I just bought a PA28-180 Cherokee and think I need a new battery. The current batt barely turns over the motor. As best I can tell it was installed in August 2006 and is a Gil G-35. That's over 11 years on the battery so I can believe it's time. After reading some it would appear replacing with a concorde RG-35A or RG-35AXC are highly preferred by this crowd and others. So, my questions are below. Go easy, new pilot, new aircraft owner so these may be obvious but I can't figure it out. 1) I seem to be reading this is legal for me as a pilot and owner to do to my own plane. Correct? 2) If I replace with the Gil with a Concorde AGM battery, do I need an STC or anything other than a log book entry. It seems that is all I need but was reading some of the concorde literature and an FAA AC 23-27 for vintage aircraft parts replacement and it says I can replace any group 35 batt with any other as long as they are within 1 lb of each other. These are NOT. 27 lbs for Gil, 29.5 for the RG-35A and 32lbs for the RG-35AXC. 3) I don't know if I need an STC or I am reading FAA/PMA wording and am confused about all of this. Any advice appreciated. 4) Do I need my W/B reworked if I do this? Maybe I am overthinking this but its just a lb or two and a battery swap. Should take 10 minutes but I keep thinking about things that might not be legal about this. Thanks in advance for any advice or experience.