Will a bad marine battery give a false reading on a built in battery charger ?  It says to check connection which i did & it still says check connection & never fully charges . 

Have the battery tested. I had an MK charger that did that on one bank. It was the charger that was bad in my case.  

Yes either a battery with a bad cell can do it and so can a charger cause it. I suggest you go to your local harbor Freight ( or auto parts store, but HF is cheapest) and get yourself one of these simple battery load testers. They sell for $22