is two independant switches plugged into the two furthest apart sockets of the Rx. As a pre flight check test each switch with the other "off" then switch both on when proved OK. Edited By PatMc on 30/07/2016 17:21:25 Quote Link to comment Share on other sites More sharing options...
trebor Posted July 30, 2016 Author Share Posted July 30, 2016 Thanks Percy, I'll surf over and have a look. The plane is a Seagull Edge but fitted with standard servos. It will be ok for me at the moment as I will not be attempting any 3D manoeuvres. Quote Link to comment Share on other sites More sharing options...
trebor Posted July 30, 2016 Author Share Posted July 30, 2016 Posted by PatMc on 30/07/2016 17:19:59: Why would it need any diodes, Percy ? It's a simple parallel battery situation. However, what would be a good idea is two independant switches plugged into the two furthest apart sockets of the Rx. As a pre flight check test each switch with the other "off" then switch both on when proved OK. Edited By PatMc on 30/07/2016 17:21:25 It's fitted with two separate switches. Quote Link to comment Share on other sites More sharing options...
PatMc Posted July 30, 2016 Share Posted July 30, 2016 Diodes are not necessary, in fact just something else that might go wrong. Quote Link to comment Share on other sites More sharing options...
trebor Posted July 30, 2016 Author Share Posted July 30, 2016 I read on Mike Reeves site that they would stop one battery discharging into the other if a fault happened. Quote Link to comment Share on other sites More sharing options...
Former Member Posted July 30, 2016 Share Posted July 30, 2016 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
PatMc Posted July 30, 2016 Share Posted July 30, 2016 I don't think Mick is an expert in battery management & technology. Quote Link to comment Share on other sites More sharing options...
trebor Posted July 30, 2016 Author Share Posted July 30, 2016 I will have to check out its Futaba Rx details. I normally use spektrum but I had a Futaba tx donated so I might as well use it. Quote Link to comment Share on other sites More sharing options...
Former Member Posted July 30, 2016 Share Posted July 30, 2016 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
PatMc Posted July 30, 2016 Share Posted July 30, 2016 Percy, I fail to see how that qualifies him in this particular area. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted July 30, 2016 Share Posted July 30, 2016 As it happens, Mick's thoughts are exactly the same as mine (and yours, Pat?) - the unreliability you recall Percy, was if you separate the receiver and power bus to the servos with separate batteries - he thoroughly endorses the parallel battery option! I use it on all my larger models. Edited By Martin Harris on 30/07/2016 20:30:31 Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted July 30, 2016 Share Posted July 30, 2016 I've been using twin packs with twin switches and no diode for a number of years. If one pack fails the other takes over. If one pack discharges early it's internal resistance and the other pack's voltage will automatically limit cross-charging to a few milliamps. I'm happy doing it this way and confident that I have resilient systems. By all means stick a diode in each lead if it makes you happy but I personally think that as Pat said, it's just another possible failure point. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted July 30, 2016 Share Posted July 30, 2016 The only time I use diodes is when using A123s with standard servos (to drop the voltage slightly). They do protect against a short circuit in a pack or its wiring but I suspect the likelihood of this is an order of magnitude less than an open circuit at the diode connection. Edited By Martin Harris on 30/07/2016 20:38:58 Quote Link to comment Share on other sites More sharing options...
Shaunie Posted July 30, 2016 Share Posted July 30, 2016 In electronics circles it is generally frowned upon to connect Nimhs or NiCds in parallel. Their low internal resistance and negative voltage characteristic at full charge can cause thermal runaway situations. Check cell manufacturers data sheets as they have warnings about this. So saying, this is really only a problem if cells are paralleled at the pack. I feel that if packs are connected through their own individual switches then the lead resistance will give sufficient isolation between the packs to avoid problems, nevertheless I would not be happy to see two packs permanently connected in parallel. Of course if a cell was to fail short circuit then there would be a very large circulating current that could cause burn damage to the wiring. Diodes would prevent this admittedly very unlikely event causing a problem. So basically, the advice to use diodes was given by someone who read the cell manufacturers data and understood it. Shaunie. Quote Link to comment Share on other sites More sharing options...
