To answer your question, I think the back-up battery is suffiecently charged
through the diode.

John,

 

As usual, Experimental airplanes are interesting in the different schemes used to meet different objectives.

 

What is the voltage you measure at aux V as the secondary battery charging level?

 

I view my setup as follows:

 

There is one main battery/alternator/regulator circuit that is the airplane's electrical system.  The battery is a working battery in providing starting power, noise ballast functions, occasional short term supplemental power when the alternator cannot provide adequate juice (motors running, lights lighting, transmitters transmitting, etc. all simultaneously), etc.  Voltage Regulation is driven by the needs of the main battery.  The system is divided is case that some catastrophically failed main element not necessary to safely getting to the ground can be isolated and the battery can directly supply, for a limited period of time, a necessary subset of elements to effect a safe landing - ergo the essential bus.  Note that there is still another backup system since I still have vacuum operated DG and horizon should the battery itself fail.

 

Now comes a dual electronic ignition dependent on electricity but divided into two independent plug igniter systems.  One is designed to run from the essential bus - either thru a diode from the main bus or directly from the main battery for a limited period of time.  The other is designed to run off the main bus or a standby battery for a limited period of time. All moot, of course, if the engine won't run for some other reason.

 

The STANDBY battery is maintained by the main electrical system but thru a diode that protects the standby battery from back feeding a failed system, but that also causes a voltage drop in the maintenance supply.  It is only a side benefit that the battery can supply full power to one part of the ignition during the heavy load on the main system when starting.  Otherwise, this battery is only to be called on in case of a total electrical system failure - just like any other standby battery.

 

The problem is that the standby battery is not being managed by a system set up for it.  Consider that most all electric airplanes probably have a main and secondary (standby) battery with the main managed off the primary alternator and regulator and the secondary maintained by a small supplementary alternator driven by the vacuum pump accessory gears.  It has its own regulator.

 

What I am looking for is a Battery-Tender like device to slow charge and maintain a STANDBY battery at the appropriate float voltage and that operates from a 12-14 VDC power supply rather than 120 VAC. This would properly keep such a standby battery ready for use when needed.