JR RD921 rx advisory.

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Roy Hill 1855
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JR RD921 rx advisory.

Postby Roy Hill 1855 » December 20th, 2008, 12:22 pm

The rd921 rx has dual battery ports, left hand port is labelled---"batt 1/bind. right hand port is labelled batt 2/data, please note,the positive pins of both batt 1 and batt2 ports are connected together, as are the negative pins,therefore the use of 2 batteries(without diodes/batt backer systems incorporated)should be discouraged, as in extreme circumstances one batt could discharge to the other through the rx tracks with possible disasterous results. The 9 channel spektrum rx is virtually identical(but red coloured) however the ports are labelled batt/bind and the other ---data . Please understand that we have been using 2switch harnesses off 1 battery to two separate rx outlets (as per "docs" suggestions) for years, but not two separate batteries without reverse flow protection of some kind. Hoping this info is of help in safeguarding your rigs ,regards, Roy ;) ps, at nowhere in any instructions or publications is this made clear :roll: pps,your comments are eagerly awaited, but dont shoot the o.a.p., im, dooin me best aer kid!!

Rob Cavell 529
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Re: JR RD921 rx advisory.

Postby Rob Cavell 529 » January 10th, 2009, 9:27 pm

Hi Roy, interesting post!
This is not a case of shooting the messenger but simply a bit of a hobby horse of mine, and what do I know?

I just wonder if the JR RD921 receiver manufacturer in question knows something the LMA have turned their back on for donkey’s years!
I refer to the practice of paralleling NiCad’s or Nmh’s which is generally frowned upon by LMA and it's extremely doubtful that any r/c system which included it without a diode system would gain approval by any inspector for the simple reason, if no other, that it has become written into folklore so to speak.

Some years ago after reading ‘Red’ Scholefield's Battery Clinic I wrote a couple of times to the journal for someone suitably qualified to answer Red’s statement and claim that there is little danger in parallel use without diodes “providing the two packs deployed are of equal number of cells, but can be of widely different capacity”.
My letters received a stony silence!

From memory "Red" Scholefield (I haven’t recapped all his history) is an American ex GEC engineer who was engaged for that company in NiCad research.
His web address where Ni-cad advice can be found is: http://www.rcbatteryclinic.com/

Here is a direct excerpt from his article on this subject, (quote):
Many pseudo battery "experts" put forth the argument that plugging two battery packs into the same receiver without blocking diodes is NOT a good thing, claiming that his creates a host of problems and the two packs will end up fighting each other or "cross charging".
These concerns show a lack in the understanding of the charge and discharge potentials involved in Ni-Cd cells. One pack cannot charge another (equal number of cells) as the discharge voltage of a pack can never be as high as the voltage required to charge the other pack. For the doubters here is an experiment: completely discharged one pack to 4.0 volts and then connected to a fully charged pack having an equal number of cells. There will be less than a 10% transfer of charge in a 24 hour period. Since shorts rarely occur in fully charged packs the risk of one pack "dumping" into one with a shorted cell are insignificant. A simple ESE pre-flight test would detect a pack with a shorted cell.
While it is a fact that the typical failure mode of a battery is for a cell to fail shorted there are some subtleties here that escape many people. First, one of the major causes of "battery" failure has nothing to do with the batteries themselves but rather with a switch or connector in the battery circuit. The dual redundancy concept is to protect against the failure having the highest probability - that being the circuit path from the battery to the power buss in the receiver. Adding more components to this path, like regulators and/or diodes isn't going to help the matter but rather adds to the probability of failure, (unquote).

From a personal point of view it’s all academic anyway, in under 20kg models I have never felt the need for the use of two receiver packs, I simply use one of about double the capacity which is appropriate for the specific current draw.
However I never fit and forget my Rx packs but always monitor them noting the auto charge cycle characteristics, any change in charge time and or premature cutting out tells you that something needs investigating, I hardly ever slow charge for that reason.

Footnote:
In the fullness of ‘age related wisdom’ I think I know the reason why LMA ‘ban’ parallel battery use without diodes in over 20kg models but if Mr Scholefield, and the manufacture of the JR RD921 receiver is right, then I would prefer it not to have been or continue to be labelled an unsafe practice for models in general.
I would go further and raise the question that had the use of parallel batteries not been generally discouraged they may well have saved more crashes than the folklore tells us they would have “caused”.

Roy Hill 1855
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Re: JR RD921 rx advisory.

