Sad to hear of your L39 loss Jack, but I really believe that we have the technology to (almost) totally avoid this sort of loss and probably all other power related losses, regardless of battery chemistry in use. I now use Weatronics 2.4 system in all my jets and this system gives me a huge range of highly useful facilities simply not available on any other radio as well as a DUAL RF link on 2.4

One of these features is the data logging of the system which is crashproof as its downlinked to an SD card in the Tx in real time. The system allows me to examine in great detail the performance of the batteries IN FLIGHT and you can plot the data on a graph or graphs, for example, voltage v current and relate that to the movement of any servo as servo positions are also logged. Its like a black box flight recorder and QAR rolled into one.

One great asset is that if you DO have a crash you have masses of REAL data to work with, no more guesswork or uncertainty AND the data to analyse your battery performance after every flight. Add to that the fact that the system will trigger an alarm on the Tx in real time if battery voltage falls below the level you have set on the Giga control software make this system absolutely brilliant and in my view, and I now have 9 of these receivers, the Weatronics, system leaves every other radio years behind what present technology can offer. I cannot imagine going back to any other system after seeing what the Wea can do.

Regards,

David.

PS As an aside you may recall I built a BVM F4 after seeing your dad's (give him my best wishes) "Screamin' Eagle" at Deland. Its still around and just been refurbished and now with an AMT NL Pegasus HPES which gives it a superb performance, even with three external tanks . I am now installing a Weatronics Micro 12 with 3 gyros in my Phantom. Out go the matchboxes, out goes the gyro (there are 3 in the receiver so it has damping on all 3 axes) and of course the external aerial is now gone. Combined with the Wea rx and the JR 12x tx with its switch assignability I have , for me at least, the perfect control system, and arguably, by far the safest.

Each to there own of course but sometimes I think so flyers tend to way over analyse and also over complicate things. There is a lot to be said for the keep it simple principal in my view. Also I believe that using over powerful servos tend to add to these sort of problems.

I know many jet flyers who use one receiver, One ordinary Nimh battery pack, a simple switch and standard servos and they have great success........... oh and a lighter plane

Regards

Dennis

You're right Nicolas. The biggest problem that I've found is that the price of the VPX pack has risen so much that it is not worth having to dismantle it anymore.
I've just ordered a few single cells from Lindinger in Austria and will build the pack my own way.

Yes and on top of this, A123 are very strong ( kind of crash proof ) and relatively resistant to charging abuses

Yes for sure. Having a proper charger that can cycle the battery and give informations on its internal resistance is for me a big plus. Only a few German chargers can do this to my knowledge.

Once again, monitoring the battery health means knowing the internal resistance of each cell composing the pack and knowing at what stage it shall be discarded.

VERY important as well:
When the internal resistance increases, the critical voltage ( voltage at which the pack will not be able to cope with the load any more ) will increase as well.
So what is true for a brad new pack in terms of number of flights and critical voltage is not true for an older pack.

Well I am a long time believer in using twin, electrically separated Rx batteries. There is absolutely no point in having twin batteries if you do not separate them, even if is is only with a diode. Otherwise a fault on one battery system (could be the switch for example) could not only flatten one battery but the other one too. After a crash the battery voltage of the "second" battery could recover because it is disconnected from the failure point.
I think that David Gladwins point is a valid one as technology is appearing that can warn us of fault conditions on board. My latest radio is the Multiplex M-Link Royal Evo Pro 16. This has telemetry capabilty built in and alarms can be set to warn the pilot of a potential problem. Reprting of signal value and Rx voltage is standard with the Pro rx's. This means that in the event of say, a flaps servo stalling for some unknown reason and pulling down the Rx voltage, the Tx is going to give you an audible warning and the Tx screen will then default to the Rx voltage screen. This at least gives you the option to quickly undo your last action.

Could you post a sample graph, David?

Heres one which will give you the idea. The top graph plots current and voltage and one can clearly see the relationship. The time base here is condensed to 1 min per division, this can be progressively expanded upto .1 sec per division for close analysis.
The second graph (you can create as many as you wish, add any or all parameters as desired) shows current against servo position, of course voltage could also be superimposed on this diagram too.

This is 2d format. With GPS you can create 3d track format and superimpose any data on THAT track diagram.

The two possible causes of Jack's crash are also addressed by the Weatronics system :
1. It automatically connects BOTH batteries with one switch on the larger receivers and;
2. It automatically performs a load check (brief application of 7 amps) at switch on. If either battery fails this check by exceeding the voltage drop which YOU set on Giga Control it illuminates a bright red LED on the switch panel.

