|The ideal backup system would be totally independant of the primary battery. This
allows complete swtiching regardless where the primary system fails. And, your receiver
will need an extra servo slot available to make the backup battery connection. You can use
a "Y" harness if a servo slot isn't available.
Basically, you are installing two complete battery packs in your airplane. This would include 2 batteries, 2 charging jacks, and 2 on/off switches. They are isolated by a diode on the backup side. The diode does two jobs; first it separates the batteries and second it sets a voltage level to kick in. A silicone diode has a voltage drop of .7 volt. Therefore the backup battery won't kick in until either the primary battery looses connection or, the primary battery voltage drops equal or below the backup battery voltage.
The backup battery doesn't need to be as big as the primary battery IE: AMP capacity. Typically, the backup is around 500 to 700 Mah. This is more than enough power to get your plane home safely. You could run the system without the diode, but I wouldn't suggest it mainly because you're simply connecting the batteries is "PARALLEL" with separate on/off switches. That's not necessarily a bad thing but you are using both batteries for "normal" flight. The use of a diode (similar to a 1N4007) will "lock" out the backup battery until it's needed.
Your OEM and most after-market on/off switches are mechanical and sensitive to vibration. Location and orientation is important. As a rule of thumb, the "on" side of your switch should be towards the rear of the airplane, and as far away as you can get it from the engine. A good place would be behind the cockpit or wing. Try to avoid the use of extensions.
This drawing illustrates some good locations for your switch(es). You'll notice the "ON" position is oriented towards the rear of the airplane.
Copyright © 2004 M. B. Fuess