Grunzster,
1) Your
current charge controller (a Morningstar SK-12), is only capable of handling a maximum panel
current of 12 amps, and you're just about there now (two Shell 85-P panels should have a max current of 11 amps)....(and it is not spec'd to handle them in series either)
So, you must get a new (bigger) charge controller.....
So, since you'll need a new controller, I'd recommend an
MPPT controller.....
The simple explanation of an
MPPT controller is that it allows the panels to operate at their maximum
power output (Max
Power Point = MPP), and provide the maximum charge current to your
batteries at their best charge voltage.....
Typically this can mean a 20% increase of actual battey charge current, vs. a standard charge controller (such as your current SK-12)......
My favorite is Blue Sky Energy
Blue Sky Energy Inc. - Industry Leaders In Solar Boost Charge Controllers and Maximum Power Point Tracking (MPPT) .....but others have reported good results from others as well....
2) If you're adding more panels to your array, and wish to use a MPPT charge controller (a VERY good idea, in my opinion), then you should try to match the new panel's voltage specs (particularly their Max Power voltage spec) to your existing panel's specs...
(Which would be about 17.2 vdc, for their MPP)
Here's a link (with your Shell 85-P specs) that might be helpful....
Shell Solar 85-P, 85 Watt Solar Panel
Many current "12-volt" panels have MPP voltages of 17.2 to 17.9 volts, and should be close enough to
work fairly well, if you cannot fit / find an exact match....
3) There's a lot more info on
solar out there.....but I think this answers your question, so I'll not bother to add more that might confuse you....
However, if you wish to see my set-up, have a look here....
Solar Panels
Fair winds and sunny skies...
John