Postby Roy Hill 1855 » January 15th, 2009, 7:34 pm

Hello Rob, apologies for lateness of reply,I have just completed a test using 2 1800mah 4.8 volt sub c nicads, 1was fully charged(5.2volts)the other was discharged to 4.4 volts, when connected in parallel with a ammeter in circuit the initial current draw between the two packs was 500ma dropping quickly to 30ma,while this is a small amount, it does indicate cross charging takeing place,however, from the safety viewpoint the use of suitable! diodes protects both packs from each other and provides the battery backer system recomended(5cell 6volt packs +diodes). IT also ensures that each pack is charged se parately,whereas to try to charge 2 packs in parallel,1may delta peak early, leaving the other one only part charged :?:
remember folks,safety is paramount
Thanks again Rob for your input----regards Roy ;)

Roy Hill 1855
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Re: JR RD921 rx advisory.

Postby Roy Hill 1855 » January 15th, 2009, 8:01 pm

to add to my last post,it appears that correctly maintained systems and meticulous pre flight checks useing direct parallel wired packs should not be a problem, but in the real world if it can go wrong it will,then the cost of 2 diodes ,or a good backer system could save your model at the very least,or dare i say it ,something a lot more precious!!! once more,safe flying ,Roy.

Rob Cavell 529
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Re: JR RD921 rx advisory.

Postby Rob Cavell 529 » January 16th, 2009, 10:10 pm

Roy Hill 1855 wrote:to add to my last post,it appears that correctly maintained systems and meticulous preflight checks using direct parallel wired packs should not be a problem, but in the real world if it can go wrong it will,then the cost of 2 diodes ,or a good backer system could save your model at the very least,or dare i say it ,something a lot more precious!!! once more,safe flying ,Roy.


Hi Roy:
That seems reasonable to me too and please be assured I am not trying to cross swords here but my point is that I am posing the question of whether Red Scholefield is correct when he says that adding diodes (cost is not an issue) to a parallel systems is adding unnecessary complexity which itself can fail.
His point is effectively that given a well maintained system (and all over 20kg model are!) the introduction of diodes to the circuit have an equal or greater chance of failure than that of the unlikely occurrence of a direct short within the cells.

My point is that I feel we have been weened on the premise of NEVER to use parallel nicads without diodes on the basis of according to Red Scholefield, a misplaced theory of the possibility of cross charging.
Personally I feel Red has made a good enough case that established practise has yet to answer and or shoot down.
In the absence of a believable argument to the contrary I would be prepared to use a parallel system as per his diagram of two separate switched circuits without diodes but I feel it would never be passed by an LMA examiner for over 20kg models because of established common practice rather than actual fact.

Like I said, by installing a common busbar in their Rx in question is Japan Radio also accepting and or saying that they too feel that diodes are not really necessary?

Roy Hill 1855
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Re: JR RD921 rx advisory.

Postby Roy Hill 1855 » January 17th, 2009, 2:37 pm

Hello again Rob,to try to clarify things, may i submit the following,for many years i have used two ports at either end of the rx,ie,batt and channel 8 to even out the power distribution,(like a household ringmain circuit), useing 2 switches but 1 battery,i have had switches go faulty,but have been fortunate battery wise.When useing larger models,it seemed prudent to use a safer system,ie,2 batteries,and a diode /backer system.this comprised of 2 6volt batts,each with 2 diodes in parallel(for extra safety),2switches into 2 separate ports,(both being charged separately)thus power was drawn from both batts at once,BUT if either failed the other would carry on, this had the added advantage of doubling the batts capacity,ie in my case 1800 mah +1800 mah=3600mah.--=---May i also add,in addition, batt monitors and suitable! diodes only were used !!

To sum up,whilst it may be feasible to use paralleled batts,i can,t quite see why? if it is just for more capacity, just use a bigger battery and 2 switches

Incidently i cannot claim that all the above idea,s are mine alone,most credits must go to certain officials,both past and present of the LMA, whose efforts made it as safe as it is today.

(gets off soap box,goes for afternoon tea) :oops:

safe flying,regards Roy.

Rob Cavell 529
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Re: JR RD921 rx advisory.

Postby Rob Cavell 529 » January 17th, 2009, 8:48 pm

Thanks Roy what you say is all good sense but we are a little at cross purposes.

