As all Cirrus pilots know, SR20s and SR22s have 2 batteries in the Cirrus electrical system (technically, they have 3, as Battery 2 is made up of 2 12 volt batteries in series with each other). Battery 1 provides power to the starter and is a backup for the entire Cirrus electrical system, while Battery 2 provides backup for the Essential Bus items.
If you look on the engine page of either an Avidyne or a Garmin Cirrus, there is a section showing the Cirrus electrical system health. With the engine running, you have voltmeters showing the voltage from Alternator 1 and Alternator 2, ammeters showing the amperage output from Alt 1 and Alt 2, and an ammeter showing the charging rate of Battery 1. There is no indication for Battery 2.
This brings up a question. How does the pilot know that Battery 2 has any kind of a charge? What happens if the whole Cirrus electrical system goes caput and all that’s left is Battery 2? Will Battery 2 have juice then?
The answer is actually rather simple. As part of the pre-flight inspection, the first step in the cabin inspection is to turn the Battery 2 switch on, then check the Essential bus volt meter. The checklist says the voltmeter should be reading between 23-25 volts. This tells you how much voltage Battery 2 currently has. The next step (after ensuring the flap lights are out) is the turn Battery 1 on. Battery 1 then powers the Main Bus and Essential Bus. The voltage showing on the Main Bus shows how much voltage Battery 1 currently has.
Easy enough right? Now you can impress friends and family alike with your Cirrus electrical knowledge!