I disagree with Rick Schrameck's claim that dual alternators and redundant
batteries are necessary for safe flight.

(Note that this post has nothing to do with airplane crashes or lawsuits.  
Let's move on and leave all that legal crap behind us...)

Rick is absolutely correct that a backup power source should be considered
essential.  However, this is normally done by picking some minimum acceptable
"emergency flight time" and sizing the battery so that "essential" electrical
needs can be supplied by the battery for that duration in the event of
alternator failure.  The typical is 30 minutes, which is arguably too short.  
It also assumes that the pilot notices the alternator failure immediately and
shuts down all nonessential functions.  

The concept is fine -- but the implementation may be weak.

Have you done an electrical power budget on your airplane?  Do you have a
list of things to turn off when the alternator fails?  Do you know how to
detect an alternator failure?  Does a little needle on one side of the panel
shift 1/4 inch to the left (probably), or do horns blare and lights flash?
(probably not)  Do you know how much time you have once the alternator fails
to get back on the ground?  What happens if all electrical power is lost --
does the radio die (who cares -- pick up the handheld) or does the engine
stop?  (Oops!  That would be bad...)  Have you done no-flap landings
recently?  Or dropped the gear manually?

Massive proliferation of redundant systems is not the answer.  Working out in
advance how to recognize and act upon an alternator failure IS the answer.

- Rob Wolf
LNC2 Mk2 IO-360