Quote Originally Posted by gderamel View Post
Also, would someone explain why the two voltages? Is the left side (#1) the Primary battery, and the right side the Emergency Backup Battery? When I do the preflight Ignition check, with the key on Right "mag" (electronic Ignition) and the Emerg. Backup Battery switch "ON" the #1 or #2 voltage drops to 12.0v (ish) and the other voltage stays at 14.4 (ish). I'm assuming the 14v is showing the charging Primary BATT and the 12v is the current state of the Backup BATT?
This is a great question, hopefully the following will answer it.

When the backup battery is engaged it isolates the backup system such that only the backup battery is connected to the right-hand (R) ignition and only the R ignition is connected to the backup battery. This is done for two reasons.

The first reason is so that the pilot can check the operation of the complete ignition backup battery system at the pre-takeoff check. During normal operation the R ignition is running from the main battery that is tied in with the rest of the electronics. The backup battery is getting charged through a diode so that a main system low voltage condition cannot discharge the backup battery. This diode is the reason why there is a slight voltage difference between the main battery and the backup battery during normal operation. When the backup switch is turned on, the two electrical systems are separated, and the backup battery is no longer getting 14.4v from the alternator and it drops it's internal voltage. If the systems weren't separated, the pilot could not be sure that the backup battery is in good state because the alternator or main battery would still be providing the power that could mask any shortcomings in the backup battery. The same reasoning that this check must be done with the key in R, so that the L ignition system doesn't mask any problems by running the engine during the check of the R ignition system and the associated backup battery.

The second reason is that isolation, once that backup switch is engaged, none of the rest of the electrical system can interfere with that backup battery powering that R ignition.

That said, you are seeing normal operation when the main battery is still charging via the alternator and is at 14.4v (ish) and the backup battery drops to 12.0v (ish) because it is disconnected from the rest of the electrical system including the alternator.

I hope this answers your question and helps others in understanding the ignition backup battery system.

I also agree that is great info on comparing the batteries, thanks for sharing.