> I hope the 50% makes sweet music now! Best "headroom" seems to 
 > apply, when the panel open circuit voltage is twice the battery voltage.
I think that HomePower Magazine had an article some time ago that indicated a 
need for 18 or 19 volt-rated solar panels to charge a 12 volt battery. It was 
significantly higher than the nominal battery voltage, but not double. The 
reason for the higher voltage was as you said, to make up for blocking diode 
and regulator losses. You do need that headroom!
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