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[David47]

Phil, I’m interested. I’ll try and pm you.

[PaulSS]

Hi Phil,

The UK LAA allows certain electronic instruments to be used as backup sources of info e.g. airspeed and altitude but they must have their own backup power source. Some manufacturers have instruments with the facility but the likes of the MGL Vega ASV-1 do not and need to have a backup battery. I've been trying to figure out such a system, so I'd be very interested to hear how you did it on a larger scale and then, maybe, I could shrink it to fit. The ASV-1 only draws 73mA even at maximum backlight so the battery does not need a huge Ah. I was trying to see how I could do it, avoiding 'backup' switches i.e. an automatic change over and think I've got that figured out but I am a bit concerned about the means of charging a small battery and think I need to add something else to my system

With the diagram in the attachment, with 12V the relay closes, wigglies flow to the backup battery and ASV-1 and everybody is happy (I think). I don't know at this stage if the B/U battery would get charged or if it needs something else there to (a)charge it and (b)stop it over-charging. When the 12V stops flowing from the main aircraft system the relay opens and the ASV-1 is automatically powered by the B/U battery.

I have no doubts that I have something wrong my theory and would welcome advice on (a) and (b) above

Edited to add: I've just realised that unless the ASV-1 has an on/off switch then this system would mean the instrument would carry on being powered even when the battery master is off and the aircraft is tucked up in bed. Not ideal. Sorry, I also noticed I missed the obvious diode needed after the relay to stop the B/U battery trying to 'back power' the aircraft system.

[Flybyjim]

I see there is a choice of 4 or 8 amp for the TCW unit, what have you used?

[PaulSS]

I've got the 4A. I'm running all my MGL gear off that (RDAC CAN, Status Monitor, iEFIS, Extender Module and Magnetometer). All up that 975mA, so 4A is more than enough

[jiott]

The Dynon/AFS EFIS has no problem being ON during engine start. Maybe its because I have their EFIS backup battery.

[PapuaPilot]

I posted information about my homemade backup battery in the "Electrical" section.