I did refrain to point out that Red Scholefields original artical is a little dated and refers to a time when Ni-cads were only available at no more than about 1200mah!
However that too is the time when the 'prejudice' to Ni-cad parallel connection was formed and still continues.
Reds parallel battery system was intended simply to increase Rx battery capacity, as opposed to splitting Rx power from servo power, at a time when all pundits including LMA (it was before the over 20kg scheme) told us never to do so without diodes and this situation remains today.

I raised the question again in case someone would be able to put my mind at rest and refute satisfactorily Reds assertions that the 'principle' of paralleling Ni-cads is not bad or dangerous practice.

Nowadays with very large capacities available we don't need to parallel them for purpose of normal Rx operation but the prejudice remains.
We now all go for what works for us, personally I have never used a parallel system and always use at least a double sized Rx battery and I've never found a model that can't carry one nor have I ever experienced a crash due to defective batteries.
Over the years I've binned probably dozens of duff batteries but have never had a crash due to battery failure, the moral of course being to continually monitor them.

Thanks again

Roy Hill 1855
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Re: JR RD921 rx advisory.

Postby Roy Hill 1855 » January 17th, 2009, 9:57 pm

Hi Rob,with regards to refuting Reds assertions,i cannot,my tests and others seem to agree with Reds findings, however because of potential problems that could occur,on my models i,ll stick to proper backer systems,imo.

Now then Rob,have a look at the spektrumrc.com/community/installation pages
they say you can put multiple batteries into any spekky rx!!Simply choose 2 batts of same size, with 2 switches,
Whilst the above statement agrees with Reds ideas,i myself cannot condone such potentially unsafe practices,allthough they do seem to answer some of your questions.I feel i should leave it there,as we appear to be approaching dangerous ground :oops:

Fly safe,Roy

stuart knowles 1611
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Re: JR RD921 rx advisory.

Postby stuart knowles 1611 » January 18th, 2009, 12:01 pm

I've read a quote recently to the effect that ' A lie will be twice around the world while the truth is still getting its boots on' apologies for any inaccuracy. I recently had a long conversation at work with a supposedly knowledgeable person who said that a particular battery powered device should be run almost flat before being re charged to avoid 'memory effect' that would soon kill the battery. The device is powered by Li-on batteries! It does rather illustrate how these things can perpetuate.

I would side with Rob / Red on this particular debate, although I have no evidence to support the assertion other that I have used the system without diodes & backers with no problems at all. There is something to be said for minimising the number of plugs, connections and black boxes in any radio system as every additional component presents a potential point of failure.

It seems to me that Rob is recognising the 'potential unsafe practices' of both ideas being discussed. Personally, unless someone comes up with the technical evidence to prove one or the other beyond doubt, I'd say its a matter of going with whatever you feel most comfortable with.

regards
st k

Rob Cavell 529
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Re: JR RD921 rx advisory.

Postby Rob Cavell 529 » January 18th, 2009, 7:27 pm

Roy Hill 1855 wrote:Hi Rob,with regards to refuting Reds assertions,i cannot,my tests and others seem to agree with Reds findings, however because of potential problems that could occur,on my models i,ll stick to proper backer systems,imo.

Now then Rob,have a look at the spektrumrc.com/community/installation pages
they say you can put multiple batteries into any spekky rx!!Simply choose 2 batts of same size, with 2 switches,
Whilst the above statement agrees with Reds ideas,i myself cannot condone such potentially unsafe practices,although they do seem to answer some of your questions.I feel i should leave it there,as we appear to be approaching dangerous ground :oops:

Fly safe,Roy


I too will leave there even though we seem now to be getting to the nub of my question.
My problem is that I believe Red Scholefield IS a genuine engineer with particular expertise in Ni cad's and he advocates there is not a problem with parallel connection.
As you have pointed out Spekky says there is not a problem and JR seem to be saying the same with the JR RD921 Rx.

The deafening official silence from LMA convinces me that having now convinced 99.99% of the modelling community of "such potentially unsafe practices" (sorry!) they are going to remain silent, safe in the 'fact' that as a result there have been no known crashes attributable to parallel connection of Ni cad's.

So many years have now passed since the no parallel policy started that I have the greatest respect for any 'don't rock the boat' stance if that's what it is, in spite of the fact that I have no way of knowing if there have been any crashes attributable to diode failure which Red says is potentially a more likely possibility.

I would never advocate the policy be scrapped but it remains that I would have preferred a qualified engineer to have come up with a reasonable argument as to why Red is wrong rather than to leave his assertions in the air! (pun).

Thanks for the contributions, the hobbyhorse just died!


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