After our alternator failure and subsequent forced landing on Jan. 22/05, we have now installed a backup 18 amp/ hr. battery and Aural warning buzzer in our RV6A.
Battery Basics
Some basic facts about typical sealed lead acid/ dry cell type batteries commonly used in aircraft should be understood before you design your electrical system. The first thing that should be noted is that the amp/ hour rating is how many amps you can pull for one hour from a NEW battery before TOTAL discharge. This rating has little meaning in the real world as most devices on your aircraft will not operate below 9-9.5 volts. As the battery ages through a number of cycles, its capacity to deliver this current diminishes also. A better rating system is the current draw to 9 volts. Our new battery has a rating of 11.1 amps for one hour to reach 9 volts. Batteries should be replaced every 1-3 years to be sure that things will operate up to snuff if the alternator fails. You should select a battery which will give you enough time to find a suitable landing area over the terrain which you normally fly with at least minimum electrics on to keep the engine running. Our standard daylight running load is around 18 amps with the avionics on and about 10-12 with just the fuel pumps, ECU, injectors, coil pack and electric turbocharger scavenge pump running. Don't forget to turn off all non-essential electrics when you detect an alternator failure as this will noticeably increase your available flight time. A PC680 battery commonly used will be essentially dead (below 10 volts)in 15-25 minutes with average ECU, ignition, fuel injector, pump and avionics loads. It should be noted that some electronic devices and ECUs will not operate properly even at the 10 volt threshold.
Wiring and Failsafe Considerations
It is my view that a backup system should not rely on any more of the primary system current path than necessary. The master contactor draws current and has been known to fail so my backup battery is simply wired to the main bus directly through a toggle switch rated for 25 amps. No relay, no diode. If the primary battery dies, I shut the master off and turn the toggle switch on. Current flows from the backup to the main bus and the primary battery is isolated from the main bus by the open master contactor. The backup battery is charged with the switch on after starting and through the taxi and runup. The switch is shut off the to isolate it during flight until it is required. The other part of this is that if you forget to turn off your master after shutdown, the backup battery will still be available to charge the primary for starting.
Aural Warning for Alternator Failure
We had a low voltage warning LED mounted right in front of the pilot which was never noticed until other devices started shutting down because of low voltage first. It is my view that it is very important to know immediately of an alternator failure when using a electric fuel pumps and non-magneto ignition. The aural warning devices installed in larger aircraft have been shown in studies to get the pilot's attention immediately despite other distractions while flying. This should be far superior to an on/off light. A flashing light would be better than the on/off warning light if that is all you can fit. I have my 95 decibel buzzer mounted about 15 inches from my head. It does not run through the intercom/ headset as this is another potential source of failure. The buzzer is triggered at 12.5 volts and comes on within 1 second after the alternator field switch is turned off. I received several E-mails from other pilots stating they never noticed the low voltage conditions through their gauges or warning lights. Only when avionics started to fail did they become aware. Sunlight and high workload/ inattention were stated as the reasons. It should be noted that most jet aircraft use aural and visual warnings as these have been proven more effective than visual alone.