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Output of alternator

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Output of alternator

Postby PKnight » Sat Feb 10, 2018 10:36 pm

While messing about with the starter problems that I experienced I did measure the alternator output. I thought I would put this as a separate topic for reference.
Configuration is J430, standard alternator on engine 33A1336. Current measurement is on positive rectified feed immediately before the battery. Voltage measurement is the voltage between the positive instrument bus and the earth instrument bus. There is a voltage drop of about 0.4 volts between the voltage at the buses and the battery. The battery was not fully charged and there was a load from instruments and other bits of about 5 amps.
Output in these circumstances was as follows:-

RPM VOLTS AMPS
1000 11.4 0.3
1200 11.5 2.5
1400 11.6 5.3
1600 11.9 8.1
1800 12.3 11.0
2000 13.6 12.0

This confirms what I knew intuitively that the alternator was not much use for charging anything below 1600rpm. Certainly if you want to put a bit of whoof back in the battery after starting you need to be running at 2000 or above for a few minutes or more.
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Re: Output of alternator

Postby Steevo » Sun Feb 11, 2018 8:02 pm

Have you done the alternator mod? This will give a charge at 900RPM.
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Re: Output of alternator

Postby PKnight » Mon Feb 12, 2018 9:20 pm

I have thought about doing the alternator mod but not yet.
As far as the aircraft is concerned there is enough electrical power for all its needs. The need for 2000+ revs to get significant charge is not really a problem provided you are aware of it and don't assume that the battery is being re-charged at tick-over.
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