Use of solar cells for inflight recharging

Jim,
It is very remarkable you have been able to do this to your batteries
for four years without damage. I wouldn't recommend it for large solar
systems.

However, for an in-flight use, where the pv is basically augmenting the
battery supply a charge controller isn't needed. With longer term
charging & sustaining an inexpensive controller is essential if you
want to get the full life out of the batteries.

Regarding your need for 250 more watts: You might want to do some math
as it looks like you are right on with what you have now. PV's today
are going for between $4 and $6 per watt, so your 250 W would cost you
around $1,000 at the bottom end not including freight, mounting, and
labor. So at $40/year savings it would take you the life of the PV to
earn back your expense, not considering present dollar value or roi on
the $1000 if you didn't spend it.

Sounds like you planned your system very, very well!