Electrical/battery gremlin?

I am having problems with my 912 ULS. It has 250 hours on the Hobbs. I have a relatively new Odyssey PC680 battery in it and I keep it on an Odyssey trickle charger. Despite the charger showing 5 bars as fully charged, the battery voltage on the second start of the day, after flying for over an hour, can drop from 12.3 to 10.4 while trying the start it. The initial start of the day goes great and it fires off immediately. I have changed the battery, the Ducati regulator rectifies and the AV5001 fuse panel and associated wiring so far. I intend to replace the battery again tomorrow. I am looking for any suggestions at this point to track down the intermittent problem. Thank you all in advance!

You don't say if alternator is charging in-flight. I assume your EMS screen shows +/- charge rate....

That large of a voltage drop could also be indicative of a loose connection somewhere in the starter circuit. It’s POSSIBLE that engine heat after flying is causing a connection to expand slightly and cause a high resistance, then make better contact after cooling. Trace all connections from the starter motor back to the battery, and don’t forget to check the ground side of the battery and starter motor. Just a wild possibility based upon your description.

Thanks for the suggestions. I will work on it this weekend. I am getting a 14 volt and +1 while in flight, so the alternator appears to be doing its job.

Try not using the trickle charger. If you fly at least once a month or so they are not needed.

Carl

Carl Froehlich said:

Try not using the trickle charger. If you fly at least once a month or so they are not needed.

Carl

Click to expand...

Actually, the Odyssey charger is not trickle charger...

I asked Odyssey the following question with their response:

The PC680 battery is used in an experimental airplane. I fly 1-2 hours a week and the rest of the time the airplane is stored in a hanger. I would like to connect my Odyssey OBC-6A battery charger when the airplane is stored in the hanger.

As I understand how the OBC-6A functions… it only charges the battery if required and then remains at zero charge level unless battery requires a charge. It seems that this would do no harm to the battery over long term.

Can you confirm that it is acceptable to leave PC680 connected to the OBC-6A for extended periods of time?
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Odyssey response…. There is no issue with leaving the OBC-6A connected to the battery indefinitely. The charger was designed exactly for applications similar to yours where the battery might not be used frequently.

The advantage of having the Odyssey battery at full charge is to allow fast/powerful engine cranking. The Rotax 9-series engines have a Sprag clutch on the starter motor vs. Bendix as used on Lycoming or Continental. Sprag clutch does not like kickback if starter motor is weak in overcoming power stroke with fixed ignition well before TDC.

Last time I bought a replacement battery it came with a no load voltage curve that said 12.8V indicated 100% capacity. Every annual I check the no load voltage and 100 amp load voltage with a Harbor Freight battery tester. I marked the voltage of a fresh battery under load on the meter for reference. I don’t remember the exact value without checking the meter at my hangar, but I think it was 10.8V under 100 amp load.