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Specifically in my case, a Sport EX.
The wiring diagram has a secondary power input on pin A15. GRT says to add a switch inline with the battery.
They also said that if primary power is "on" it should keep the backup charged. I asked them twice for specifics and they don't "get" what I'm asking...
Did you connect the battery positive to both A14 (primary) AND A15 (secondary) for it to charge, or is 12V available at A15 without a backup battery connected (or connected).
Just trying to figure out if I need to make two connections to the backup battery positive, or just wire it to A14 and be good....
I'm working on this myself. I wasn't planning on a backup battery since my engine (o200) doesn't need one to operate. I was just planning to run with one battery but now I'm second guessing this decision. The reason I'm wondering is because I understand it's best to not have the EFIS powered while cranking the engine due to fluctuating current levels. Much like the radio bus. The problem is there's no way to monitor oil pressure once the engine does fire up until the EFIS is powered and boots up. I don't think that takes long but still a moment or two. Now I'm wondering if it's best to power the EFIS off the second battery during startup to monitor oil pressure. I wasn't planning to install any other gauges besides the EFIS.
The install manual for the SPort 10.1 and EX manual say the power inputs are isolated so I understand this to mean you can have multiple power inputs available at the same time. I didn't read anywhere in the 10.1 manual that the EFIS would charge the secondary batteries, maybe I messed it. I don't know if that answers your question or added more.
GRT has been less helpful here. Sport EX harness pin A14 is the regular 12V input. A15 is the secondary power buss. GRT said to add the backup batt with a switch here.
I then asked if that circuit would charge the backup battery in normal operation. They answered back, yes, you need a switch on the backup battery. *sigh* The Sport EX manual does say that the EFIS automatically detects and uses the higher power found on the busses. So if primary were 11.9 but secondary was 12.8, I'm assuming it will then draw from the backup battery.
Maybe he was confused, I'm suspicious that the EFIS would charge the secondary battery. Especially since it says it draws a mac of 2A.
I'm guessing the switch is so that the EFIS doesn't use the secondary bat and potentially drain it.
I had a similar setup with my Sonex, but using MGL instead of GRT.
Was a similar setup; switched battery and the battery was charged when system voltage was (?) 12V or more. You need a switch on the backup because - think about it - if you are done with your flight and you turn off the master, the EFIS thinks you lost all power so draws from the backup battery. In effect it will keep the EFIS on until it dies.
The more I think about this, the more I'm - in ignorance - thinking that like software, EE designers use from a "library" of proven circuits. I'm convinced that GRT will charge the battery. Nothing special is required - 12V on the line into the battery will in effect act as a trickle charger.