|The reapairs are done and I'm getting ready for my second flight (probably Saturday). As you probably recall my first flight ended with a power failure and a hard landing 15' short of the runway. I traced the power failure back to a popped alternator output C/B and a low battery. The engine quite only 20 minutes into the flight. So, there are two basic changes in this flight. First I will fly always within easy glide distance of the runway, period! The second change is to put in a un-missable low voltage warning system and an emergency engine electrical system. The Low Voltage Warning system is a Bob Nuckles design built for me by a fellow EAA member, Dave Bristol. This circuit lights a sunlight readable pulsating LED right in front of my face when the voltage drops below 13.0. It also automatically De-energizes a relay and connects the engine ECU and fuel pump directly to the highest of the two batteries. So, if I have an electrical problem I can turn everything off and the engine will continue to run (just like a mag at least until both batteries are gone). So the question is how long is this? So I did a test last night. I ran the engine for 30 minutes at the lowest electrical load I could (had to have the fans on for ground ops) which was about 25 amps. Then I turned everything on and ran until the engine started stumbling. The electrical load was about 54 amps and it continued to run another 10 minutes. The voltage at onset of engine roughness was 9.0 volts. I integrated the amps over time and got 22.1 amp-hrs from full charge to 9.0 volts. (Link to analysis) So I figure with everything off except the emergency engine circuit (15 amp load), it should run 88 min. The two 17 amp hr batteries totaling 34 A-H only produce 22 A-H to the 9.0 volt cut off. Sounds pretty good to me. The catch: ok given this why did the engine quite after only 20 minutes? I was pulling 35 amps during that flight. Over 20 min that would be only 11.7 A-H. I'm sure one battery was full and the other probably 3/4 full. I figure that should have been about 19 amp hrs or 33 min at 35 amps. Wonder where the other 15 minutes went?
At any rate, the test last night gave me confidence that the engine runs well and the electrical system is much more robust that before. So I'm off!