With the 2 12 VDC battery setup, the essential bus could be fed from 24 volts or either of the 12 VDC batteries with the proper switching and isolation

 

question for the math wiz's:

 

how much does your total MTBF improve when you go from 2 power subsystems to 3?

 

e.g. suppose the MTBF of a battery in the first two years of its life is 1 per 10,000 hours

 

Then, assuming for a moment that switches and contactors don't count, the alternator MTBF is a don't care because it's way higher than the battery.

 

So you have two batteries, now you are talking at 1/(10,000 * 10,000) failure rate.

 

Is that good enough?

Colyn,

 

Nope.  Your sophisticated computation is in error.

 

If one was relying on a single battery that only fails once per 10,000 hours and one was the recipient of that failure, one would not be happy (a theorem).  If you had 2 batteries in series, the failure rate is greater (2/10,000 or 1/10,000 + 1/10,000 or certainly no better than 1/10,000), not like twin engined aircraft where the engines are in parallel).  If one 12V battery failed, then the 24 VDC system would fail (not like a twin where one can limp along with one power source still working).

 

I have tired of using "one", I am going back to using "you all".

 

The alternator does count because that is the most frequent electrical system failure in commercially available airplanes - of course, such aircraft have cheap or no alternator/bus monitors and so the operator only notices a failure when the battery finally dies.  The charging regulator failure rate is important, too.

 

It would be wise to be able to monitor both batteries and the alternator/regulator in the system Marv is considering - sometimes this is easily done by just switching the sensor to the monitored source.

 

Otherwise, land when in doubt or uncomfortable.

 

PS: I have 2 electronic ignitions running one off the main bus and the other off the essential bus.  If there is a total electrical failure, I can switch in the small dedicated backup battery to one ignition.  BTW, the bus monitor display is switchable betwixt main, essential and backup and a check of each reading is part of the preflight.  The ignition system is at the top of my electrical system failure pyramid.  Those of you all with magnetos, skip this PS - you all have other problems.