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”.
ps, at nowhere in any instructions or publications is this made clear 
pps,your comments are eagerly awaited, but dont shoot the o.a.p., im, dooin me best aer kid!!
