kbecker,
I've been navigating this battery issue for a while now. There are two STCs out there that I'm aware of to install a Concorde battery for us.
Concorde has one - SA01118WI, and
Wilco has one - SA00897WI.
The Wilco STC has some Concorde batteries on it the Concorde STC does not. If you select a Concorde battery that is on the Wilco STC, Wilco will give you the STC (a $100 savings) if you buy the battery from Wilco.
I'm not sure how the Concorde STC works.
The Concorde battery website has a good airframe/competitor search page at
www.concordebattery.com (look under the "Search by your Aircraft" tab at the top of the page) but I suspect it might only show the batteries Concorde owns the STC for ... not the Concorde batteries Wilco owns the STC for.
I have a Concorde RG24-15M I'm working on getting a Field Approval for. Concorde did not include the RG24-15M battery in their STC, Wilco did - but Wilco did not include the Reims assembled Skymasters in their STC. I have an FT337GP 1976 P model and the RG24-15M is a perfect fit in the battery box and uses the original hold down clamp. My battery box is Part No. 1470090-10. This battery does not require venting, so no lid, or vent tube, is needed, as I understand it (I'll be surer about this once the Field Approval is granted.) The RG24-15M is a 13.6-amp hour battery, so it will give me 3.6 more amp-hours of energy than the currently specified Gill G-242 battery, and no acid to deal with!
The fitment in the battery box might be an issue, depending on the battery you select. Concorde alludes to this on their search page. A call to Concorde could help you ferret that out. The RG24-15M and the RG24-16 are remarkably close to the same size. there does appear to be a difference between the top of the two batteries though. That might affect the hold down clamp.
Once you select a Concorde battery and STC to use, and prior to your purchase/installation, I suggest you run it past your IA to make sure you've covered all the bases, so you don't wind up with a problem during your Annual Inspection.
Best wishes,
Rick