(Reposting by popular demand 
)
I lost electrical power resulting in a forced landing and substantial damage to my RV-4 as mentioned in the Battery Failure post: https://www.homebuiltairplanes.com/forums/threads/battery-failure.51170/
Link to more specific threads:
https://www.homebuiltairplanes.com/forums/threads/n214fl-forced-landing.51158
As mentioned, I had considered the battery as backup for the alternator and the alternator the backup for the battery. But that didn't work for me when battery failed open.
I had pulled back on the throttle from about 5,000 RPM down to about 4,000 RPM (which should be roughly 8,000 alternator RPM -- still plenty to produce full voltage) and ran for at least 20 seconds at that before the failure. So RPM reduction was not the direct trigger of the failure, although fairly rapid decent (3,700 to 2,700) did cause temperature change under the cowling: 190F to 170F alternator output air.
I removed the alternator and tried to bench test it with a 1/2HP drill motor. It does deliver 12 amps into a 1 ohm test load.
12V from battery is needed to the "field" terminal when first spinning up the alternator, but can be removed once the alternator is outputting about 6V, or so. With battery disconnected there is considerable ripple on the B+ alternator output.
Looking at the attached (abbreviated) schematic you can see that there is a relay between the alternator B+ and the battery. The purpose of the relay is over-voltage protection. If the transistor in the alternator ever shorts to ground, the field winding in the alternator will get full battery/alternator voltage and the alternator will potentially output 100's of volts -- hopefully somewhat shorted to ground by the battery, but could take out all electronics, including the engine controller, fuel injectors and spark igniters. The OVP module shorts relay power to ground if voltage exceeds a set point (about 16V). This will then trip the 5A breaker that supplies voltage to the over-voltage relay. It's then up to the battery to supply needed electrical power for the engine to run.
In my case the 5A breaker did not trip! So why did the alternator stop delivering power? We can speculate that the alternator and/or voltage regulator temporarily failed due to heat. However, another possibility is that the OVP circuit did trigger on the ripple spikes from the alternator after the battery failed open and the relay lost power faster than the circuit breaker could trip. With voltage to alternator regulator and field winding being pulled to ground and no voltage from battery to re-energize the relay, the alternator would no longer produce power.
A diode between the breaker and alternator field terminal would have allowed the alternator to continue producing power (output from the three small diodes in the alternator not pulled to ground by the OVP module would have allowed the alternator to produce power and then re-energized the OVP relay. The OVP module might have continued to trigger but alternator would have continued to produce power -- even if very noisy due to no battery filtering -- perhaps too noisy for the engine controller to operate?
So I still recommend a backup battery, as described in the referenced thread, even if you have two alternators!
Best would have to be an alternator with external regulator, do away with the relay in the B+ lead, replacing it with a big diode (my original configuration). The OVP module killing the field voltage would prevent continuous over voltage from the alternator. The big diode will prevent a shorted alternator from shorting the battery.
But, still a backup battery for the engine bus that can be separated from main electrical bus (battery and alternator).
)I lost electrical power resulting in a forced landing and substantial damage to my RV-4 as mentioned in the Battery Failure post: https://www.homebuiltairplanes.com/forums/threads/battery-failure.51170/
Link to more specific threads:
https://www.homebuiltairplanes.com/forums/threads/n214fl-forced-landing.51158
As mentioned, I had considered the battery as backup for the alternator and the alternator the backup for the battery. But that didn't work for me when battery failed open.
I had pulled back on the throttle from about 5,000 RPM down to about 4,000 RPM (which should be roughly 8,000 alternator RPM -- still plenty to produce full voltage) and ran for at least 20 seconds at that before the failure. So RPM reduction was not the direct trigger of the failure, although fairly rapid decent (3,700 to 2,700) did cause temperature change under the cowling: 190F to 170F alternator output air.
I removed the alternator and tried to bench test it with a 1/2HP drill motor. It does deliver 12 amps into a 1 ohm test load.
12V from battery is needed to the "field" terminal when first spinning up the alternator, but can be removed once the alternator is outputting about 6V, or so. With battery disconnected there is considerable ripple on the B+ alternator output.
Looking at the attached (abbreviated) schematic you can see that there is a relay between the alternator B+ and the battery. The purpose of the relay is over-voltage protection. If the transistor in the alternator ever shorts to ground, the field winding in the alternator will get full battery/alternator voltage and the alternator will potentially output 100's of volts -- hopefully somewhat shorted to ground by the battery, but could take out all electronics, including the engine controller, fuel injectors and spark igniters. The OVP module shorts relay power to ground if voltage exceeds a set point (about 16V). This will then trip the 5A breaker that supplies voltage to the over-voltage relay. It's then up to the battery to supply needed electrical power for the engine to run.
In my case the 5A breaker did not trip! So why did the alternator stop delivering power? We can speculate that the alternator and/or voltage regulator temporarily failed due to heat. However, another possibility is that the OVP circuit did trigger on the ripple spikes from the alternator after the battery failed open and the relay lost power faster than the circuit breaker could trip. With voltage to alternator regulator and field winding being pulled to ground and no voltage from battery to re-energize the relay, the alternator would no longer produce power.
A diode between the breaker and alternator field terminal would have allowed the alternator to continue producing power (output from the three small diodes in the alternator not pulled to ground by the OVP module would have allowed the alternator to produce power and then re-energized the OVP relay. The OVP module might have continued to trigger but alternator would have continued to produce power -- even if very noisy due to no battery filtering -- perhaps too noisy for the engine controller to operate?
So I still recommend a backup battery, as described in the referenced thread, even if you have two alternators!
Best would have to be an alternator with external regulator, do away with the relay in the B+ lead, replacing it with a big diode (my original configuration). The OVP module killing the field voltage would prevent continuous over voltage from the alternator. The big diode will prevent a shorted alternator from shorting the battery.
But, still a backup battery for the engine bus that can be separated from main electrical bus (battery and alternator).
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