Looking for trouble shooting advice for a power up issue on a TWTT Sportsman with an 3500S. Yesterday I turned on the unit with the master switch to check battery power and oil temperature as I was preheating the engine. I then turned the Master switch off and the unit went down without running on the backup battery as it usually does. I assumed the backup battery was no longer charged and so I turn unit on again with the master while having the trickle charged on the battery- 2 amp. I left the unit on for about an hour and when shutting down the master the backup batteries now kept the unit powering down as previously down. All things seemed good and possibly just week backup batteries.
After prepping for flight I turned on using master, started the engine, powered up all avionics and taxied to the fuel station. Shut down with master, all was normal. After fueling I turned on the master but this time the EFIS did not fire up. I was able to start the engine, power up all instruments and taxi back to the hanger. Again, on several attempt the 3500S did not power up when turning on the master. There is power everything and no breakers trips. Looking for advice.
