ORIGINAL: cmoulder
If you plan to fly 3D in the future, the use of an IBEC (ignition battery eliminator curcuit) will cut the weight of your model by anywhere from 4 to 6 ounces (HUGE) depending upon what ingition battery/switch combo you are using.
The [link=http://www.tech-aero.net/ultra-ibec]Tech-Aero Ultra IBEC[/link] is a very lightweight and reliable unit. I have been using one in a Focus II pattern model with a SAP 180 (RCExel ignition) and have had nary a glitch in over 300 flights. And it was super easy to set up. Uses about 200 mAh/flight with pattern flying, which is the total consumption for the Rx/servos and ignition combined. 3D is probably a bit higher.
Bob's right on the mark with the Tech Aero IBEC. Ed came up with an outstanding little gizmo that does exactly what it's supposed to. The filtering Ed uses in the device just about eliminates any RFI from the CDI unit getting back to the RX. Cost is about what you'd pay for the extra battery and switch harness
Installed as instructed, ignition is activated from the TX (retract channel is convenient On-Off switch), and requires no extra switches or anything like that in the model. The LED light is fairly bright and can be seen easily in daylight.
It eliminates the need for a separate, TX activated "OPto Kill Switch", a second battery as Bob stated, and a second switch. Also, if your TX allows preset default settings in the event of signal loss, you can preset the TX to kill the IBEC (retract channel or other). Of course, if for some reason the linkage came off on the carb in flight and your engine just wouldn't die, rather than waiting for the tank to be burned off as one would have to do in a glow application, just flip the switch assigned on the TX when you are ready to land.
It is an excellent safety device and I highly reccommend it. Between the two of us (Bob and me) we have about 500 flights on them, trouble free. Many other guys are running the device and by now the flights must be several thousand