Almost all conceivable data is streamed back to the Tx (parameters are sampled at .1 sec intervals !) and stored on the Tx SD card, (and integral SD card in the larger DRs) however on the Micros, Smart and Clever receivers, there is no current data as they have no regulators.

All this data (and programmability) is also a feature of the tiny 8ch Smart receiver which weighs all of 16 grams with an integrated gyro. It is a full range dual receiver with downlink and one is now installed in my Savex L39/ Wren 44 combo.

Brilliant equipment.

Plumb's assertations about powerful servos does not seem to be true. I tested a range of JR servos from 537s through to 6301 (33kg/cm) and 6311HVs (36.5 kg/cm at 7.4 v) all at 6v and all drew currents of around .12 amps with a 1 kilo load at 1.5cm arm EXCEPT the MP80T which is a powerful, 25kg/cm, brushless servo which drew significantly less current , around.05 amps than all other brushed servos, digital and analogue. In summary it seems correct to say that current draw is a function of torque actually being delivered rather than the torque a servo is capable of producing, certainly in the lower torque ranges.



Regards,

David.

I have 6 of these pack welded by myself. Been using them on 3 different planes for more than 3 years now. Done around 400 flights with these packs. Not a single problem.
Now the first packs are reaching the end of service...

Those numbers are fundamentally wrong. Servos draw very short sharp current spikes, something most DVM's cannot cope with, hence the faulty data.
A 25Kg servo drawing less current than a modern RX (50mA!), come on now, that's just not cricket old chap...

Great Info in your posts. A123's certainly have my vote. Why anyone would use a Lipo for anything but EDF is beyond me.

I was one of these guys that follow the traditional setup, one battery one receiver. Ontil i received a lockout after regaining control for some unknown reason and landed my mind was shot shaking and nervous i was done for that day. Thats when I Install two receiver and two battery. Another day i was in a flying a knife edge config at the same spot the rudder went neutral then back to full deflection. I must admit that flield is plague with RF problems. For me any Turbine jet or RC plane over 16lb should use two batterys (Opinional two receiver) Sometime keeping it simple does not mean using a double A batterys for the receiver or CA elevator servos formers.

Just my opinion

I have been using pairs of A123 batteries with 2 heavy duty switches. Never had a problem but only concern is I don't have any isolation if one battery were to go south. Been thinking about some form of diode arrangement to provide isolation but then I would have to ask - which has higher chances of failing - an A123 pack or a diode?

PaulD

Jack, I appreciate your chart, but lipo voltage discharge percentage below 7.4v is not relevant because anything below that is below the batteries nominal voltage. I recharge or replace all my packs at 7.6v and use the "80%" drain rule across all my lipo batteries, whether it be a 2s rx pack or 12s set-up in a large heli.

To the A123 fans, When flying IMAC, I used A123 packs, but playing the guessing game on voltage got annoying. There are pros and cons to each battery type and no "best" battery. As Jack has wisely stated, the user needs to understand how to use their packs.

My opinion two batteries pack connected directly to the receiver is better than diode isolation. Under heavy load a diode can either short (witch is better) or open. Remember if there is something onboard that would cause such circuit load, there is no way you would be able to land that plane unless you have two receiver.

P.S Two battery redundancy protect your system if one battery pack cell goes high in resistance (open).

Just my opinion

Moerig, These are figures I obtained using a Hangar 9 DVM which IS quite accurate, (checked against 2 other ammeters) with a steady sustained load. Of course there are spikes and larger currents during movement particularly under load, but they are , I believe, useful as comparitive if not absolute terms.

Values for a JR synth receiver, no servos, .02 amps, Weatronic Micro 12, .11 amps, no servos.

OK, lets see your values.

That said, what REALLY counts is in-flight performance. I am just about to test a PST Revision with a big Weatronics receiver. I have marked all channels and servo outputs on the receiver software so I will be able to see exactly how loads change (every .1 sec) with various servo movements, particularly those on the flaps, by using the Giga Control to plot all values graphically.

Those who dont believe in two receiver batteries should talk to one of my Australian friends ! He was the Captain of the Qantas 744 which lost almost ALL electrical power descending into BKK. (Galley water getting into the E + E bay corrupting the GCUs) Never happened before on a jumbo with 4 generators (and you can't start the APU in the air) It did that day, and the standby battery (all 96 pounds of Nicad) saved the day and allowed them to get one genny on line and land the aircraft safely !

Phil says things got quite "sporty" for 20 minutes !

Regards,

David.

Jack great info....just reinforces my switch to A123 and LiIo and getting away from Lipo except for the turbine ECU.

