The backup battery does not power the magnetometer. It seems to me that the only time the backup battery would be used is in the event of a loss of the ship's electrical power due to a failure of an alternator, master switch, battery, master relay or otherwise. In that case, the HSI would be useless irrespective of a working G5 backup battery.
If I am correct that the backup battery is useless upon a loss of electrical power, I believe that Garmin is in bad faith by so disclosing.
You're
not correct. You simply don't understand the system architecture and how it works.
Further, there's nothing being done in bad faith here.
Two things here, just to make sure we have them all covered: the G5
ADI will still present an attitude solution even if the external GPS input is lost. It is "degraded" but accurate enough you're unlikely to be able to discern the difference. The unit uses the GPS signal to "assist" the attitude solution.
The HSI: if you want your magnetometer powered by a backup source of some kind, get a backup power source for it. I'm not personally aware of any installations regardless of brand in which the magnetometer receives its own unique source of backup power (someone feel free to correct me if I'm wrong) so for the Garmin ecosystem, you'll meed another source of electrical power, if that's what you want. In the modern era we're seeing new single-engine piston aircraft being produced with dual alternator installations and of course light twins have always had two sources of electrical power available, one generator or alternator per engine. The ultimate backup is the main battery itself of course.
Since those probably aren't the solutions you envisioned, here's the way it actually works. (I'm assuming your aircraft is set up in the most common way in terms of its electrical system: a main battery and an alternator, usually a 50a or 60a unit, along with associated buses, CBs, solenoids and switches.)
In the event you your aircraft's alternator fails, and subsequently your main battery is then depleted:
1) Your G5 ADI will continue working as you'd expect for as long as the internal backup battery has sufficient charge.
2) Your G5 HSI will continue working as you'd expect for as long as the internal backup battery has sufficient charge, however, instead of heading the unit will display ground track. You'll still be able to navigate normally on the device, however you'll have to ascertain wind correction angles the old fashioned way.
In practice, in a real emergency, this is not a big deal. You'll still be able to fly approaches, maintain SA on your general heading, etc. Don't forget to reference your magnetic direction indicator -- probably your compass.
By the way, this can all be found in the product documentation. Highly recommended reading.
