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.
