Large Model Association Forum

Single-battery systems for petrol models
I recently noticed on an Internet forum some discussions on the use of a single battery to power both the radio and the CDI spark ignition in a petrol powered model. This flatly contravenes the usual rule that ignition and radio circuits must be kept separate and must use separate batteries. The following note is seeking comments on the validity of the idea concerning the risks or otherwise of interference.
There is now a move to small petrol engines, the latest being a 9 cc unit from NGH, which invites use in models around 60 inches span, which suggests that it could have a wide appeal. The problem that arises is that although engines may be smaller the ignition module and battery remain the same size, and become a significant burden in the smaller model. To be able to dispense with the separate ignition battery and power the engine from the flight battery would make the accommodation of the petrol engine much easier.
The problem with use of single battery is that the spark may generate radio interference, a real threat to older 35 MHz systems. However I have heard that any naturally generated noise – e.g. spark - -will never generate a frequency greater than 400 MHz (or something around that value). While 400 MHz will obviously affect 35 MHz systems it is way below 2.4 GHz and should be undetectable on 2.4 GHz radios; at least that is the theory. Is it true?
I carried out a simple experiment by altering a well used model to run both engine and radio on a single battery and then did some range testing. The set-up used a JR DSX9 transmitter and a Spektrum AR7000 receiver powered by a 2300 mAh LiFe battery pack made from two A123 cells. This was connected by a Y-lead, one arm going to the radio via a 6 V regulator and the other arm to the ignition module via another switch and a 5V regulator. The engine was a DLE 20 running a 16 * 8 propeller at up to 8000 rpm with a tick over around 1700 rpm. The battery type was chosen because of its known ability to meet a demand for heavy currents without voltage drop. I would not consider this approach with any Nickel chemistry battery system.
The first range test was for the radio alone with the ignition circuit disconnected. This gave a range check still working at 70 yards, indicating a signal strength more than 5 times the minimum required. The test was not carried beyond the 70 yard limit. The second test had both the radio and the ignition connected, and the engine running through its full rev range. The result was the same, without any suggestion of a problem. At the full distance the engine was under complete control and the aircraft responded correctly at full power.
It appears from this that the model would be safe to fly with a single battery—unless I am missing something. There are on the market UBEC and IBEC devices (Ignition Battery Elimination Circuit) though they are not reliable according to some opinions. The problems they have may be the result of taking the ignition power from a receiver output in order to control the ignition from the transmitter. Though this may be desirable it may be asking too much. If the use of a Y lead to provide separate power lines to radio and ignition overcomes this danger, thus making single battery use safe, it would offer a significant advantage to models able to use small petrol engines.
My model has, for the present, reverted to the proven two-battery arrangement. Before attempting any flight tests I would welcome any views on the safety of this approach.

Hi Clive,
Along the same lines as this about 3 years ago I changed my 2 jet models over to single battery with no problems. They are both Wren MW44 powered and small as jets go, I use a 3300/3600mhr NimHys 4 cell. Both are flown on Spektrum radio.
I am also considering changing my petrol models to single battery but like you will use Lipo or Life with a Y lead, cannot really see there being a problem.

Barrie

Seems off to me...

For the cost of a battery ? £5

Your ignition unit can't drag your RX battery down and loose your model...

What benifit is there in doing this ??

Can't make any sense at all...

I cannot understand the gain. It's a bit like putting all your eggs in one basket.

You could wire up with two batteries that both supply RX & ignition, so the loss of either wouldn't mean you lost either engine or radio.

If weight was an issue, two 2S 1100mAh A123's could be used for about the same weight & capacity as one 2s 2300mAh's.

I can see the point of the comments about using two batteries, and the small difference that would make to weight. The problem I have is with small petrol engines, for instance the NGH GT9 (9 cc, or 0.54 cu.in.) The difficulty is lack of space. The ignition components take up a lot of room in the nacelle for this size of engine in relation to the size of the aircraft, in spite of being able to use smaller fuel tanks than glow engines need. To be able to ditch the volume of an extra battery make a useful difference.

If it were just weight it would be less significant. What I am looking for most is to see if I have missed something vital. I understand the usual attitude to hte use of only one battery; I just want to tap into the experience of others to see if someone has tried it and found it actually to cause trouoble, or if anyone has tried it and can confirm it as OK.

My thanks to Barrie King, that’s just what I was looking for.