PatMc Posted July 30, 2016 Share Posted July 30, 2016 Shaunie, the internal resistance of lipos is much lower than nimhs or nicads & they're much more likely to suffer a thermal runaway, yet they are often used in parallel. Personaly I think the advice to use diodes was given by someone who didn't understand what the manufacturers data meant when applied in a real world situation. Quote Link to comment Share on other sites More sharing options...
Shaunie Posted July 30, 2016 Share Posted July 30, 2016 Sorry PatMc, but LiPo characteristics are very different and of course can be happily connected in parallel. You seem to have a very strong opinion on this subject, I can only suggest you pull up some manufacturer data sheets and take a look. In the interests of this being a friendly forum, I will often back down, in this case I stand by my previous post. Shaunie. Quote Link to comment Share on other sites More sharing options...
steve c Posted July 30, 2016 Share Posted July 30, 2016 I use the two batteries and two switch method on all my planes and have done for many years and thankfully I do as a couple of months ago I had a enolope nimh loose power from one battery (my fault )but the 2nd battery kept it going so if I didn't have two my lovely hangar 9 bf109 would be no more so I will carry on using two batteries steve Quote Link to comment Share on other sites More sharing options...
PatMc Posted July 30, 2016 Share Posted July 30, 2016 Shaunie, at the time nimhs were taking the place of nicads in most applications I downloaded the series of data sheets produced by Panasonic & held the pdf's in my PC for reference. This was partly because I was tired of hearing the much repeated mythology being pumped out, mainly it seemed, originating from one UK battery building & electric power specialising company. The need for diodes in parallel connected nickel batteries was one such piece of "expert" advice. I no longer hold the Panasonic data & have no intention of researching anew something that would have no more than academic interest to me. BTW I do have practical experience of using paralleled nicads & nihms for at least 15 years in various thermal gliders where a single large capacity battery wouldn't fit. Some were directly paralleled a few had twin switches fitted & not all batteries were the same capacity. On a few occasions a battery dropped a cell mid flight leaving the other to cope but never once any fault causing over current between a pair. Finaly, in the friendly forum theme, I think were going to have to agree to disagree on this issue. Quote Link to comment Share on other sites More sharing options...
trebor Posted July 30, 2016 Author Share Posted July 30, 2016 Now what do I do ? Leave it as it is or change it Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted July 30, 2016 Share Posted July 30, 2016 I first started using the parallel method on the (internet) advice of Red Schofield who was the USA service manager for Sanyo batteries. Seems like reasonable credentials and I am fairly sure that I remember him explaining how the normal failure mode of nickel cells made them extremely unlikely to fail in any spectacular manner during use - and in any case, the far more common switch or wiring failure would far outweigh any unlikely cell shorting failure. I would certainly not advise permanent connection/charging in parallel which is a recognised no-no! Separate switches and charging should always be adopted. Edited By Martin Harris on 30/07/2016 23:04:53 Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted July 30, 2016 Share Posted July 30, 2016 Posted by trebor on 30/07/2016 22:44:50: Now what do I do ? Leave it as it is or change it I would happily leave it! Quote Link to comment Share on other sites More sharing options...
Shaunie Posted July 30, 2016 Share Posted July 30, 2016 PatMc, let's agree to disagree then. Maybe things have changed but it was considered industry best practice when I started in electronics almost 40 years ago and I see no real reason to change my view. Remember that failing to follow best practice does not automatically cause disaster, it just means that when boundary conditions occur what you may have got away with you may not get away with. What I can't understand is why you are so dead set against a pair of diodes, if you use Schottkys then the voltage drop is very small, you have little to lose and it may just make a difference. I am talking Rx packs here not massively high current applications like flight packs of yore where any voltage drops were a significant problem. As far as the OP is concerned, separate packs separate switches, use diodes if you wish. Shaunie. Quote Link to comment Share on other sites More sharing options...
Geoff S Posted July 30, 2016 Share Posted July 30, 2016 On one of my bigger models (actually it only has a Zenoah 26cc engine so not big by some standards) I fitted a single sub C NiMH 2000 mAH pack but had 2 switches in parallel. I felt switches were far more likely to fail than a battery pack. I think If I opted to use to battery packs I'd OR them with a pair of Shottky diodes just for my own feeling of confidence although I guess the most common failure mode would be open circuit die to a wiring fault in which case the diodes wouldn't help but they wouldn't harm either. Geoff Quote Link to comment Share on other sites More sharing options...
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