Hey Dantley,

That is exactly the point of the chart. We were Lions users, where the no fly voltage under a 1amp load is 6.9V (according to Duralite and confirmed by the chart). If you apply the same rules to Lipo's, then you will run out of power.
Net, as you also mention, use whatever you like, I guess they are all good. Just make sure you understand how they work, what their no fly voltage is, etc, etc...

Let me know next time you come up to woodland...

Jack

Hi Dantley,
The Li-Ion batteries are also 7.4 volts nominal.
The issue is that due to the different internal resistance between Lipos and Lions, their voltage under the same load are quite different.
At 7.4 volts under 1 amp load, Lions still have about 85% of its capacity left, while Lipos have only 50% (and both are 7.4 nominal).

Regards

Jack

Hi Jack, I recharged my Li-ions at 7.6v too.

What time of day do you fly at Woodland. Last time we were there the sun was in our face most of the day.

There's a lot of myths about two battery systems but has anyone bothered to actually test the scenarios & publish the results?

Fortunately the answer is YES!

In the Feb/March 2001 issue of RC Jet Int. Paul Mitchell bench tested lots of dual battery siuations. For the 'model' part of the test he used a 9 channel Futaba radio with a servo test function which cycles all servos slowly & continually to the extremes of their travel, i.e. nine servos at once, no flight loads but I doubt we would operate nine servos at once over their whole travel.

The simulated 'flying session' was six ten minute flights with a 30 minute wait between flights for a total flying time of one hour.
Batteries were 1200 mAh 4 cell packs.

With one battery flat & one fully charged the result was the servos ran faultlessly for the one hour and the discharged pack showed a slight gain in voltage. This suggested there was some cross charging but it was insignificant over the one hour period.

Most importantly with one fully charged pack and one pack with three charged cells & one shorted the result was the same except the faulty pack did not increase in voltage, it started & finished at 3.9 volts.

That's a good enough result for me, try to get a copy of the article. - John.

Paul, this is THE question: how simple does it have to be to stay reliable ?

Personally, and given the testing protocol I use for my batteries I would rather say that the diode would be the weak point.
A battery does not die suddenly: you see a sudden consequence of a slow degradation of the internal components. A thorough monitoring of the battery internal resistance will show an impending problem.

In case of one cell short-cutting on a pack, the voltage will drop from 6,6V ( on A123 ) to 3,3V. The internal resistance will drop significantly ( from 30 mOmh to around 20 mOhm ) thus the remaining pack will see the shorted pack as a load and will start to slowly discharge into it. I have made an experience with a 1 cell pack and a 2 cell pack plugged in parallel on a Rx. The current transfer on A123 is around 0,5A. This is an equivalent of two additional servos being plugged to the system. So I believe that if the power source is sized with sufficient buffer, a cell shortcut will be handled correctly by the remaining pack. Note that this experience does not truly simulate the reduction of internal resistance of a shorted 2S pack but seems close enough to me.

Also note that I have not had a single shorted cell event for the last 15 years.

+1

I tend to agree. Since switching to the A123's, this has been my set-up and I have had no problems. When I charge my A123's I allways check the cell voltages to see how they are balancing. I have not seen a problem but do expect that it would be evident during charging. I also have a cell voltage meter that will show individual cell voltages when I check the pack's charge condition.

The Diode would likely be the weakest link.

PaulD

Not Really. The diode is probably the most reliable device ever invented and I have never had one fail in 20 years. Can they fail - yes! But invariably because their limitations have been exceeded, large spikes on the supply rail can take them out. But your Rx is full of small signal diodes, so if this happens you have bigger worries.
I always at least double the voltage and current requirements before buying diodes as these are simple devices that are small are very chaep. Don't skimp.
As for A123's I have not had a single problem with them so far. On this basis I could say I will save money and just fit one battery.
However, for me I look at it like an insurance policy. I really hope my wife doesn't need to pull out my life insurance policy, but it is there just in case one day she does....
We seem to have some crystal ball gazing going on here, based only on personal experience, and taking no account of wider experience or what might happen. It is up to each individual, but I prefer the peace of mind that my set up provides, based on wider experiences over a modelling lifetime.
I have had cells go short circuit (I am describing an internal short with in the cell to avaoid any confusion) and crash a model with a single battery. With Nickel based cells this was a common mode of failure and it has nothing to do with gradual cell degradation. I have also suffered a cell going open circuit. This is very rare but it happened to me, again in the days before I relied on twin batteries.
I have witnssed a fire caused by a shorted switch harness that would not have happened if there had been a diode in the system.
So I will continue to fit diodes in the hope that I never need them.

It seems like you want your diode to act like a fuses?. If you use the correct (A) diode it will become a piece of wire if overload. Example like a car radio or CB radio when the polarity reverse