Hi Clive,

I've got 2 models running RCGF 15cc engines, both are 46 size models with 2 x 4 cell AA packs in stacked on top of each other, normally near the back to balance the model out.

Yes is can be interesting finding homes for them

The ignition unit's thankfully arn't very heavy so they can normally squash in the engine bay - would be nicer to have a smaller HT lead in these cases.

Chris

One of my reasons to change to a single pack was that I found the 4 cell AA type packs very unreliable so wanted to use sub C packs which are more difficult to find room for in smaller models. Since changing to this type of pack I have had no problems at all. It seems common sense to me if one will do why use two.
Barrie

Backup=Failsafe =Safety

Tony, we are not talking backup it is about one on radio and one on ignition or ecu,
Barrie

Precisely Barrie. With only one battery you have no backup. If it dies then so does your model and maybe someone else too.

Yes Tony but in this case if one battery is on radio and the second is on ignition there is still no backup! The point was would it work! Backup can then be provided as well if required.
Barrie

Interesting how this question of space in a fuselage persistently tries to become a discussion of backup - which was not the original idea.

Battery quality inevitably influences the need for backup. NiCd batteries are, alas, effectively a thing of the past, replaced by NiMH with their poorer hold on a charge, and more delicate charging needs. Having used LiFePo4 for some time I can recommend them as having a performance superior to other types. Less heavy than Nickel for the same charge content, much safer than LiPo, capable of giving enormous currents, therefore resisting voltage drop under load, and with correspondingly high charge currents. The A123 cells are outstanding. If there is a need to balance single battery use against backup then I suggest giving LiFePo4 a very close look.

Back to the original idea, Barrie is the one whom I regard as being on topic. Has anyone else direct evidence of single battery use, not just theory?

Barrie King wrote:Yes Tony but in this case if one battery is on radio and the second is on ignition there is still no backup! The point was would it work! Backup can then be provided as well if required.
Barrie

OK Barrie. for safety there should be two batteries for the radio and one for the ignition. If you have size weight problems then it can be helped by using 900Ma AAA batteries. smaller, lighter and will give enough for several flights. I have never had a problem using NiMH cells and if you charge before each flying session then holding a charge for long periods of time is not an issue. My 2900Ma AA cells are charged at one amp and have NEVER been a problem. I switched on my transmitter after several months non use and it still showed 10.5 volts.

Sorry Clive but I had to answer Barries point.

Hi Tony, yes agree with what you say but I do not know any club flyers that use dual packs on radio but I know some that have had failures with AA packs. I am going to try A123 cells next. They will power radio, ignition, lighting, retracts, and a camera!
Barrie

So when was the last time you had a back up battery in your TX then.
Surly thats just as likely to fail as a flight pack.

Si

Simon, a TX battery pack is not subject to the fluctuating and sometimes very high demands of an RX pack. It has a constant low current drain and therefore a much easier life. Also it is not subject to the vibrations that can cause a less than perfect solder joint or connector to fail. However I did once have one fail.... nearly forty years ago

We still seem to be hopelessly distracted by BACKUP.

The whole point of starting this discussion was to consider the use of the SAME battery, in a suitable configuration, for both radio and petrol engine ignition. The question of backup is an entirely separate issue. As well as the usual form of backup for the radio battery with separate ignition power it would be entirely possible, if desired, to have a common battery for radio and ignition and then another common battery as a backup, given enough space.

Backup is another story. Perhaps it would have helped if I had begun under a different title, for instance “Common Battery for Radio and Ignition”. The point at issue here was intended to be whether the principle of the common battery situation has been explored and if there is any experience of the set-up, not how many batteries can dance on the point of a pin.

Please can we start a separated thread for those who want to thrash out matters of backup, and stop confusing the main subject – practical evidence of common battery usage?

Clive Hall wrote:We still seem to be hopelessly distracted by BACKUP.

Please can we start a separated thread for those who want to thrash out matters of backup, and stop confusing the main subject – practical evidence of common battery usage?

Good point and sorry

Now go onto the High-Alpha Forum site and ask the same question as those guys have been doing just this for ages to save weight in there 3d machines and they seam to have had no problems at all.

Simon

Hi Simon Willey

Thanks for the pointer, that’s exactly what I have been trying to find out, and it fits well with my own ground-test results.

Next step: some flight tests. I now see that they should be safe.