Batteries - Mixing Different Ah
Ken, I''d sure recommend reading the appropriate chapter in Calder''s _Electrical & Mechanical Guide for Boatowners_ (I may have murdered the title a bit), as it''s excellent and a solid understanding of boat electrics is almost essential for boatowners these days.
You shouldn''t mix battery chemistries, in part because the charging regimen for each type is somewhat unique. As for adding gel-type batteries to your existing bank to bulk up capacity, the critical issue does not seem to be batteries of varying capacity but rather age of the batteries. IOW putting a new, same-type battery in an older/used battery bank can rob the bank of some of its performance. Having said that, if you think the gels are lightly used and well cared for, it''s probably not a disqualifier.
You''re gonna end up with a pretty big battery bank: have you thought thru the ''other side'' of the equation, namely, how you get the same amp/hrs into the bank? Efficiently? While protecting the health of the bank? This almost never receives the initial attention it deserves when increasing bank capacity.
It is *very* hard to beat wet cell 6V golf cart-type batteries for value. And if you have the height in your battery compartment, I don''t know of a better space&weight for performance choice than the tall Trojan 6V wet cell batteries. Huge amounts of amp/hrs in a relatively small, affordable and long-lived battery.